Mitigation of Marine Aggregate Dredging Impacts ... - Cefas - Defra

Marine 

Aggregate Levy 

Sustainability Fund 

MALSF 

Mitigation of Marine Aggregate 

Dredging Impacts – Benchmarking 

Equipment, Practices and Technologies 

against Global Best Practice 

MEPF Ref No: MEPF 08/P33 Project Date: April 2010

Benchmarking Equipment, Practices and Technologies 

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Report No: 10/J/1/06/1309/0996 

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1.1� Context........................................................................................................................... 5� 3 

1.2� Approach........................................................................................................................5� 3 

1.3� Outputs .......................................................................................................................... 6� 5 

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2.1� Introduction ....................................................................................................................9� 6 

2.2� The dredging cycle......................................................................................................... 9� 6 

2.2.1� Review of impacts general to all shipping.................................................... 11� 8 

2.2.2� Impacts associated with transit phase of dredging cycle............................. 14� 11 

2.2.3� Impacts associated with loading phase of dredging cycle ........................... 16� 13 

2.2.4� Impacts Associated with discharging phase of the dredging cycle.............. 28� 25 

2.3� Conclusions ................................................................................................................. 29� 26 

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3.1� Description of the current English fleet ........................................................................ 30� 28 

3.1.1� Introduction .................................................................................................. 30� 28 

3.1.2� Vessel Age................................................................................................... 33� 31 

3.1.3� Design.......................................................................................................... 35� 33 

3.1.4� Crew............................................................................................................. 38� 36 

3.1.5� Size.............................................................................................................. 39� 37 

3.1.6� Power and Productivity Rates...................................................................... 40� 38 

3.2� UK Technology ............................................................................................................ 42� 40 

3.2.1� Dragheads ................................................................................................... 42� 40 

3.2.2� Dredge pumps, impellers and pipes ............................................................ 44� 42 

3.2.3� Screening and Loading................................................................................ 52� 50 

3.2.4� Overflow control........................................................................................... 55� 53 

3.2.5� Discharge Systems...................................................................................... 60� 58 

3.3� Dredging Practices, Mitigation and Regulatory Framework......................................... 63� 61 

3.3.1� Dredging Practices....................................................................................... 63� 61 

3.3.2� Mitigation...................................................................................................... 67� 65 

3.3.3� Regulatory Framework................................................................................. 70� 68 

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4.1� Description of current world fleet ................................................................................. 73� 72 

4.1.1� Introduction .................................................................................................. 73� 72 

4.1.2� Vessel Age................................................................................................... 73� 72 

4.1.3� Design.......................................................................................................... 74� 73 

4.1.4� Crew............................................................................................................. 76� 75 

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4.1.5� Size.............................................................................................................. 76� 75 

4.1.6� Power and Productivity Rates...................................................................... 79� 78 

4.2� World Technology ........................................................................................................ 79� 78 

4.2.1� Dragheads ................................................................................................... 79� 78 

4.2.2� Dredge pumps, impellers and pipes ............................................................ 92� 91 

4.2.3� Screening and Loading................................................................................ 97� 96 

4.2.4� Overflow control........................................................................................... 97� 96 

4.2.5� Discharge systems..................................................................................... 104� 103 

4.3� Dredging Practices, Mitigation and Regulatory Frameworks ..................................... 105� 104 

4.3.1� Dredging Practices..................................................................................... 105� 104 

4.3.2� Mitigation and dredging practices .............................................................. 110� 109 

4.3.3� Regulatory Frameworks............................................................................. 117� 116 

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5.1.1� Technology ................................................................................................ 124� 123 

5.1.2� Dredging Practices, Mitigation and Regulatory Framework....................... 127� 126 

Conclusions................................................................................................................................ 132� 131 

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�� Methods to estimate overflow losses........................................................................ �� 144 

�� Air pollution mitigation................................................................................................�� 148 

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Figure 1-1: Summary of project approach.......................................................................................................6� 4 

Figure 2-1: Schematic description of an entire dredging cycle .....................................................................10� 7 

Figure 2-2: Sound spectrum of dredging noise recorded at varying distances and ambient noise level (from 

Defra, 2003) ..................................................................................................................................................17� 14 

Figure 2-3: Summary of direct and indirect impacts associated with dredging .............................................19� 16 

Figure 2-4: Schematic diagram showing the likely re-colonization rates for benthic communities of estuarine 

mud, sand, gravel and rocky reefs (Newell and Seiderer, 2003) ..................................................................21� 18 

Figure 2-5: Direct impact of the dredging process ........................................................................................22� 19 

Figure 2-6: Indirect impacts of the dredging process ....................................................................................27� 24 

Figure 3-1: Typical 1950s Sand Dredger. .....................................................................................................32� 30 

Figure 3-2: The newest vessel dredging English Licence Areas – DEME’s vessel “Charlemagne” (Image 

courtesy of BMAPA)......................................................................................................................................34� 32 

Figure 3-3: Ages of vessels in the current fleet (years).................................................................................34� 32 

Figure 3-4: The "Donald Redford" - a converted grab dredger. (Image courtesy of BMAPA).......................35� 33 

Figure 3-5: Example of TSHD with an aft configuration - the "Sand Falcon". (Image courtesy of BMAPA). 36� 34 

Figure 3-6: Example of TSHD with forward configuration - the "City of Westminster". (Image courtesy of 

BMAPA). .......................................................................................................................................................36� 34 

Figure 3-7: The TSHD “City of Chichester” – a 2,300 Tonne Vessel serving South Coast ports. (Image 

courtesy of BMAPA)......................................................................................................................................40� 38 

Figure 3-8: Engine room on the "Sand Fulmar". (Image courtesy of BMAPA)..............................................41� 39 

Figure 3-9: Simple single visor draghead. (Image courtesy of BMAPA). ......................................................42� 40 

Figure 3-10: California style draghead. (Image courtesy of BMAPA). ..........................................................43� 41 

Figure 3-11: Inboard double casing pump. (Image courtesy of BMAPA)......................................................45� 43 

Figure 3-12: Vertical hydraulic transport .......................................................................................................46� 44 

Figure 3-13: Mixture density as a function of the mixture velocity at minimum pump inlet pressure ............48� 46 

Figure 3-14: Mixture density and production at the vacuum limit..................................................................49� 47 

Figure 3-15: Influence of pump depth on density and production for two cases of dredging at 50m depth..50� 48 

Figure 3-16: Specific energy for hydraulic transportation (suction)...............................................................51� 49 

Figure 3-17: Fixed single screen on the “City of Cardiff”. (Image courtesy of BMAPA). ...............................53� 50 

Figure 3-18: Rotating screening tower on the “Arco Dijk”. (Image courtesy of BMAPA)...............................53� 50 

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Figure 3-19: Sand level in hopper .................................................................................................................54� 52 

Figure 3-20: Determining optimal loading time .............................................................................................55� 53 

Figure 3-21: The three phases of loading and overflow................................................................................56� 54 

Figure 3-22: Velocity and concentration distribution in a hopper ..................................................................57� 55 

Figure 3-23: Settling velocity as a function of the particle size .....................................................................58� 56 

Figure 3-24: Screen and spillway overflows on the "Arco Dijk". (Image courtesy of BMAPA). .....................59� 57 

Figure 3-25: Rejects from the underside of the screen deck combine in the vertical overflow weir on the 

“City of Chichester”. The combined flow then exits through the bottom of the vessel. (Image courtesy of 

BMAPA). .......................................................................................................................................................59� 57 

Figure 3-26: The "Donald Redford" being discharged by shore crane. (Image courtesy of BMAPA). ..........60� 58 

Figure 3-27: Modern drag scraper system on the "City of London". (Image courtesy of BMAPA)................61� 59 

Figure 3-28: Bucket wheel discharger on the "Sand Heron". (Image courtesy of BMAPA). .........................61� 59 

Figure 3-29: Grab discharger on the "City of Chichester". (Image courtesy of BMAPA). .............................62� 60 

Figure 3-30: 26m forward boom conveyor. (Image courtesy of BMAPA)......................................................63� 61 

Figure 3-31: Example of Microplot display showing Active Area boundary (red line); dredging lanes (blue 

boxes); contaminant symbols (individual red crosses and diamonds) and exclusion areas (solid red). 

(Image courtesy of HAML). ...........................................................................................................................64� 62 

Figure 3-32: Example of dredging intensity plot from the EMS system (from The Crown Estate, 2009). .....65� 63 

Figure 3-33: Regulatory procedure for obtaining an English aggregate licence (Marine and Fisheries 

Agency, 2009). ..............................................................................................................................................72� 70 

Figure 4-1: Design configurations of TSHDs of the world fleet .....................................................................74� 73 

Figure 4-2: Practical results of improved hull shapes: top the "Lange Wapper" (without bulb); bottom the 

"Charlemagne" (with bulb). (Image courtesy of BMAPA)..............................................................................75� 74 

Figure 4-3: Operator position on TSHD "Brabo". (Image from IHC, 2007). ..................................................76� 75 

Figure 4-4: Modern IHC draghead. ...............................................................................................................80� 79 

Figure 4-5: The cutting process in granular sediments .................................................................................81� 80 

Figure 4-6: Jet production and density as a function of the jet pressure.......................................................84� 83 

Figure 4-7: Comparison between specific energy for cutting and jetting ......................................................85� 84 

Figure 4-8: The Wild Dragon draghead on board the TSHD “Xin Hai Long” demonstrating the high power 

water jets (from IHC, 2005). ..........................................................................................................................87� 86 

Figure 4-9: Yield indicators comparing the Wild Dragon draghead (left) and conventional draghead (right) 

when dredging the Yangtze (from IHC, 2005)...............................................................................................87� 86 

Figure 4-10: 7.2m wide draghead on the TSHD "Seriyu-maru" (from Yano et al., 2006). ............................89� 87 

Figure 4-11: The Vic Vac dredge head (image used with permission of J.F. Brennan Co., Inc.). ................90� 89 

Figure 4-12: Isometric view from underneath the Vic Vac dredge head (image used with permission of J.F. 

Brennan Co., Inc.). ........................................................................................................................................90� 89 

Figure 4-13: Schematic of the Tornado Motion eddy pump (image used with permission of Tornado Motion 

Technologies)................................................................................................................................................91� 90 

Figure 4-14: Tornado Motion suction head (image used with permission of Tornado Motion Technologies). 

......................................................................................................................................................................91� 90 

Figure 4-15: Cross-section through a conventional dredge pump (from IHC, 2004). ...................................93� 92 

Figure 4-16: Cross-section through a high efficiency pump (from IHC, 2004). .............................................93� 92 

Figure 4-17: Increased output when using IHC high efficiency pumps and impellers in different substrates 

(from IHC, 2004). ..........................................................................................................................................94� 93 

Figure 4-18: The Punaise Submerged Dredging Pump. ...............................................................................95� 94 

Figure 4-19: The Aft Centre Dredge Pipe on the TSHD “Seriyu-maru”.........................................................96� 95 

Figure 4-20: Telescopic overflow pipe (HAM 317) ........................................................................................98� 97 

Figure 4-21: Loading production as a function of the inflow concentration ...................................................99� 98 

Figure 4-22� Influence of water temperature on settling velocity.......................................................99� 98 

Figure 4-23: Anti-turbidity overflow system (from Ofuji and Ishimatsu, 1976).............................................100� 99 

Figure 4-24: Sketch showing the low turbidity valve (Jan de Nul, 2003).....................................................101� 100 

Figure 4-25� Water level in hopper with or without the influence of the green valve .......................102� 101 

Figure 4-26: Sketch showing the "Green Pipe" system (Jan de Nul, 2003)................................................103� 102 

Figure 4-27: Comparison tubidity plume dispersions (Jan de Nul, 2003) ...................................................104� 103 

Figure 4-28: Screen display of the IHC Dredge Track Presentation System (from Mallee, 2000)..............106� 105 

Figure 4-29: Example of EMS record from a Dutch monitoring system (from Sutton and Boyd, 2009)......108� 107 

Figure 4-30: Analysis of dredging intensity in Køge Bugt, Denmark (from Sutton and Boyd, 2009)...........108� 107 

Figure 4-31: On board dredge display for the Dredge Specific System (from Minerals Management Service, 

2004) ...........................................................................................................................................................109� 108 

Figure 4-32: Silt curtain in Alberta, Canada, showing high sediment concentrations contained on the right 

hand side of the curtain...............................................................................................................................112� 111 

Figure 4-33: An example of the cumulative effect of multiple environmental windows applied to the same 

dredging project ..........................................................................................................................................114� 113 

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Figure 4-34: Principles of feedback monitoring (from Jensen, 2006) ............................................116� 115 

Figure A-1 Ideal settling basin according to Camp ................................................................... 144 145� 

Figure A-2� Removal ratio according to Camp including the influence of turbulence ................... 145 146� 

Figure A-3� Velocity and concentration distribution during loading (2DV model )..........................148� 147 

Figure B-1� NOx limits (from www.dieselnet.com) ........................................................................150� 149 

Figure B-2� Power consumption of various transport means 

� (from Gabrielli and Von Karman, 1950) ...................................................................... 153 152 

Figure B-3� Comparison of Energy Index for different means of transportation. ...........................154� 153 

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Table 2-1: General and dredging specific impacts generated by TSHDs .....................................................11� 8 

Table 2-2: Composition of World Fleet (2007). .............................................................................................12� 9 

Table 2-3: MARPOL Annexes, Waste Categories and Types of Waste .......................................................13� 10 

Table 2-4: Number of recorded marine pollution incidents involving the English fleet (BMAPA, 2009)........13� 10 

Table 2-5: Indicative port to port times (does not include discharge) and transit times for three regional 

cases of the English dredging industry. ........................................................................................................14� 11 

Table 2-6: Proportion of total fuel consumed during transit for long haul and short haul cases ...................15� 12 

Table 2-7: Typical vessel capacities and loading times for UK aggregate dredging regions. .......................16� 13 

Table 2-8: Approximate fuel consumption during dredging...........................................................................16� 13 

Table 2-9: Processes contributing to dredging noise during loading (Thomsen et al., 2009) .......................17� 14 

�� Pennekamp, 1996)........................................................................................................................................25 22 

Table 2-11: Approximate fuel consumption during discharge .......................................................................28� 25 

Table 2-12: Matrix of relative sensitivities associated with the different phases of the dredging cycle.........29� 26 

Table 3-1: Operators and ships of the UK Fleet............................................................................................33� 30 

Table 3-2: Examples of third-party vessels that dredge on English licence areas........................................33� 31 

Table 3-3: Fuel usage and CO2 emissions for the English fleet (BMAPA, 2009)..........................................37� 35 

Table 3-4: General crew structure of English marine aggregate dredgers ...................................................38� 36 

Table 3-5: Sizes of vessels in the English dredging fleet (Clarkson Research Services Ltd, 2009) .............39� 37 

Table 3-6: Approximate production rates for two English dredging regions..................................................41� 39 

Table 3-7: Reduction in dredging hours 2006-2008 (BMAPA, 2009)............................................................66� 64 

Table 3-8: Production and dredging hours 2006-2008 (BMAPA, 2009)........................................................66� 64 

Table 4-1: Total world dredging fleet (Clarkson Research Services Ltd, 2009)............................................73� 72 

Table 4-2: Mean sizes of the world fleet and the maximum and minimum examples (Clarkson Research 

Services Ltd, 2009) .......................................................................................................................................77� 76 

Table 4-3: Currently commissioned and proposed megadredgers, as of November 2009...........................78� 77 

Table 4-4: Reported maximum and minimum engine, generator and dredge pump powers (Clarkson 

Research Services Ltd, 2009).......................................................................................................................79� 78 

Table 4-5: Examples of high productivity rates .............................................................................................79� 78 

Table 4-6: Results of including knives and water jets on a draghead (from Brogdon Jr et al., 1994). ..........88� 87 

Table 4-7:Suspended sediment concentrations with and without an Anti-turbidity Overflow System on the 

TSHD "Kairyu Maru" (from Herbich and Brahme, 1991). ............................................................................101� 100 

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Report No: 10/J/1/06/1309/0996 

Marine aggregate dredging in English waters has been carried out in the same way, on a similar 

scale and using similar equipment for around 3 decades. Production of marine aggregates during 

this period rose initially, has remained relatively static for the last 20 years and has the potential 

to rise in the future. Elsewhere in the world during this time dredging engineering, materials and 

practices have changed; dredging activity has intensified and understanding of the marine 

environment and dredging impacts has advanced significantly. Currently dredging best practice is 

not necessarily shared between developers, or regulators and their environmental assessors, as 

a result of competition, culture and knowledge. In addition the English marine aggregate industry 

is relatively isolated – commonly new information on dredging practice or technology is not 

circulated amongst marine aggregates businesses which are firmly focussed in England. 

Consequently the review of practices and technologies will result in a range of new information 

for all stakeholders. 

This project will therefore review current marine aggregate dredging in English waters, and 

current English marine aggregates practices and technologies are benchmarked against 

dredging techniques used in other parts of the world. This project examines dredging best 

practice, case studies and equipment from around the world and assesses whether they are 

applicable, necessary or realistic for the English industry. 

Quantitative information about novel technologies is typically commercially confidential, difficult to 

obtain and quantitative comparisons are therefore often impossible. This project is not intended 

to provide solutions; however it will indicate where opportunities for developing and improving 

practice may exist. It is also important to note that this report will not look at the mitigation of 

potential effects far from dredging areas. These can not be mitigated by alteration of dredging 

technology and are beyond the scope of this study. 

The equipment and methods used to achieve recovery and transport of marine aggregates are 

intimately associated with the environmental impacts of the process. This project will review and 

benchmark long standing English equipment, technologies and methods and identify 

opportunities to develop aggregate dredging mitigation and management techniques where 

appropriate. Benchmarking therefore offers a clear potential to evolve marine aggregate 

extraction and promote environmentally sensitive practices through a clear understanding of 

global best practice and available technologies. This project will assist the industry in assessing 

the ‘fitness for purpose’ of its current technology and mitigation strategies. It will also provide 

regulators and advisors with confidence regarding the best practice of aggregate dredging and 

the proportionality of dredging proposals. 

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This project will produce a position statement of the technologies and techniques in use by the 

English dredging industry and compare these with worldwide technologies and techniques. 

The approach the project will follow is summarized in Figure 1-1. 

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Outputs include: 

Report No: 10/J/1/06/1309/0996 

1. A summary of the technologies, capabilities and practices of the current English and 

world dredging fleets. 

2. Enhanced understanding of global practice and available technologies and 

confidence in whether current English technology and mitigation is proportionate 

3. Potential future investigation areas for development. 

4. Identification of risks - a global view of dredging will assist regulators and their 

advisors in assessing marine aggregate dredging proposals, and the industry in 

proposing ‘fit for purpose’ mitigation strategies 

5. Improving business performance through improved environmental and industry 

performance, potential efficiency improvement, investment guidance and application 

acceptability. 

This report will provide a high level review of the opportunities available and identifies knowledge 

gaps for further consideration. 

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Marine aggregate dredging results in a range of impacts on the environment and on ecosystem 

goods and services (Austen et al., 2009). Some of these impacts are general to all types of 

shipping while some are specifically a result of the operation of a marine aggregate dredging 

vessel. However, not all of the impacts specific to dredgers occur at all phases of the dredging 

cycle. This chapter will therefore define the dredging cycle, and give an overview of the 

environmental impacts associated with all shipping and specific to dredging vessels. It will 

provide an overview of which dredging specific impacts occur at which phase of the dredging 

cycle, and assesses the relative importance of these at the different phases. 

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The English marine aggregate business involves the movement of a relatively low value product 

(sand and gravel) from where it occurs offshore, to markets where it is required. The total 

aggregate dredging cycle may be divided into components, each component of which will have 

different environmental impacts. These components are: 

� Transit; 

� Loading; 

� Discharge. 

The cycle is shown schematically in Figure 2-1. English marine aggregate dredging involves, in 

most cases, the dredger sailing between multiple wharves and multiple licences. 

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The profit equation for the English marine aggregate industry involves maximising the individual 

cargo size and the number of cargoes delivered per year and minimising the costs, particularly 

fuel. The optimisation of cycle time and speed whilst maintaining good environmental 

performance is therefore critical to the economics of the business. However, with the exception of 

deliveries to some Continental ports, most dredging cycles are governed by the tides and so 

most are defined in blocks of 12 hours. Because cycle times are governed so strongly by tides 

improving production rate is not such an issue for the English industry since this would often 

simply increase the amount of time a vessel was idle waiting to enter a tidal port. Improving 

production rates is a much stronger driver for the maintenance and capital dredging sectors 

where minimising time equates much more strongly with minimising costs. 

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Empty vessel sails from 

wharf to Licence Area 

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Sediment is dredged from 

the Licence Area and 

loaded into the vessel 

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Full vessel sails from 

Licence Area to wharf. 

Often a different wharf from 

its origin 

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Offloading the cargo from the 

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Smaller vessels with short steaming distances achieve 12 hour cycles and deliver over 400 

cargoes per year. Larger vessels with longer steaming times can sometimes achieve 24 hour 

cycles if the dredging time is limited to approximately 4 hours as sometimes occurs on the East 

Coast when sandy cargoes are required. More typically they require 36 hour cycles, and the 

largest vessel with very long steaming times can take 48 hours to complete a cycle – for example 

if delivering a cargo into a Thames port from Licence Areas around the Isle of Wight. Typically 

these larger vessels will deliver between 200 and 230 cargoes per year. 

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This section of the report will give an overview of the environmental impacts associated with the 

dredging process. The environmental impacts of dredgers can be divided into impacts that are 

common to all types of shipping, and those that are specific to dredgers (Table 2-1). 

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Emissions Direct impacts of substrate removal 

Waste Indirect impacts of the sediment plume 

Noise generated by propulsion Noise generated by dredging process 

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General shipping impacts are present during all phases of the dredging cycle, while impacts 

associated with substrate removal and generation of sediment plumes will only occur during the 

loading phase of the cycle. 

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Marine fuels generate CO2, nitrogen oxide (NOx) and sulphur oxide (SOx) which are all 

greenhouse gasses. Vessel exhaust gasses can be scrubbed to remove SOx, and NOx 

emissions can be reduced through altering combustion process in the engine by changing the 

temperature in the engine. Adjusting the engine specification has the potential to reduce 

the NOx emissions by up to 20% while the introduction of selective catalytic recombiners (SCRs) 

can potentially reduce the levels by 90% while optimizing engine performance. The currently 

available NOx and SOx abatement technologies may be mutually exclusive – particularly 

the use of catalytic recombiners which require inlet gas temperatures much higher than the 

exhaust temperature of scrubbers. Further information on controlling pollution can be found 

in Appendix B. 

The IMO working group on Bulk, Liquid and Gas (BLG) has estimated the quantities of fuel used 

and emissions generated by the world’s shipping fleet based on the Lloyds Register, and Table 

2-2 below shows the composition of the world fleet in 2007. 

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Number Gross Tons Number Gross Tons 

Shipping 9,732 181,668,000 51,687 770,980,000 

Dredging 371 1,018,000 1,309 2,719,000 

Offshore supply 514 1,175,000 3,626 4,678,000 

Tugs 1,863 564,000 12,544 1,328,000 

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The BLG estimates total fuel consumption in 2007 by the world fleet of approximately 370 million 

tonnes – composed of 285 million tonnes of HFO and 85 million tonnes of marine distillates, and 

this translates into approximately 1100 million tonnes of CO2. Table 2-2 shows that the numbers 

of dredging vessels relative to general fleet is small. 

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The International Convention for the Prevention of Pollution from Ships (MARPOL) was adopted 

on 2 November 1973 at IMO and covered pollution by oil, chemicals, harmful substances in 

packaged form, sewage and garbage. 

The MARPOL Convention is the main international convention covering prevention of pollution of 

the marine environment by ships from operational or accidental causes. It is a combination of two 

treaties, adopted in 1973 and 1978 respectively, and updated by amendments through the years. 

Regulations set by MARPOL (73/78) and Directive 2000/59/EC of the European Parliament on 

port reception facilities for ship-generated waste and cargo residues also stipulate that ports 

should provide reception facilities for vessels to safely dispose and manage the various types of 

waste designated. MARPOL designates marine waste under 6 Annexes (Table 2-3). 

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I Oil Covers all types of wastes from the carriage of oil: As 

II Noxious liquid 

substances in bulk 

III Harmful substances 

carried by sea in 

packaged form 

fuel, engine room slops, cargo (tank washings) or dirty 

ballast water 

Chemical wastes derived from bulk chemical 

transportation, including residues and mixtures 

containing noxious substances 

Hundreds of specific substances defined by the 

International Maritime Dangerous Goods Code 

IV Sewage from ships Raw sewage retained in holding tanks for disposal in 

port or outside 12 nm. Partially treated sewage retained 

in holding tanks for disposal in port or outside 4 nm 

V Rubbish from ships Rubbish includes domestic (food and packaging) and 

operational (maintenance, cargo and miscellaneous) 

wastes 

VI Air pollution from ships Sets limits on SOx and NOx emissions from ship 

exhausts and prohibits deliberate emissions of ozone 

depleting substances 

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National regulations require port authorities and terminal operators to provide reception facilities 

for ships waste to allow vessels to manage their obligations under MARPOL. All dredging vessels 

working on English licence areas fulfil their obligations under the regulations. 

Information on recorded marine pollution incidents involving English marine aggregate vessels 

has been published for the last three years as part of the industry’s commitment to environmental 

reporting (BMAPA, 2009). Table 2-4 shows the number of recorded marine pollution incidents 

from 2006-2008. 

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Incidents (all minor hydraulic leaks) 6 0 6 

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General noise generated by shipping is dominated by the ship engine and propeller and these 

are of a low frequency relative to higher frequency dredging specific noise generated by 

aggregates rising up through the suction pipe, movement of the draghead on the seabed and 

splashing from the spillways (Parvin �� 2008). 

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Marine aggregates in the UK are produced by 11 companies – the majority of which are 

members of the British Marine Aggregates Producers Association. The BMAPA member 

companies have recognized that “the marine environment in which it operates is sensitive, and 

accepts that it has a responsibility to manage its operations in ways that minimise any effects on 

the marine environment and on its other users” (BMAPA, 2006). 

Transit times for UK aggregate dredging areas vary on the geographical location of the dredging 

area compared with the originating / deliver wharves. Long transit times can mean an increased 

impact with increased fuel consumption relative to the tonnage dredged. Table 2-5 below shows 

indicative 2-way transit times for three regional case studies. These times are port to port times 

and do not include time in port when the vessel is discharging. This is covered in Section 2.2.3. 

�� 

�� 

�� 

Bristol Channel 12 50 6 

South Coast 12 70-80 8-9.5 

East Anglia/Eastern 

English 

London 

Channel & 

25 70 20 

�� 

�� 

� �� 

Fuel consumption for a dredging vessel varies depending on the scale of the vessel and the 

phase of the dredging cycle. The English dredging industry has, for the last three years, been 

reporting their fuel consumption figures as part of their ongoing ‘Strength From The Depths’ 

sustainable development report (BMAPA, 2009) and energy consumption has been studied as 

part of The Crown Estate’s research programme (Kemp, 2008). Based on data from these 

reports Table 2-6 shows the estimated proportion of fuel consumed during transit for two 

dredging cases – long haul (larger ships) or short haul (smaller ships). 

11


� �� 

Report No: 10/J/1/06/1309/0996 

�� 

>3000 tonnes Long haul e.g. 

East Anglia / EEC 


�� 

� � 

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The time taken by a dredging vessel to load a cargo is controlled by a range of variables 

including the size (capacity) of the vessel, the power of the dredge pump, the amount of 

screening taking place and the composition of the seabed sediment. Given this, Table 2-7 

summarizes some indicative values for typical vessel capacities and loading times for five 

regions of activity for the English aggregate industry. 

�� 

�� 

Bristol Channel 750 2-4 

South Coast 750 2-4 

East Anglia 2750 5-8 

Eastern English Channel 5000 5-8 

Humber 4500 5-8 

13 

�� 

�� 

� �� 

� �� 

Based on data from BMAPA (2009) and Kemp (2008) the proportion of fuel consumed during 

loading has been estimated for two dredging cases – long haul (larger ships) or short haul 

(smaller ships) (Table 2-8). 

�� 

�� 

�� 




Report No: 10/J/1/06/1309/0996 

�� 

Collection 

noise 

Pump 

noise 

Transport 

noise 

Deposition 

noise 

Ship/machinery 

noise 

Noise arising from collection of sediment from the sea floor e.g. the operation 

of the drag head 

Noise from the pump driving suction through the dredge pipe 

Noise of the sediment being lifted from the sea floor to the dredger i.e. for a 

TSHD the noise of the material in the dredge pipe 

Noise associated with the placement of sediment within the hopper 

Noise associated with the ship machinery, including propeller and thruster 

noise. 

�� 

Noise associated with dredging is predominantly of low frequency – below 1 kHz (Thomsen et al., 

2009); however within this, the noise derived from aggregates rising up through the suction pipe, 

the movement of the draghead on the seabed and splashing from the spillways are of higher 

frequencies than those generated by the ship engine and propeller (Parvin �� 2008). 

Greene (1987) and Richardson et al. (1995) published studies describing the noise emitted by 

five vessels during dredging in the Beaufort Sea. These measurements were made in the early 

1980’s. In these studies the TSHDs produced the loudest noise during loading and the greatest 

range at which the dredging noise could be detected above background noise was 25 km. 

Defra (2003) took hydrophone measurements of noise produced by the TSHD Arco Adur 

operating at Licence Area 328 to the east of Great Yarmouth (Southern North Sea) and at 

Licence Area 366 on the Hastings Shingle Bank in the Eastern English Channel. The noise 

produced was predominantly of low frequency, below 500 Hz (Figure 2-2). 

�� 

�� 

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Report No: 10/J/1/06/1309/0996 

Their findings also showed that: 

� At low frequencies (2 kHz), higher levels of noise were generated by full suction 

dredging activities. 

Parvin et al. (2008) found that the 100 m long TSHD City of Westminster, powered by two 1950 

kW engines, loading at 2.5 m 3 s -1 and dredging at an average speed of 2 knots, increased 

underwater noise in close proximity to the vessel over a broad range of frequencies (20 Hz – 80 

kHz approx.) and that the dredging noise would fall below the ambient noise level (and so not be 

audible) at a range of 6 km. This distance is substantially less than that measured by Richardson 

et al. (1995) and Thomsen et al. (2009) suggests that this is because the English Channel has 

significantly higher levels of background noise than the Beaufort Sea, Alaska. 

Thomsen et al. (2009) also concluded that the available data indicated that the noise generated 

during the loading phase was not as noisy as seismic surveys, pile driving or sonar; but louder 

than ship transits (including TSHD transits) and should be viewed as a medium impact activity. It 

was also concluded that the noise generated during loading is potentially of bigger relevance for 

seals and marine fish than for cetaceans, as the overlap between the emitted frequency 

spectrum and the bandwidth of hearing is bigger in seals and fish than for cetaceans (Thomsen 

et al., 2009). 

A new MALSF funded project (MEPF 09 / P108) has been commissioned which aims to produce 

a baseline dataset for underwater noise associated with a range of UK dredger types, deposit 

types and screening conditions. 

� �� 

The impacts of loading of marine aggregates in English waters are based upon the physical 

disturbance impacts related to the removal of coarse sediment substrate and can be divided into 

the direct impacts of substrate removal and the indirect impacts of the generated sediment 

plume. These are summarized in Figure 2-3. 

15


Report No: 10/J/1/06/1309/0996 

�� 

16


� �� 

The direct impacts of dredging result from the removal of the substrate in the licensed dredging 

area and are summarised in Figure 2-5. Dredging will alter the local bathymetry by directly 

removing sediment, and the length of time that dredging features (dredge trails) will remain 

distinct on the seabed depends on the sediment type and the transport potential of local tides 

and waves (Diesing et al., 2006). For example, at an experimental dredged gravel site off Norfolk 

in 25 m of water, dredge tracks appeared to have been completely eroded within three years of 

the cessation of dredging (Kenny and Rees, 1994, 1996; Kenny et al., 1998) while erosion of 

dredge tracks in areas of moderate wave exposure and tidal currents have been observed to 

take from three to more than seven years in gravelly sediments (Limpenny et al., 2002; Boyd et 

al., 2003a, 2005; Cooper et al., 2005). In relict deposits, such as those typically dredged by the 

English industry, the gross alteration of the seabed topography and bathymetry is likely to be 

permanent and this means that while individual dredge trails will be eroded over time, the wider 

depressions will typically remain. 

Report No: 10/J/1/06/1309/0996 

Alteration of local seabed topography can potentially impact the hydrodynamics, and before a 

licence is granted for aggregate extraction, potential changes in local waves and current patterns 

are assessed through site-specific modelling studies. Changes in wave heights and current 

direction after dredging can result in localized changes in erosional and depositional patterns 

(nearfield effects), and possibly even in shoreline changes (farfield effects). It was concluded by 

Van Rijn et al. (2005) that, in general, dredging has little influence on the macroscale current 

pattern and in most cases the current pattern is only changed in the direct vicinity of the 

dredged area. 

A further direct impact of English dredging practices is the potential alteration of the substrate 

over time. This is because screening of sediments is common in order to meet a specific sand 

and gravel requirement for the market. Hitchcock and Drucker (1996) and Newell et al. (1998) 

estimated that, during typical loading of a dredged cargo at some extraction areas in the UK, up 

to 1.6 – 1.7 times the total cargo is discharged into the surrounding water column as a 

consequence of the screening process. Clearly, estimates such as these are site�specific and 

will vary in relation to the grain size of seabed sediments, the grading required for the cargo, and 

the efficiency of the dredger. However over time, the progressive removal of the original sandy 

gravel or coarse sands, and their replacement by finer sandier sediment fractions through 

screening activities, may result in a gradual fining of the sediment within the extraction areas. In 

some instances, this increase in fine sand may be temporary because of the reworking 

capabilities of tides and waves. By altering the size distribution of the seabed from coarser to 

finer particles, the seabed has the potential to provide alternative habitats to other fauna which 

may play a similar functional role to the fauna that have been lost through the dredging process 

(Cooper et al., 2008). 

Dredging also has an obvious direct impact due to removal of benthic organisms and habitat 

within the dredging areas, along with infaunal and epifaunal organisms that are incapable of 

avoiding dredging. A number of studies have investigated the recovery of benthic communities 

following dredging (Blake et. al., 1996; Newell et al, 1998, 2004; Boyd et al., 2003b, 2004) and 

17


Report No: 10/J/1/06/1309/0996 

have shown that community diversity and abundance can recover within several years. Newell 

and Seiderer (2003) also summarize recovery rates of benthic communities post-dredging for 

different substrate types (Figure 2-4) with sandy substrates typically recovering within 2 to 4 

years. 

�� 

�� 

The Minerals Management Service (2004) suggests that though re-colonized communities may 

be similar in terms of total abundance and species diversity, their taxonomic composition, in 

terms of dominant species and species abundance, is often very different from pre- to postdredging. 

Benthic resources are important in the food web for commercially and recreationally 

important fishes and invertebrates, and they contribute to the biodiversity of the pelagic 

environment. Their removal can therefore lead to reduced species richness, biodiversity and food 

sources within the dredging areas – and subsequent potential effects on spawning, stock 

recruitment and displacement of organisms. 

18


� 

�� 

� 

�� 

Report No: 10/J/1/06/1309/0996 

�� 

� 

�� 

� 

�� 

�� 

�� 

�� 

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19 

�� 

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� �� 

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Dredging leads to the production of plumes of suspended material, partly from the disturbance of 

the seabed by the draghead but mainly from the outwash of sediment from the dredger’s 

screening towers and the hopper spillways. These plumes can have indirect impacts (i.e. impacts 

outside the dredging area itself) on the environment and these are summarized in Figure 2-6. 

The spatial extent and transport of the plumes depends on the sediment particle size, total 

quantity of material suspended, velocity of discharge, and the local hydrodynamics (Hitchcock 

and Drucker, 1996; Hitchcock and Bell, 2004). 

The water flowing overboard from the vessel contains sediment but because the coarse particles 

settle faster than fine sediments, the overflow discharge contains relatively more fines that the 

sediment loaded. The largest part of the sediment flux flowing overboard will settle close to the 

dredging vessel because the sediment water mixture has a density larger than the ambient seawater. 

This creates a density driven plume (also called the dynamic plume) which transports 

sediment directly towards the seabed. The remainder of the overflow sediment that remains in 

suspension (the so-called source term) is the passive plume. Reliable and calibrated models are 

not yet available for these plumes, however a PhD research study is being undertaken at 

Technical University Delft to develop a model to quantify the magnitude of this source term (De 

Wit, 2009). 

The dynamic plume is driven by initial momentum or buoyancy due to density differences 

between the plume and ambient water. The density and velocity in the plume is governed by the 

bulk behaviour of the plume and not by the individual fall velocity of the particles. In contrast, for 

a passive plume the density difference between the plume and surrounding water is negligible 

and the individual settling velocities of the particles determine the settling velocity of the plume 

itself. The dynamic plume behaviour is mostly present in the near field of the dredger, while the 

passive behaviour is observed in the far field (Spearman et al. 2004). An exact location of the 

transition between near field and far field cannot be given and depends on the operational 

process conditions. From an experimental study (Winterwerp, 2002) it appeared that near field 

overflow plume can be either dynamic or passive depending on a bulk Richardson number and a 

velocity ratio, defined as: 

� � 

Where : 

� � � 

� �� 

�� 

� � 

� � � � � � 

� � � �� 

� 

� 

� � � 

�� 

� 

relative density difference [-] 

� � 

density of overflow [kg/m 3 ] 

� � 

density of ambient water [kg/m 3 ] 

� velocity of the mixture in the overflow [m/s] 

� 

� velocity of the ambient water [m/s] 

� 

20


� 

Report No: 10/J/1/06/1309/0996 

For high values of the Richardson number in combination with low values of the velocity ratio 

dynamic plumes and density currents along the bed developed. For the opposite situation (low 

Richardson number and a high velocity ratio), the overflow mixed over the total water column and 

a passive plume developed. 

The (vertical) flow velocity of the sediment particles in a dynamic plume is governed by the bulk 

velocity of the total plume and is much higher than the individual settling velocities of the 

sediment particles in a passive plume. The velocity in a dynamic plume is of the order of [m/s] 

while the velocity in a passive plume will be order of [mm/s]. The higher transport capacity of the 

dynamic plume transports more sediment towards the seabed compared with the passive plume, 

hence measured turbidity in the vicinity of the dredger will be lower in that case (Van Parys 

et al., 2001). 

Apart from the flow situation in the overflow as described in Equation 2-1 the following near-field 

processes are also important in the vicinity of the vessel: 

1. Influence of entrained air on the plume behaviour. 

Entrained air can cause a lift effect by upward buoyant forces (air-lift effect), this can lift fine 

particles to the water surface, where passive plumes of fine sediment particles are formed. 

2. Influence of the TSHD vessel itself. 

When the plume enters the propeller wake of the vessel the mixing of the sediment in the plume 

is enhanced because of the high turbulence level in the wake. The transition from active to 

passive plume is therefore accelerated. 

3. Instationary behaviour of the plume 

The overflow discharge is far from stationary, this generates separate sediment puffs or clouds 

instead of a continuous plume. These individual clouds are more prone to passive behaviour 

than a continuous plume. The instationary behaviour therefore accelerates the transition from 

dynamic to passive behaviour. 

Field studies of plume concentrations and behaviour are limited, however some data have been 

reported. Wakeman et al. (1975) described turbidity field studies during the 1974 maintenance 

work at Mare Island Strait (San Francisco Bay, USA). He reported concentrations 50 m behind 

the TSHD of 210 mg/l at the surface without overflow and 75-350 mg/l with overflow. 10 m below 

the water surface those values were 230 mg/l without overflow and 165-870 mg/l with overflow. 

Bernard (1978) suggested that plumes with concentrations of 200-300 mg/l may extend behind a 

TSHD for distances up to 1200 m. 

Willoughby and Crabb (1983) found concentrations close to the dredger of about 500 mg/l near 

the bed and 50 mg/l near the surface when dredging sand (0.25 mm) when background sediment 

concentrations were approx. 5 mg/l. Within approximately one hour of dredging ceasing the 

concentrations in the plume fell to background values and about 90% of this reduction occurred 

within the first 20 minutes. Given the local current velocity of 0.6 m/s, the major proportion of the 

dredge suspended material settled within 600-700 m downcurrent of the TSHD. 

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Pennekamp (1996) and Kirby and Land (1991) performed field measurements in several Dutch 

harbour basins which had small ambient currents


� 

Report No: 10/J/1/06/1309/0996 

Dieppe, where the indirect effects of sand discharged from the dredger were as great as the 

direct effects of extraction on macrobenthic species (Desprez, 2000). Newell et al. (2002, 2004) 

and Robinson et al. (2005) both found evidence of suppression of benthic biomass beyond the 

margins of dredging areas. They conclude that this is due to the indirect impact of sediment 

generated by screening. 

It should also be noted that the natural background conditions will also have an influence on the 

impacts associated with generated plumes – where habitats are subjected to natural disturbance 

by bedload transport (such as off the East Coast of England) impacts of plumes will be minimised 

relative to areas that are otherwise stable (such as the Eastern English Channel). Similarly, 

sedimentation and resuspension caused by dredging in deposits of clean, mobile sands are 

generally thought to be of less concern, because the fauna inhabiting such areas tend to be 

adapted to naturally high levels of suspended sediment resulting from wave and tidal current 

action (Millner et al., 1977; Newell et al., 2002; Cooper et al., 2005). 

In certain circumstances fine sediments liberated by dredging may actually result in more positive 

impacts. One study of a fine�sediment site in Moreton Bay, Australia, demonstrated enhanced 

abundance of benthic invertebrates adjacent to dredged subtidal sandbanks, which may have 

been linked to sedimentation of plume material (Pointer and Kennedy, 1984). 

23


Report No: 10/J/1/06/1309/0996 

� 

�� 

�� 

� 

�� 

� 

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24 

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2.2.4 Impacts Associated with discharging phase of the dredging cycle 

� �� 

As in the cases for transit and dredging described above, the fuel consumption for the dredging 

vessel during discharge varies and based on data from BMAPA (2009) and Kemp (2008) the 

proportion of fuel consumed during discharge has been estimated for two dredging cases – long 

haul (larger ships) or short haul (smaller ships) (Table 2-11). 

� �� 

Report No: 10/J/1/06/1309/0996 

�� 

�� 

�� 




�� 

Report No: 10/J/1/06/1309/0996 

Table 2-12 below is a matrix summarizing the relative sensitivities associated with the different 

phases of the dredging cycle. 

� � �� 

�� Transit Loading Discharge 

Time as 

proportion of 

cycle 

High Moderate Low 

Fuel Consumption 

/ Emissions 

High Moderate Low 

Noise Level Low High Low 

Direct impact Seabed None High None 

Benthos None High None 

Water Column None Moderate None 

Indirect impact Seabed None Moderate None 

Benthos None Moderate None 

Water Column None High None 

Interaction with 

other users 

Moderate High None 

�� 

Table 2-12 shows that the transit time relative to the overall dredging cycle time is high; while fuel 

consumption and emissions are high for both transit and loading but relatively low on discharge. 

Noise levels are relatively high during loading due to the additional noise generated by the 

draghead and dredging equipment but relatively low during transit and discharge when the noise 

generated by a dredging vessel is no different to that generated by ships of similar size. Direct 

and indirect impacts on the seabed only occur during the loading phase of the dredging cycle 

while the potential sensitivity for interaction with other users is also highest during the loading 

phase. Those sensitivities that can be most influenced by technological and practical changes, 

and hence the focus for the benchmarking process, are the direct and indirect impacts caused by 

the dredging process itself. 

26


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27


�� 

Report No: 10/J/1/06/1309/0996 

�� 

�� 

�� 

Whilst the dredging of marine aggregates has been carried out in various ways for hundreds of 

years, it is only over the last 35 years or so that it developed on an industrial scale, to the point 

today where the business produces around 25 million tonnes per year from UK waters. Originally 

dredging rights and profits from the trade belonged to the Lord High Admiral of England but in 

1594 the privilege was transferred by Lord Howard to Trinity House. In the river Thames, an act 

of Elizabeth’s reign gave the Corporation exclusive rights to supply dredged ballast to vessels in 

the River Thames from London Bridge to the sea. 

Dredging from the seabed around the UK coast has been taking place for at least two hundred 

years – on 14 th June 1736 the Reverend William Gosling of Canterbury recorded “...I have got a 

piece of a huge bone (I suppose from an elephant’s thigh bone) petrified, which the dredgers of 

copperas stones fishes out of the sea.” Another cleric, the Reverend James Parker of Northfleet 

in Kent, patented Roman Cement (sometimes called Parker’s Cement) in 1796 which was a very 

rapidly setting hydraulic cement particularly suitable for wet or under-water work. Roman cement 

was made from copperas stone being broken up, burned in kilns and ground into powder and 

was shipped to all parts of the UK and Northern Europe. 

In 1826, the Topographical Dictionary of England recorded in its entry for Harwich “...about one 

hundred small vessels and boats are employed in or near the harbour dredging for stone for 

making cement. The manufacture of copperas from stones, which are found in abundance on the 

shore, was carried on here in the seventeenth century...” and by 1835 there were five cement 

factories in Harwich with “some five hundred men employed in dredging the stone and 

manufacturing the cement”. The removal of “several hundred thousand tons” of stone from 

Beacon Cliff caused changes in the strength of tides which threatened to silt up to the mouth of 

Harwich Harbour and in 1845 removal of stone was immediately stopped by The Commission on 

Harbours of Refuge. The consequence of this was to see a marked increase in dredging until, by 

1850, up to 400 smacks from Kent ports, each with a crew of three or four, were dredging stone 

from the West Rocks off Walton with some off Brightlingsea and off Hythe. However, by 1890 the 

industry had died out as Roman cement was replaced by the cheaper chalk based Portland 

cement. 

In the early days, both aggregate and maintenance dredging was the by way of “spoon and bag” 

which both Holland & Italy claim to have originated but is thought more likely to have been 

introduced in western Europe and Britain by the Phoenicians or Romans who, as their empire 

expanded, brought the practice of dredging with them. The 1829 edition of the London 

Encyclopaedia gives an exhaustive description of spoon and bag dredging or “ballast heaving” 

which allowed two to four men lift up to sixty tons of ballast in a tide from a depth of some three 

fathoms. The ballast or stone was scooped up in a leather bag which had its mouth held open by 

28


Report No: 10/J/1/06/1309/0996 

an iron hoop attached to a pole, which pivoted, allowing the bag to be swung inboard with its 

load. Vessels were limited in the water depth they could safely work in to about 10 metres and 

often suffered from stability problems that resulted in a number of vessels capsizing and sinking. 

This eventually led the regulating authorities to produce the dredger stability criteria that are still 

in use today. 

The first suction dredger used in the UK sailed from Bristol on the 15 th June 1912. This was the 

49 ton, steam driven suction dredger “City of York” and she dredged a cargo of sand from the 

Bristol Channel and returned to Bristol and thus marked the start of the modern British aggregate 

dredging industry. The industry’s first purpose built aggregate dredger was named “Portway” 

after the riverside road linking Bristol with Avonmouth and launched at Charles Hill & Sons’ 

Bristol yard in 1926. The innovative design of the 297 ton vessel saw her fitted with wing tanks 

for fuel oil, allowing her to double up as a bunkering barge. In 1930 the 1920-built coaster “Jolly 

Marie” was converted to a suction dredger and was the first to have a hydraulic discharge 

capability. Re-named the “Sandholm”, with a cargo capacity of 300 tons, she continued in the 

trade until 1962. 

The development of maintenance or ‘capital’ dredgers for such tasks as channel clearing and 

land reclamation progressed separately from that of that of the aggregate dredger by way of all 

manner of craft such as the water harrow, mud mill, scrapper dredger, grab dredger, & bucket 

dredger. The first hydraulic (suction) dredger using a centrifugal pump for dredging spoil was 

thought to have been that invented by M. Bazin in 1864 and it is this dredging method which was 

eventually adopted by the “City of York” for aggregate dredging. A paper published in the 

Scientific American of 1882 records �� 

�� 

��. 

Technical advances in the trade have continued apace since 1912 with over 250 ships and some 

50 companies having been involved in the industry which has variously been centred in the 

Bristol Channel, Solent / South Coast, Southern North Sea, Liverpool Bay, River Dee, River 

Clyde and a well established trade on Lough Neagh in Northern Ireland. By the 1950s, slurry 

pumps which allowed water to be pumped at sufficient velocity to entrain large volumes of 

sediment of 150 mm diameter were common and the modern marine aggregate dredging 

industry began to develop (Figure 3-1). 

29


� 

Report No: 10/J/1/06/1309/0996 

�� 

Until 1965 vessels were, in the main, reliant on shore cranes to discharge them but then the first 

successful dry self discharging systems were developed, using the dragline principle. By the 

early 1970s vessel capacities has increased above 2,000 tonnes and steady development has 

continued to the present day. This has resulted in the current UK fleet of 26 vessels of various 

sizes, having a variety of discharge systems, and the most modern vessels capable of dredging 

the deepest approved seabed deposits. Currently there are 8 UK based operators of these 

vessels (Table 3-1). 

�� 

�� 

Britannia 

Aggregates 

Britannia Beaver 1991 2775 4800 

CEMEX UK 


DEME Building 

Materials 

Sand Falcon 1998 5022 8700 

Sand Fulmar 1998 4000 6920 

Sand Harrier 1990 2700 4671 

Sand Heron 1990 2700 4671 

Sand Weaver 1974 2400 4152 

Welsh Piper 1987 790 1367 

Charlemagne 2002 5000 8650 

Hanson 

Arco Adur 1988 2890 5000 

Aggregates Arco Arun 1987 2890 5000 

30



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Arco Avon 1986 2890 5000 

Arco Axe 1989 2890 5000 

Arco Beck 1989 2600 4500 

Arco Dart 1990 700 1250 

Arco Dee 1990 700 1250 

Arco Dijk 1992 5100 8800 

Arco Humber 1972 4800 8000 

Donald Redford 1981 510 880 

(Fareham) Norstone 1971 1075 1860 

Severn Sands Argabay 1985 620 900 

Tarmac Marine 

Dredging 

Westminster 

City of Cardiff 1997 1300 2300 

City of 

Chichester 

1997 1300 2300 

City of London 1990 2775 4800 

City of 

Westminster 

1990 3000 5200 

Sospan 1990 700 1200 

Dredging Sospan Dau 1978 1400 2400 

�� 

A small number of third party vessels are also used on an occasional basis by English aggregate 

dredging companies when operational need dictates. Table 3-2 shows an example of third party 

vessels that also dredge on English licence areas. 

�� 

�� 

Reimerswaal 1994 1600 2740 

Orisant (now renamed as DC 

Vlaanderen 3000) 

2002 2600 4460 

Brabo 2007 11630 20000 

�� 

�� 

The mean age of the fleet is currently 21.8 years with vessels ranging between 8 years old in the 

case of the “Charlemagne” (Figure 3-2) to 39 years old in the case of the “Norstone”. It should 

however be noted that the “Charlemagne” is not an exclusively English operating vessel – of 

these the youngest are CEMEX UK Marine’s vessels “Sand Falcon” and “Sand Fulmar” which 

are both 12 years old. 

31


� 

Report No: 10/J/1/06/1309/0996 

�� 

�� 

A large number of the vessels were built in the late 1980s and early 1990s and the majority of the 

vessels in the fleet are a few years younger than the mean (Figure 3-3). 

�� 

32


�� 

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The English marine aggregates dredging fleet is comprised entirely of Trailer Suction Hopper 

Dredgers (TSHDs). All but two of the vessels currently in service were built as dedicated marine 

aggregate dredgers – the smallest vessel, the “Donald Redford”, was originally built as a 

maintenance grab dredger for the Manchester Ship Canal (Figure 3-4), while the newest (and 

one of the largest ships) the “Charlemagne” (Figure 3-2), was built as a multipurpose vessel 

capable of undertaking maintenance work as well as dredging marine aggregates. 

�� 

There are two typical configurations that have been favoured by English TSHDs - 60% of the 

vessels have the accommodation and wheelhouse aft (Figure 3-5) and the remainder have them 

located forward (Figure 3-6). This latter arrangement has the accommodation well separated 

from the main propulsion and discharge machinery noise, gives the possibility of a much more 

flexible discharge system arrangement and unrestricted forward vision but has the disadvantage 

of somewhat restricted accommodation space and more uncomfortable vessel motions for the 

crew. They are designed to dredge on relatively shallow licence areas, close to markets and 

operate on quick turnarounds 

33


Report No: 10/J/1/06/1309/0996 

�� 

�� 

�� 

The “City of Westminster” (Figure 3 6) is an example of an ‘A’ Class equivalent vessel. There are 

7 ‘A’ Class equivalent vessels and they form the backbone of the English dredging fleet. They 

comprise the Hanson Aggregates Marine vessels “Arco Adur”, “Arco Arun”, “Arco Avon” and 

“Arco Axe”, the Tarmac Marine Dredging vessels “City of London” and “City of Westminster” and 

the Britannia Aggregates vessel “Britannia Beaver”. There are minor differences in engine 

configurations, however all the ‘A’ Class equivalents have capacities between 2700 and 3000 m 3 

34


� �� 

Report No: 10/J/1/06/1309/0996 

and were all built between 1986 and 1991 at the Appledore Shipyard in North Devon. The 

CEMEX UK Marine vessels “Sand Harrier” and “Sand Heron” show similar capacities but are 

slightly different and are not designated as ‘A’ Class equivalents. 

Generally, a ship’s fuel consumption varies as the third power of the speed up to about 12 knots 

but beyond this it can vary as the forth power or more so is a significant disincentive to increasing 

output by faster speeds, thus making increasing size the best solution to increased output when 

the wharves can accept such vessels. 

The smaller vessels serving the South Coast and South Wales areas have service speeds of 

only 9/10 knots but even so, because the dredging areas are often less than 20 miles away and 

the loading and discharging times are around two hours, they are capable of delivering two 

cargoes per day – one on each tidal cycle. 

The largest vessels however can be steaming over 130 miles each way to and from the dredging 

grounds, take over 4 hours to load and discharge and often have to fit into 12 hour tidal windows. 

As steaming is a large proportion of their cycle time they usually need to have a service speed of 

at least 12 knots to match the tidal cycles when the wharves, such as most of them on the 

Thames, are tidally restricted. On the Continent, however, such restrictions do not apply so this 

gives greater freedom to choose the most economical speed. 

� �� 

BMAPA (2009) reports fuel consumption and emissions figures for English dredging vessels as 

part of an ongoing sustainable development and environmental reporting programme. Reported 

figures separate out smaller vessels with cargo capacities of less than 3000 tonnes from larger 

vessels with capacities greater than 3000 tonnes. Smaller vessels typically supply local wharves 

from near-shore licence areas on short cycle times. Larger vessels typically dredge cargoes from 

more offshore areas and supply more distant wharves and typically supply a cargo every 24- to 

48-hours (BMAPA, 2009). Table 3-3 shows the fuel consumed and CO2 emitted for the two 

categories of vessel. 

�� 

�� 

3000 tonnes 2.308 7.363 

�� 

� �� 

35 

�� 

�� 

Fuel has now become a very significant operating cost. With widely varying unit costs and the 

diverse vessel operating patterns it is difficult to generalise, but it would not be unusual to have 

fuel costs being between 25 and 40 % the operating cost of a vessel. All UK operators, however, 

are using high grade gas oil or marine diesel fuel despite the fact that savings of up to 40% could


�� 

Report No: 10/J/1/06/1309/0996 

be achieved by the use of low grade residuals fuels. However, there are issues to be addressed 

before this could be considered viable – the availability of low sulphur fuel for use in coastal 

waters, the increased capital equipment cost, somewhat increased maintenance, and the ability 

to deliver the fuel in the required quantities and to locations where the vessels operate. 

All vessels of the UK fleet operate on a 24 hour a day, 7 day a week basis, with the majority of 

the crews working a 2 or 3 week on/off rota. The work can be very intense, with some vessels 

delivering more than 400 cargoes per year, so modern vessels have a good standard of crew 

accommodation, comparable with “deep sea” vessels of similar size. 

Crew numbers are not proportional to vessel size but relate to the number required to effectively 

maintain and safely operate each particular vessel and vary from 7 to 8 on the smaller vessels to 

11 to 12 on the larger vessels. 

Table 3-4 shows the general crew structure of English aggregate vessels: 

�� 

Master 

1 st Mate 

2 nd Mate Additional Mate 

Chief Engineer 

1 st Engineer 2 nd Engineer 

2 x ABS Additional ABS 

Cook 

�� 

36


�� 

Report No: 10/J/1/06/1309/0996 

The fleet ranges in capacity from about 880 tonnes to 8800 tonnes (Table 3-1) while Table 3-5 

summarizes the lengths, breadths and dredging draughts for the English fleet. It should be noted 

that several vessels in the English fleet have been modified, post-build, to maximise their 

capacity. Sand Falcon was lengthened, and some of the Hanson A class vessels have had 

raised cargo combings installed (together with retrospective bottom dump valves to mitigate 

stability concerns) (Russell, ��). 

�� 

Britannia Beaver 100 17.4 6.3 

Sand Falcon 120 19.5 7.9 

Sand Fulmar 100 19.5 7.9 

Sand Harrier 99 16.5 6.4 

Sand Heron 99 16.5 6.4 

Sand Weaver 96 16.7 6.1 

Welsh Piper 69 12.4 4.4 

Charlemagne 100 20.8 8.5 

Arco Adur 98 17.4 6.7 

Arco Arun 98 17.4 6.7 

Arco Avon 98 17.4 6.7 

Arco Axe 98 17.4 6.7 

Arco Beck 100 17 6.5 

Arco Dart 68 13 4.1 

Arco Dee 68 13 4.1 

Arco Dijk 113 20.1 7.7 

Arco Humber 107 20 8.2 

Donald Redford 53 11 3.4 

Norstone 67 12.5 4.5 

Argabay 58 9.4 3.9 

City of Cardiff 72 15 5.1 

City of Chichester 72 15 5.1 

City of London 100 17.4 6.3 

City of Westminster 100 17.4 6.7 

Sospan 57 10 3.6 

Sospan Dau 70 14.3 3.2 

37 

�� 

��


� 

Report No: 10/J/1/06/1309/0996 

As the industry has matured the vessel size and configuration has become better matched to the 

commercial requirements and the physical constraints of the receiving wharves. For example, 

ships serving Langstone Harbour on the South coast and some South Wales ports have been 

designed around the largest practical ship dimensions for these ports - 72m length and 5m 

draught resulting in a maximum cargo capacity of about 2,300 tonnes (Figure 3-7). 

�� 

�� 

This contrasts with some Thames and Continental wharves which can accept vessels up to 120m 

long and with 8m draughts. Vessels serving these locations have, therefore, been built around 

these parameters and can deliver in excess of 9,000 tonnes per cargo to these locations. 

�� 

The smaller, older, vessels such as the “Norstone” have single propulsion engines of about 500 

kilowatts (kW), two small generators to supply the domestic load and an inboard mounted diesel 

driven dredge pump of about 350 kW. 

As vessels have become bigger, the main propulsion load and the powering for the dredge pump 

and discharge system have inevitably greatly increased so that today the largest vessels such as 

the “Chalemagne” and the “Sand Fulmar” have two 2,800 kW main engines, each coupled to a 

propeller and a large alternator of up to 2,000 kW capable of supplying all the vessel’s power 

requirements. During normal steaming each engine drives a propeller but when dredging or 

discharging, one of the main engines also supplies the power via its generator for that duty 

(Figure 3-8). 

38


Report No: 10/J/1/06/1309/0996 

� 

�� 

Even the smaller modern vessels like the “City of Cardiff” and the “City of Chichester” have had a 

very significant uplift in power with these 2,300 tonne vessels now having two 1,360 kW engines. 

The large power requirements of these relatively small ships are a result of their relatively 

inefficient hull design which was necessary to obtain the required annual capacity within the 

length and draught constraints of their trading area. They also have relatively powerful dredge 

pumps to minimize cycle times and also have two 400 kW thrusters that are necessary for quick 

and safe manoeuvring in the constricted channels they have to navigate. 

Production rates for dredgers depend on a number of variables including the pump power and 

the nature of the sediment dredged. Productivity rates can be estimated for different dredging 

regions based on information presented earlier in Chapter 2.2 and two cases are presented in 

Table 3-6 below. 

�� 

�� 

�� 

39 

�� 

�� 

Bristol Channel 750 2 – 4 190 – 375 

�� 

�� 

Eastern English Channel 5000 5 – 7 625 – 1000 

��


� 

�� 

�� 

Report No: 10/J/1/06/1309/0996 

With the exception of the sand dredged in the Bristol Channel, the North West, and one area on 

the South Coast where the material tends to be in thick, clearly defined deposits, dredging is 

performed by trailing the draghead at the end of the dredge pipe over the seabed at speeds 

varying from 0.5 to 2 knots. The primary source of production is erosion of sediment around the 

edges of the draghead caused by water being sucked into the system. No consolidated deposits 

are dredged by the English industry, although some may be compacted. 

The UK aggregates industry uses dragheads such as the single visor draghead (Figure 3-9) for 

dredging fine, free-flowing material such as sand where the sediment is easily entrained in the 

water flow and no special arrangements are required to maximise production. The California 

draghead (Figure 3-10) however is required when the grain size is larger and the sediment may 

be slightly compacted on the sea bed. It has two independently hinged visors that allow it to 

better follow the seabed contours. These also provide an increased area where water can enter 

the system and provide the necessary erosion to entrain sediment from the sea bed, into the 

water flow, which is subsequently pumped up the dredge pipe. Production at these dragheads is 

therefore based completely on the principle of erosion and vertical hydraulic transport (Figure 

3-12): a flow of water is created between the movable visor of the draghead and the sea bed, 

and this flow erodes the seabed sediment which is subsequently pumped up the dredge pipe. In 

the single visor draghead this water flow enters mainly on backside of the visor, while in the 

California draghead it mainly enters the sides of the visor. 

�� 

40


Report No: 10/J/1/06/1309/0996 

� 

� 

�� 

The sediment production as a result of hydraulic entrainment is proportional to the momentum of 

the water flow 

� �� 

� 

Where � is the momentum ([kg/s]), � is a non dimensionless constant ([s/m]) depending on the 

particle size. � is the discharge through the suction pipe ([m 3 /s]) and � the entrance flow rate into 

the draghead ([m/s]). 

The discharge � is a given quantity for a certain hopper and depends on the power of the dredge 

pumps installed. The entrance flow rate � can be increased by decreasing the inflow area below 

the edges of the draghead. Decreasing this area does however have an influence on the 

pressure difference over the draghead. Inside the draghead an under pressure must be present 

to accelerate the entering fluid. According to Bernouilli’s equation this pressure difference �� is 

proportional to the square of the velocity of the entering fluid: 

� 

� � 

�� 

� � � � � � � � �� 

There are limits to the pressure difference over the draghead. Due to the pressure difference the 

draghead is pushed onto the seabed and if this force is too high the draghead is more or less 

anchoring the hopper on the seabed. Too much propulsion power is then needed to drag the 

head over the seabed at sufficient speed. Propulsion can even be insufficient to move the ship 

41


� 

Report No: 10/J/1/06/1309/0996 

forward at all. Another disadvantage of an excessive value of pressure difference is the negative 

effect on hydraulic transport. The pressure difference over the draghead can be seen as an 

hydraulic loss in the system. More pumping power is needed and it will affect the pressure at the 

inlet side of the dredge pump and can reduce production due to the vacuum limit (see 3.2.2 

below). 

� �� 

The natural movements of a vessel due to the action of waves and the natural undulations of the 

seabed mean that a mechanism to control the draghead is needed to prevent it being lifted from 

its position. If the draghead is lifted too high from the seabed the sediment / water mixture tends 

to contain too much water while if it is too low or at full weight, the draghead does not extract the 

optimum quantities of bottom material at the best solids/water mix. 

The swell compensator on board a TSHD is designed to cope with uneven bottom profiles and to 

compensate vertical vessel motion at the gantry e.g. the IHC swell compensator is designed to 

compensate vertical motion of up to 6 meters (IHC, 2009a). Every swell compensation system is 

designed specifically for the individual vessel, hardware and range of conditions in which it will 

work and is basically a spring device integrated in the hoisting wire system. It consists of a set of 

pressurised air vessels connected to a hydraulic cylinder between sheaves and the 

compensation levels necessary are prepared especially for different suction tube arrangements 

on individual dredgers. 

�� 

Until the mid 1980s all dredge pumps were fitted inside the vessel and the largest had a power 

input of about 1,000 kW, pumping through 850 mm dredge pipes. Because of the resistance to 

flow between the draghead and the pump the dredging depth with this arrangement is limited to 

about 33 metres i.e. above this length resistance to flow in the pipe makes it impossible for the 

pump to produce sufficient flow to entrain and transport the sediment. These large pumps were 

also often used for “pump out” discharging and were occasionally of “double walled” construction 

allowing the main pump casing to be made of an extremely hard, wear resistant, material which 

unfortunately is also very brittle. The consequence of this could be to flood the ship if it fractured 

– hence the need for a secondary casing (Figure 3-11). 

42


Report No: 10/J/1/06/1309/0996 

� 

�� 

The inboard mounted pumps are capable of pumping very large quantities of water but at a 

relatively low bulk density of 1.25 tonnes per m 3 from >50m water 

depth using 700 mm dredge pipes, saving weight, space and costs. Such pumps are capable of 

pumping sandy sediment at rates in excess of 2500 tonnes per hour and give a depth capability 

not achievable with inboard pumps, or alternatively give 40% higher pumping rates in shallower 

water. 

The capacity of the dredge pump is dependant on the required loading rate which is affected by a 

number of factors such as water depth, particle size and the degree of screening. As a general 

rule, however, the loading time will vary from less than 2 hours for an “all in” sand cargo in 

shallow water to more than 6 hours for a heavily screened gravel cargo in deep water. 

The majority of dredge areas off the East Coast, along the South Coast and on the West Coast 

lie in water depths of 15 to 33 m and therefore just within the theoretical capability of inboard 

43


� 

Report No: 10/J/1/06/1309/0996 

dredge pumps. The recently permitted Eastern Channel licenses have depths in some places in 

excess of 50 m and require vessels with high power underwater pumps. In order to dredge these 

deposits vessels have undergone modification with longer dredge pipes and new outboard 

pumps fitted. 

The ability to entrain sediment at the draghead is governed by the pump vacuum limit and its 

pumping power. The energy needed to transport the sediment / water mixture towards the 

hopper can be divided into the energy needed for lifting the mixture towards the surface against 

gravity and the pipeline flow resistance. 

�� 

� 

� 

44 

� 

� 

� 

�� 

�� 

� �� 

The pressure at the inlet side of the dredge pump can be calculated as follows. At the water 

surface (Figure 3-12 point 1) the atmospheric pressure �� is present. At the seabed (Figure 

3-12 point 2) the pressure is equal to the atmospheric pressure plus the pressure due to the 

water depth : �� Moving from point 2 to 3 (Figure 3-12) the pressure in the flow 

changes due to a difference in height and flow resistance. The pressure at the inlet side of the 

pump is: 

� 

In which: 

� � � � � �� 

� � � � 

� � � � � � � �� 

� pressure [N/m 2 ] 

� � mixture density [kg/m 3 ] 

� water depth [m]


Report No: 10/J/1/06/1309/0996 

� � suction depth [m] 

� mixture velocity [m/s] 

� friction factor [-] 

The friction factor includes the pipe wall friction and additional friction due to entrance losses, 

bends, etc. 

This equation shows that the pressure at the suction side of the pump will decrease with 

increasing density and velocity of the mixture sucked from the seabed and with an increasing 

level of difference between the pump and the seabed. If the pressure at the inlet side of the 

pump drops below a certain level the pump is cavitating, the efficiency of the pump will decrease 

and production reaches a limit. This is called the vacuum limit. If the minimum pressure is 

denoted as �� the following equation for the density as a function of the other quantities can be 

derived: 

� 

� � � �� 

� � 

� 

�� 

� � � � � � � �� 

� � 

�� 

� � 

In Figure 3-13 the density is plotted as a function of the mixture velocity u for two different values 

of the suction depth. For this example the following values are chosen: 

� � � � �� 

: 4 [m] 

� �� : 15 [kPa] 

� �� : 100 [kPa] 

� : 2.5 [-] 

45


Report No: 10/J/1/06/1309/0996 

�� 

1600 

1500 

1400 

1300 

1200 

1100 

1000 

0 2 4 6 8 10 

�� 

46 

50 m 30 m 

�� 

Figure 3-13 shows the mixture density decreases with increasing dredging depths and increasing 

velocity. From this figure however it can not be concluded that production decreases with mixture 

velocity since the production is proportional to the product of density and velocity. The suction 

production using the above example data is shown in Figure 3-14 which shows that production 

reaches a maximum at a particular velocity value, and subsequently begins to decline. The 

maximum production is decreased at greater suction depths. 

All the points in the lines in this graph have one thing in common i.e. pressure at the inlet side of 

the pump is minimal and cannot be reduced further. Therefore increasing the available power of 

the pump drive system will not have an effect on this maximum production – it is minimum 

pressure (in dredging often called ‘vacuum’) that will limit production.


�� 

Report No: 10/J/1/06/1309/0996 

1600 

1500 

1400 

1300 

1200 

1100 

1000 

0 2 4 6 8 10 

�� 

50 m 30 m prod 50 m prod 30 m 

�� 

Despite the fact that the vacuum limits production there are other ways of optimizing production. 

The first important factor is to ensure that the dredger is working at the maximum possible 

vacuum production - often mixture velocity is too high and by lowering the velocity mixture 

density increases and likewise production. Many modern TSHDs are equipped with process 

control systems that can regulate pump speed to arrive at the optimal vacuum limit production. 

Another factor that has a strong effect on the production and that follows directly from the mixture 

density equation is the submerged depth of the pump �. Increasing the depth of the pump 

increases the absolute pressure in the pump and therefore the pressure drop up to minimal 

pressure due to friction and lifting of the mixture is much larger. This is indicated in Figure 3-15 

which shows the vacuum production at a dredging depth of 50 m for two values of pump depth. 

Increasing the pump depth from 4 to 15 m below water level has a very large effect on 

production. In the 4m depth case the suction pump was placed on board the vessel and its 

maximum depth was therefore limited by the draught of the vessel. The only way to increase the 

submerged depth � is to place the pump on the suction pipe which explains the increasing 

popularity of under water dredge pumps on recent TSHDs. 

47 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

��


� 

Report No: 10/J/1/06/1309/0996 

� 

�� 

1800 

1700 

1600 

1500 

1400 

1300 

1200 

1100 

1000 

0 2 4 6 8 10 

�� 

k = 4 m k = 15 m 

�� 

� �� 

The specific energy required can be determined for the hydraulic transportation process from the 

seabed to the hopper and is again defined as the ratio between power and production. For 

hydraulic transportation the power needed is: 

� �� 

The production is [kg/s]: 

�� 

The ratio is the specific energy and is equal to: 

� �� 

� � � 

�� 

� 

� 

48 

2.5 

2 

1.5 

1 

0.5 

0 

�� 

� � � � � � � �� 

�� 

�� 

�� 

�� 

�� 

� � � � �� 

�� 

� 

�� 

� � � � �� 

�


In which: 

Report No: 10/J/1/06/1309/0996 

� water depth [m] 

� length of suction pipeline [m] 

� friction factor [-] 

� �� empirical constant depending on particle size [s/m] 

The pressure loss is proportional to the transport distance, so specific energy will linearly 

increase with water depth. If we normalize the specific energy with water depth (� [Jkg -1 m -1 ]), this 

quantity can be written as: 

� 

�� 

�� 

� � �� 

� � � � �� 

� � � � �� 

� 

� � � 

49 

� � � � � � 

The first term is the contribution due to vertical transportation against gravity, which is 

independent of the process parameters. The second term is the friction resistance. This term is 

dependent on the operational parameters of sediment concentration and mixture velocity. 

�� 

The specific energy according to equation (3-9) is shown in Figure 3-16 as a function of the 

mixture velocity and volume concentration. This shows that a low specific energy is obtained at 

��


Report No: 10/J/1/06/1309/0996 

high concentration and that for particular concentrations the energy is lowest at an optimal 

velocity. 

�� 

Common to almost all marine aggregate dredgers is the need to have the ability to screen off 

either the over or undersize from the dredged material and reject it back into the water. 

Screening is the mechanical separation of particles by physical size, generally by passing the 

base material over a uniformly perforated surface. Dredged sediment is passed over a steel or 

plastic mesh and is a static version copied from the land based minerals industry. 

Screening is important as it allows the dredger to modify the sand:gravel ratio in a cargo to 

satisfy customer demand. It also allows vessels to economically work marginal deposits by 

increasing the gravel ratio through increased rejection of sand. This reduces the environmental 

impact of dredging by reducing the amount of licensed seabed required, but reduces productivity 

of the vessels, increases the fuel consumption per tonne of aggregate landed and increases the 

wear and tear. The screening process can also be turned around to reject large particles instead 

of sand. 

There are two types of screen – the original arrangement and still used on the “Arco Dee”, “Arco 

Dart”, “City of Cardiff” and “City of Chichester” is a fixed screen box at one end of the hold and a 

longitudinal chute which distributes the aggregate along the length of the hold (Figure 3-17). The 

second arrangement consists of screens mounted on towers on one side of the hold that rotate to 

distribute the sediment. Some vessels in the English fleet have had screen towers retro-fitted 

from the original static screen boxes e.g. all the Hanson Aggregate Marine Ltd ‘A’ Class ships. 

The screen tower is much more flexible and does not interfere with the discharge system but is 

more complicated and expensive to maintain (Figure 3-18). It does, however, allow more efficient 

filling of the hold and results in greater screening efficiencies on account of the greater screen 

area. 

50


Report No: 10/J/1/06/1309/0996 

� 

� 

�� 

�� 

51


Report No: 10/J/1/06/1309/0996 

The design of the equipment (especially the hopper) has an influence on the settling of sediment. 

The most important factors are: 

� Hopper Length L 

� Hopper Width B 

� Hopper Depth H 

� Loading system 

� Overflow System 

The length � and width � are most important regarding the geometrical quantities. The product of 

these geometrical quantities is the hopper area which forms with the discharge � the overflow 

rate. The ratio of the overflow rate and the settling velocity is the most decisive factor regarding 

the overflow losses as will be explained in detail in Appendix A. The depth of the hopper is less 

important. This is counter-intuitive since it might be argued that the flow velocity in the hopper 

would decrease with increasing depth. This would be true if the flow distribution in the hopper 

was uniform, which is not so because the flow is driven by gravity (density currents) and 

concentrated near the bed. The mixture is distributed into the hopper by either the static screen 

box or by rotating loading towers. The loading system must be efficient when loading fine or 

coarse grained sediments. 

The settling velocity of fine-grained sediments is low, therefore the effect on settling efficiency is 

most important. The turbulence level produced by the loading system must be as low as possible 

and the inflowing mixture should be evenly spread over the width of the hopper and with a low 

flow velocity. In addition the distance between the loading structure and overflow should be as 

large as possible. 

In the case of loading coarse sediments overflow losses do not play an important role since 

settling velocity of this type of sediment is large. Due to this high settling velocity the sediment 

settles at the location where it enters the hopper and spreading the mixture over the length of the 

hopper becomes a more important issue to avoid large piles of sediment in the hopper with 

enclosed pools of water (which reduce the effective load) (Figure 3-19). 

�� 

52


Report No: 10/J/1/06/1309/0996 

�� 

During the loading phase overflow losses often increase and loading production decreases with 

time. A typical graph of the load in a hopper during a hopper cycle is shown in Figure 3-20. The 

cycle starts when the hopper is fully loaded and starts sailing to the discharge area. Next the load 

is discharged (red line drops to zero). The third phase in the cycle is sailing empty to the 

dredging area. The last phase in the cycle is the hopper loading. After reaching the overflow level 

loading production decreases due to increasing overflow losses. 

�� 

� 

� 

� �� 

�� 

�� 

� 

� � �� 

�� 

�� 

53 

�� 

�� 

� � �� 

The cycle production is defined as the discharged load divided by the cycle time and minimum 

costs are associated with a maximal cycle production. The cycle production is equal to the 

tangent of the angle of a line from the origin to the point on the loading graph where the loading 

phase is ended. On the graph two lines are drawn – one line is drawn to the load associated with 

the maximum draught (the end of the loading graph), the other to a point where the line is 

tangential to the loading curve. In this case the angle � is larger, indicating that the cycle 

production is larger. It is therefore not always economical to load the hopper to maximum 

draught. The maximum cycle production is sometimes reached at less than maximum load 

depending on the shape of the loading curve. An optimum as shown in Figure 3-20 can only be 

determined if this line shows a curved appearance. The maximum cycle production is reached at 

maximum draught in case the loading curve is a straight line (which is not uncommon). 

�� 

During dredging a mixture of sediment and water is discharged in the vessel hopper with the 

intention being that the sediment settles in the hopper and the excess water flows overboard to 

maintain vessel stability. Generally a part of the incoming sediment does not settle in the hopper 

during dredging, but flows overboard with the excess water. Depending on the particle size 

distribution (PSD) of the sediment, the hopper geometry and other process parameters this


Report No: 10/J/1/06/1309/0996 

overflow loss can reach values up to 30-40 % of the total volume of sediment pumped into the 

hopper. The loading time of a dredging vessel increases with increasing overflow losses, and the 

finer fractions of the particle size distribution (PSD) are present in the overflow mixture and a 

sediment plume can be generated. 

The loading process can be divided into three phases. 

�� 

� � 

� �� 

When loading starts on board of a TSHD, the hopper is generally partly filled with water, with the 

water level inside the hopper equal to outside. During the first phase the level of the mixture in 

the hopper rises until the overflow level is exceeded and overflowing starts. The flow pattern 

inside the hopper is strongly influenced by density currents with the density of the mixture 

discharged in the hopper being greater than that of the fluid inside the hopper. The mixture flows 

down towards the hopper bed below the inlet section due to this density difference. In the inflow 

section, flow velocity and the level of turbulence is high near the hopper bed and, as a result, 

sedimentation in this section is lower compared with downstream (in direction of overflow) 

locations. As a result a scour hole develops at the inflow section after some time and from the 

scour hole the mixture flows to the other side of the hopper as a density current. This implies that 

the flow is concentrated near the bed and flow velocities are much higher than for a situation 

where uniform flow would exist. This phenomenon is shown in Figure 3-22 

54


Report No: 10/J/1/06/1309/0996 

�� 

� �� 

During this phase the level of the mixture in the hopper is often constant because the level of the 

overflow is often constant. The discharge through the overflow is equal to the inflow and a 

density current is present near the bed for the duration of the second phase. Sediment settles 

from this current and the sediment bed rises gradually. Normally a certain proportion of the 

particles will not settle and remain in suspension. Therefore concentration in the suspension will 

increase during time and likewise the concentration in the overflow. Often both concentration in 

the suspension in the hopper and overflow mixture will increase during time until the settled bed 

reaches the overflow level and maximum load is reached. 

� �� 

In the final phase the top of the density current reaches the water surface. Soon after that 

moment a free water flow (like a river flow) develops and the flow velocity increases 

considerably. The increased velocity reduces sedimentation and the outflow concentration 

increases strongly. If loading is continued the outflow concentration becomes equal to the inflow 

concentration, and the total influx of sediment disappears in the overflow. 

Overflow losses are influenced by the characteristics of the equipment, sediment properties and 

operational conditions. The losses are generally defined by the term cumulative overflow loss� 

�� which is the ratio between the total mass of sediment flowing overboard and the total mass 

of sediment discharged into the hopper: 

�� 

�� 

� 

� 

� 

� 

� 

�� 

� � 

�� 

� � 

�� 

55


Where: 

Report No: 10/J/1/06/1309/0996 

�� Inflow discharge [m 3 /s] 

�� Overflow discharge [m 3 /s] 

�� Volumetric inflow concentration [-] 

�� Volumetric overflow concentration [-] 

� Loading time [s] 

Another term often encountered is loading efficiency � , defined as: 

� �� 

During loading the sediment particles should settle in the hopper, the settling velocity increases 

with particle diameter and Figure 3-23 shows the settling velocity as a function of the particle 

diameter. The line ‘CD iteration’ results from direct solving of the equilibrium equation of a 

submerged particle. Natural sediment does not consist of one particular particle size, but of 

different sizes and the particle size distribution is therefore important. The shape of particles also 

influences the settling velocity with flattened particles, like shells, having a large surface area 

relative to their volume. This results in more resistance and therefore settling velocity will be 

lower than a spherical particle of the same size. 

�� 

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Unlike screening, overspill is a process necessary to maintain the vessel stability and the 

resulting discharges result in a silt plume developing around the vessel (Figure 3-24). On many 

English TSHDs the overflow is over the side of the hopper. Although this is a simple structure it 

has some disadvantages. The level in the hopper is less adjustable and the overflow location 

relative to the loading position is more difficult to adjust and often too small leading to large 

overflow losses. On three of the more recently built vessels, however, (“City of Cardiff”, “City of 

Chichester” (Figure 3-25) and “Charlemagne”) both the screening system rejects and the hold 

overflows pass out through the bottom of the vessel. This encourages a density plume to form, 

which takes more of the returned sediment back to the seabed, rather than allowing it to disperse 

passively – thus, potentially, reducing indirect impacts. 

�� 

� 

�� 

�� 

�� 

57


�� 

Report No: 10/J/1/06/1309/0996 

Only one vessel in the UK fleet is not fitted with some form of self discharging system (the 

“Donald Redford”) and this is discharged by shore cranes (Figure 3-26). All the other vessels are 

fitted with “dry” discharge systems with 35% of these also fitted with wet “pump out” systems. 

These have a high power demand and require the bottom of the hold to be tapered resulting in a 

loss of volume and raising of the cargo centre of gravity but this matches well with a bucket 

wheel dry discharge system which requires a similar shape. Whilst the wet system is now little 

used for discharging to wharves, mainly due to the problems of dealing with the resultant silty 

water, it is regularly used for beach replenishment work. 

� 

�� 

The first dry discharge systems were drag scrapers based on those used in land based mineral 

operations. They proved to be relatively simple and robust but result in high wear rates on the 

ships’ structure, have a high energy demand and are labour intensive as they are difficult to 

automate. Notwithstanding this, 8 systems remain in use today and a typical system has an 

average discharge rate of 1,800 tonnes per hour (Figure 3-27). 

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Report No: 10/J/1/06/1309/0996 

� 

�� 

Subsequently, the bucket wheel system was developed which was also adapted from land-based 

systems. It gives an almost constant discharge rate of 2,000 tonnes per hour and has a relatively 

low energy demand. The required shaping of the hold bottom is not a disadvantage when 

combined with a pump out system and the system causes little structural damage to the ship but 

it is labour intensive as it has not been successfully automated. There are currently 9 of these 

systems in service (Figure 3-28). 

� 

�� 

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Report No: 10/J/1/06/1309/0996 

One older vessel, the “Norstone”, has been retrofitted with an excavator based system 

discharging to a receiving hopper and a short ship to shore boom conveyor. This has proved to 

be a cost effective modification by decreasing discharge time, reducing damage to the vessel 

and removing the reliance on shore-based facilities and labour. 

The most recent development has been the discharger with a grab the full width of the hold and a 

shuttling receiving hopper. Introduced in its original form in 1990 on two 1,300 tonnes vessels 

(the “Arco Dee” and the “Arco Dart”), it was refined into a fully automated 1,200 tonne per hour 

system on two 2,300 tonnes vessels in 1999 (the “City of Cardiff” and the “City of Chichester” 

(Figure 3-29)). In 2002 a 2,500 tonnes per hour system was installed on the “Charlemagne” with 

another order placed for a sister vessel in Q2 2008. This system is easily automated, has little 

impact on the vessels’ structure and allows a variety of ship to shore conveyor arrangements. 

� 

�� 

The ship to shore conveyor arrangements on most vessels are generally designed to match the 

wharf receiving facilities they expect to operate into and vary between a single 26m conveyor 

forward (Figure 3-30), to a single 15m conveyor aft to two 20m conveyors aft. 

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Report No: 10/J/1/06/1309/0996 

� 

�� 

�� 

�� 

� �� 

Positioning on all English aggregate dredgers utilizes Differential GPS, which is a satellite based 

positioning system. Differential GPS is the use of differential correction data for the satellites in 

view, to remove the errors in the position due to atmospheric effects and satellite clock errors. 

The correction data is generated by base stations, which are sited at accurately known locations. 

The UK base stations are operated by Trinity House and comprise 14 ground-based reference 

stations providing transmissions with coverage of at least 50 nautical miles around the coasts of 

the United Kingdom and Republic of Ireland. It is an open system - available to all mariners - and 

is financed from light dues charged on commercial shipping and other income paid into the 

General Lighthouse Fund. The correction data is formatted in the industry standard RTCM SC- 

104 and is available on a continuous basis. Positional accuracy within 1 or 2 metres is achieved. 

� �� 

All vessels dredging English licence areas are fitted with an AIS (Automatic Identification System) 

which the International Maritime Organization (IMO) requires for all vessels over 299GT. The AIS 

transponder transmits the vessel name, position, speed and course is intended to help ships 

avoid collisions, as well as assisting port authorities to better control sea traffic. 

Dredging management and navigation within Dredging Licence Areas on board all English 

dredgers is controlled through a Microplot system which allows the boundaries of dredging 

licence areas, active areas and dredging lanes to be displayed electronically (Figure 3-31). 

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Report No: 10/J/1/06/1309/0996 

�� 

�� 

�� 

The Microplot is linked to the ship’s DGPS system and the ship position can be displayed on 

Microplot, in real time. It is also possible to include offsets to the positional data from the DGPS 

so that the position of the draghead is displayed, rather than the ship position, which allows the 

vessel’s Master to accurately dredge within previously defined boundaries. Known targets such 

as wrecks or areas of contamination can be added as symbology allowing these to be avoided. 

The vessel Master is also able to electronically note any new areas of contamination or target 

contacts and inform the dredging fleet via the shore-based office. 

� �� 

All vessels dredging on UK licence areas are required, by The Crown Estate, to have onboard an 

Electronic Monitoring System (EMS) for recording vessel activity. The EMS consists of a secure 

computer on the ship’s bridge, which logs data from sensors on the dredge gear and positional 

data from the GPS. The EMS automatically records the date, time and position of all dredging 

activities and provides a secure read out of the location of the dredging vessel every thirty 

seconds when the dredging equipment is deployed, and every thirty minutes otherwise. 

These dredging data are supplied monthly to The Crown Estate for analysis and approximately 

500,000 data points are analysed for irregularities (i.e. time gaps in the data or indications of out 

of area dredging) every month. During 2008 approximately 20,000 hours of dredging were 

recorded and approximately 0.03% of this dredging was out of area/zone (The Crown Estate, 

2009). Since the introduction of EMS over one and a quarter million kilometres of dredging tracks 

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Report No: 10/J/1/06/1309/0996 

have been analysed. In addition to analyzing dredging performance the EMS data can be used to 

measure the annual extent of dredging activities, dredging intensities within individual licence 

areas (Figure 3-32) and the cumulative footprint (i.e. the total extent of dredging activity) over a 

period of time. 

�� 

BMAPA and The Crown Estate are committed to transparency and have been reporting the area 

licensed for dredging under the ‘Area Involved’ initiative. This included a commitment to publish 

an annual report detailing the extent of dredging within regional licensed areas using analysis of 

EMS data. Every year since 1998, therefore, they have jointly produced the Area Involved report. 

In addition a five-year review report summarizing the period 1998-2002 was published in 2005, 

while a ten-year review report (summarizing the period 1998-2007) was published in 2009. 

� �� 

Over the last decade vessels of the English dredging fleet have altered on-board dredging 

practices in order to reduce the environmental effects of the dredging. Examples include limiting 

the number of vessels able to dredge in a region at any one time and loading as efficiently as 

possible, in order to decrease loading time and hence reduce the time the environment is directly 

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impacted and that noise is generated. Table 3-7 shows the total hours dredged by vessels in the 

fleet for the period 2006-2008. 

� �� 

Hours dredged 28,686 26,340 22,985 

�� 

Reduced (or prohibited) screening has been instigated when dredging some license areas in 

order to minimize the effects of the plume. Table 3-8 combines data on total marine aggregate 

production and hours dredged from to calculate the tonnages landed per hour dredged. 

� �� 


aggregate 

production 

64 

�� 

20.29mt 20.64mt 19.75mt -4.31% 

Hours dredged 28,686 26,340 22,985 -12.74 

Tonnes landed 

per hour dredged 

707.41tph 783.57tph 859.12tph +9.64 

�� 

A reduction in dredged hours of nearly 13 percent at a time when total production dropped by just 

over 4 percent suggests less intensive screening, and as a result the tonnage landed per hour 

dredged increased by over 9 percent (BMAPA, 2009). BMAPA members are also committed to 

reviewing all production licence areas over a rolling five year period, and to surrender areas no 

longer containing useful resources of sand and gravel. 

� �� 

Key to mitigating the impacts of dredging on other sea users is an effective mechanism for liaison 

e.g. the success of the majority of mitigation measures related to the commercial fishing industry 

in the UK relies heavily on communication between the two industries (Royal Haskoning, 2004). 

As part of the fisheries liaison process BMAPA, in conjunction with The Crown Estate, produces 

twice-yearly active dredge area charts which define the areas where dredging will take place. 

These are widely distributed in association with local marine and Fisheries Agency officers and 

Active Dredge Zones are not altered until advance notices to local groups have been made. Biannual 

fisheries liaison meetings also take place on the south and east coast. 

An area where potential conflicts with other users may become more apparent is in the proximity 

of dredging licences to regions where offshore marine renewable energy interests will be 

developed. There is a long-term issue with the routing of cables from developing wind farms to 

the coast, but there is also a potential for operational impacts on dredging vessels (BMAPA, 

2009). Dredging vessels require access to and from their licence areas and the ability to navigate


Report No: 10/J/1/06/1309/0996 

within the area while dredging. To mitigate against operational impacts BMAPA has generated a 

series of charts, available to offshore wind developers, showing the extent of dredger transit 

routes between licence areas and ports relative to round 1, 2 and 3 offshore wind sites. 

� �� 

To reduce potential archaeological impacts dredging companies observe a code of practice on 

marine aggregate dredging and the historic environment developed jointly by BMAPA and 

English Heritage. This protocol means that finds of archaeological interest are reported to the 

Secretary of State, the Crown Estate Commissioners, DCMS, English Heritage and to relevant 

County Archaeologists. In the event that features of archaeological interest are encountered in 

the course of dredging, companies comply with the Merchant Shipping Act 1995 in respect of 

reporting and ownership of wrecks including notification of the Receiver of Wrecks. Dredging 

exclusion zones are implemented around features of acknowledged archaeological importance 

identified from the assessment of existing geophysical data and these may be reviewed in light of 

archaeological assessment of new data. If any previously unreported wrecks become apparent 

within the boundaries of the dredging permission area precautionary exclusion zones, defined in 

consultation with independent marine archaeological consultants and English Heritage, are 

instituted around them. 

�� 

The environmental impacts of English dredging may potentially be reduced by development of 

practices and mitigation techniques. These techniques can involve spatial and temporal 

mitigation and application of suitable dredging practices. 

� �� 

Potential impacts of dredging can be mitigated by reducing the total area of seabed dredged i.e. 

through spatial mitigation. At a high level, all licence areas managed by English aggregate 

dredging companies are reviewed on an annual basis and unproductive zones are relinquished 

to minimise the area under licence. Thus the total area of seabed licensed decreased by 387.09 

km 2 between 1998 and 2007, with the greatest reductions of area occurring within 12 nm of the 

coast. This net change was due to a relinquishment of 749.84 km 2 combined with 362.44 km 2 of 

new Licenses (The Crown Estate, 2008). The industry reports the extent of licensed and dredged 

area through the ongoing ‘Area Involved Initiative’ which started in 1999. Data from 2007 

represented the 10 th anniversary of the initiative and an additional review document was 

produced to illustrate trends in the licence and dredged areas since 1998. The annual documents 

also include data on the area of cumulative dredge footprint (BMAPA, 2009). 

Dredging within individual licence areas is also controlled through voluntary zoning to minimise 

the areas of seabed actually being worked (spatial footprint directly impacted). Thus Active 

Dredge Zones (ADZs) are defined within a License Area, and these ADZs are themselves 

subdivided into dredging lanes. Zoning is a requirement of the Marine Mineral Guidance Note 1: 

Guidance on the Extraction by Dredging of Sand, Gravel and Other Minerals from the English 

Seabed (also known as MMG1; Office of the Deputy Prime Minister, 2002). MMG1 describes the 

policies and procedures for marine minerals dredging in English waters and provides guidance to 

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regulators, their scientific advisors, industry and wider stakeholders as to how marine minerals 

extraction should be undertaken so that it is consistent with the principles of sustainable 

development. The zoning guidance states that “the intention is to limit the extent of groups of 

active dredge Areas whilst at the same time minimise the impact of multiple dredging activities 

where necessary” (ODPM, 2002). Zoning plans and ADZs are agreed with the regulators. 

Zoning an area potentially mitigates the effects of the dredging by reducing the area of direct 

removal of biomass. It also allows the majority of a License Area to be undredged, or undergoing 

recolonisation, at any particular time. MMG1 also requires an operator to work areas to economic 

exhaustion – so allowing uninterrupted recovery once extraction has ended (ODPM, 2002). Initial 

recolonisation of an area may be more rapid than if a larger area was being dredged. Dredging in 

lanes means that strips of undredged habitat may be left between adjacent lanes and this may 

mitigate the effects of the dredging since colonising species may have shorter distances to travel 

and recolonisation rates may increase. The rate of recovery of species diversity depends on the 

complexity of the fauna and the controls on recruitment of larvae and settlement. Longest 

recovery rates would be for slow-growing species that do not have planktonic larvae and rates 

are also long for sites that are repeatedly dredged. Minimizing the area dredged will therefore 

minimize these effects. 

Another means of mitigating the effect of dredging on marine habitats, and a standard condition 

on all English licenses and dredging permissions, is to leave the seabed post-dredging in a 

similar state to its pre-dredged condition e.g. if the seabed was a sandy gravel pre-dredging 

attempts will be made to leave the seabed as a sandy gravel following cessation of dredging. In 

addition sediments should not be dredged completely but 0.5 m thickness of resource sediment 

should be left after cessation of dredging. 

The English dredging industry makes efforts to maintain seabed condition through targeted 

screening and dredging and regular monitoring. Newell and Seiderer (2003) report that many 

species do not re-colonize regularly, and even though the seabed may remain in a similar 

condition to its pre-dredged condition there may still be a significant interval before all the species 

components are present in the community. 

Zoning reduces the area over which screened sediments are returned to the seabed and zones 

are positioned, where possible, parallel with the directions of tidal currents to ensure that bedload 

and plume transport is rapidly incorporated into any regional sediment transport pathway. Zoning 

to target sediments with low silt and clay contents also mitigates against the potential effects of 

plumes by reducing the amount of fine sediment introduced into the water column. This mitigation 

technique, however, can only be properly implemented following commencement of dredging, 

when further knowledge about the location of fine sediment has been gained. Zoning of English 

License Areas means that of the approximately 1300 km 2 of licensed seabed between 222.6 km 2 

(1998) and 134.5 km 2 (2004) was actually dredged in any one year (The Crown Estate, 2009). 

Dredging also has the potential to create conflict with other potential users of the sea area 

particularly fishermen. Such conflicts may result in diminished access to fishing areas and a 

reduced catch. Spatial mitigation actions to reduce effects on fisheries can include exclusion 

zones to avoid important fishery sites. Other mitigation actions also include activities to facilitate 

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communication and cooperation among potentially conflicting users such as appointing fishery 

industry liaisons to facilitate communications and providing advance notice to other users of 

changing dredging. 

� �� 

The potential effects of dredging may have an increased environmental impact during particular 

times of the year e.g. during spawning periods. The English aggregate dredging industry 

mitigates against these effects by adopting temporal mitigation measures i.e. through minimizing 

or avoiding screening (or prohibiting dredging entirely) on certain license areas during particular 

environmental windows. Temporal mitigation may take place during certain phases of flood or 

ebb; during the Spring-Neap tidal cycle, or over particular seasons. As an example, mitigation of 

the potential effects of benthic boundary layer plumes on breeding areas for crab has been 

achieved by dredging only when the tidal stream transports sediments away from sensitive 

areas. 

� �� 

The marine aggregate industry in the UK has significant sums of money in increasing knowledge 

of the marine environment and in enhancing the understanding of the impacts of aggregate 

dredging. It has done this through internal research projects, the Environmental Impact 

Assessment process and most significantly through the Aggregate Levy Sustainability Fund 

(ALSF). In 2002 the Government imposed a levy on all primary aggregates production (including 

marine aggregates) to reflect the environmental costs of winning these materials. A proportion of 

the revenue generated was used to provide a source of funding for research aimed at minimising 

the effects of aggregate production. This fund, delivered through Defra, is known as the 

Aggregate Levy Sustainability Fund (ALSF). The Marine ALSF (MALSF) is currently administered 

by the Centre for Environment, Fisheries and Aquaculture Science (Cefas) through the Marine 

Environmental Protection Fund (MEPF) and English Heritage. 

The objectives of the ALSF in the marine environment are to reduce the environmental footprints 

of marine extraction. Priority areas for funding have been identified in line with five main aims. 

� To develop and use seabed mapping techniques to improve the evidence base of 

nature, distribution and sensitivity of marine environmental and archaeological 

resources relevant to marine aggregate activities. 

� To increase understanding of the effects of aggregate dredging activities, including 

noise, and their significance. 

� To develop monitoring, mitigation and management techniques, underpinned by 

scientific research. 

� To research and understand the socio-economic issues associated with aggregate 

dredging activities. 

� To promote co-ordination and establishment of sustainable archives for the 

dissemination of research related to these aims to a wide range of stakeholders. 

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The Marine ALSF supports a wide range of projects that have addressed a range of scientific and 

socio-economic fields including exploring the ecology, geology and heritage of the seabed 

around the UK; environmental protection and impact assessment, protection of the historic 

environment, recoverability, mitigation, technological advances and the raising of public 

awareness. Since 2002, over £20 million has been invested in projects increasing the knowledge 

and understanding of marine environmental resources through applied, science led research. 

This directly supports marine planning and decision making through the provision of robust stateof-the-art 

evidence. The MALSF has commissioned projects that have addressed a range of 

scientific and socio-economic themes. 

The MALSF seeks to apply the “collect once, use many times” principle to the data collected and 

recognizes that data collected for a project specific purpose may have wider applicability, if 

accessed by other scientific disciplines and be of use beyond the requirements of any single 

project. Accordingly, Cefas (2008) produced a summary report for the MALSF which examined 

current practices in MALSF research and provided recommendations on how they may be 

adapted to maximise the value of the data collected in future, for the benefit of all stakeholders. 

For transparent planning and management of the marine environment, the information and 

knowledge gained should be readily available to all ‘stakeholders’ – from Government through to 

the general public. The MALSF aims to make the results of the projects accessible through a 

range of communication and dissemination materials and activities. 

The increasing amount of knowledge gained as a result of the MALSF has allowed development 

to commence of a Marine Aggregate Extraction Risk Assessment (MARA) Framework to explore 

the potential of an approach for assessing hazard probability and consequences for receptors 

and to present these in the context of the overall risk arising from dredging (HR Wallingford, 

2007). 

�� 

The English marine aggregate industry is licensed commercially by The Crown Estate, which 

owns the seabed to the 12-mile territorial limit and holds the non-energy mineral rights out to 200 

miles as part of the hereditary possessions of the Sovereign. Under The Crown Estate Act 1961, 

The Crown Estate Commissioners have a duty to maintain and enhance the value of the estate’s 

assets and to secure revenue from them. It receives a royalty for every tonne of aggregate 

extracted from licensed areas. Companies seeking to dredge are obliged to obtain authorization 

from the Government and a license from The Crown Estate, before they can legally operate 

(Gubbay, 2005). 

Before dredging can take place on an English licence area the dredging company must obtain 

consent, termed a dredging permission. The dredging permission process in England is 

administered by Defra’s Marine and Fisheries Agency. In 2007, the Environmental Impact 

Assessment and Natural Habitats (Extraction of Minerals by Marine Dredging) (England and 

Northern Ireland) Regulations 2007 (the Marine Minerals Regulations) were introduced which 

provided a statutory framework for the environmental impact assessments required for proposed 

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marine minerals extraction. The Marine & Fisheries Agency carry out the licensing and 

enforcement duties resulting from the introduction of the regulations. The procedure to follow is 

summarized in Figure 3-33 below. 

The English regulatory process is transparent, puts the environment first, and involves significant 

consultation to issues that need addressing in the environmental impact assessment. The 

process is supported by Government Guidance – primarily Marine Mineral Guidance Note 1: 

Guidance on the Extraction by Dredging of Sand, Gravel and Other Minerals from the English 

Seabed (MMG1) (ODPM, 2002) which describes the policies and procedures for marine minerals 

dredging in English waters and provides guidance to regulators, scientific advisors, industry and 

wider stakeholders as to how marine minerals extraction should be undertaken so that it is 

consistent with the principles of sustainable development. 

An environmental statement and supporting studies are widely circulated to stakeholders, with 

the applicant then tasked with overcoming concerns. Unless the applicant can demonstrate 

that the environmental impacts are acceptable at the end of the process, then the administering 

government department will not award a dredging permission. Consent decisions are 

accompanied by a schedule of legally enforceable conditions that form an integral part of 

any Crown Estate production licence to dredge and must be adhered to by the licensee 

(BMAPA, 2009). 

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� 

Report No: 10/J/1/06/1309/0996 

�� 

Applicant supplies 

necessary information 

to MFA 

�� 

�� 

MFA consult widely 

statutory and other 

stakeholders 

EIA or AA 

required? 

Applicant applies to MFA for 

pre-application advice from 

Cefas (recommended) 

Applicant to pay fee to MFA 

MFA to instruct Cefas to 

engage with applicant and 

can supply formal scoping 

opinion if required 

Applicant to complete fully 

worked up ES 

Applicant provides application 

form, fees, ES and all other 

necessary supporting 

information to MFA 

MFA make decision 

based on consultation 

responses 

� 

� 

�� 

�� 

70 

Screening determination 

sent to applicant and 

published 

Decision sent to 

applicant and 

published


Report No: 10/J/1/06/1309/0996 

71


�� 

�� 

�� 

Report No: 10/J/1/06/1309/0996 

This Section of the report will describe the characteristics of the current world fleet of dredgers 

and compare these with the situation in the English fleet. In order to make comparison easier it 

will mirror the structure of Chapter 3.1 and focus on: 

� Vessel age; 

� Design; 

� Crew; 

� Vessel size; and 

�� 

� Power and Productivity Rates 

The most recent data available estimates that there are in excess of 1300 vessels in the current 

world dredging fleet (Clarkson Research Services Ltd, 2009). These vessels are divided into a 

number of different classes of dredging vessel as shown in below. 

�� 

Backhoe / Dipper / Grab Dredger 243 

Barge Unloading Dredger 19 

Bucket Ladder Dredger 61 

Cutter Suction / Bucket Wheel Dredger 420 

Special Equipment Dredger 32 

Suction Dredger 40 

Trailer Suction Hopper Dredger 494 

TOTAL 1309 

�� 

Table 4-1 shows that TSHDs make up 38% of the world’s dredging fleet, and the remainder of 

Chapter 4.1 will concentrate on these 494 vessels. 

�� 

The age of TSHDs in the world fleet varies greatly. The oldest TSHD in the world fleet is the 

TSHD “Westerschelde”, owned by Dutch Dredging, which was built in 1935 and is 74 years old. 

The mean age of vessels in the world TSHD fleet is 24 years. The data are skewed, however, by 

the number of older vessels working in the developing world, since vessels that have reached the 

end of their working life in Europe or North America are often sold to operators in emerging 

72


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markets to continue working. A better comparison with the English dredging fleet would therefore 

be to examine the fleets of the major European dredging companies i.e. Van Oord, Jan de Nul 

N.V., Dredging International and Boskalis. The oldest vessel is Van Oord’s vessel “HAM 350” 

which is 35 years old, and the mean age of the vessels in the fleets of these four companies is 

11.1 years. This is considerably younger than the mean age of English TSHDs (21.9 years). 6 

further vessels are due to be launched in 2010 – 2011 which is likely to reduce the mean age 

further. 

Data on configurations are available for 413 of the world fleet (Bert Visser’s Directory of 

Dredgers, 2009) and are summarized in Figure 4-1 below. 

�� 

It can be seen that for those vessels where data are available the majority have an aft design 

configuration, while approximately 20% of the vessels have a forward configuration. There is also 

a small percentage (2% - 10 vessels) that have a midships bridge and accommodation block. 

Presumably this is to reduce movement and increase crew comfort but places an obvious 

restriction on the capacity of the dredge hopper. 

Modern ship design can also play a part in altering the sailing resistance (and hence fuel use) of 

a ship. The resistance to a ship’s sailing depends on its speed, the way water flows around the 

hull and the waves the ship makes as it moves. Different hull designs can significantly alter the 

sailing resistance with the bow design being of particular importance. Many modern TSHDs have 

a "bulb" (i.e. artificial nose) mounted on the hull at the waterline which alters the flow of water 

around the hull, and resistance is reduced because of the reduction of induced waves. Fuel 

savings of up to 10% can be made – but this is only optimized for one draught. Figure 4-2 shows 

two TSHDs – one with a conventional bow and one with a bulb. It can be seen that the waves 

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Report No: 10/J/1/06/1309/0996 

induced by the ship with a bulb are significantly smaller. The design of the ship’s stern can also 

have an effect on sailing efficiency with carefully designed sterns resulting in an ideal approach 

of flow to the propellers resulting in less vibration and higher propulsion efficiency. 

� 

�� 

�� 

Ship propellers are reasonably efficient in transforming mechanical engine power into thrust 

power with efficiencies mostly in the range of around 70%. Propeller efficiency can be limited by 

conditions imposed by the hull and the ports the vessel serves, for example propeller diameter is 

restricted by the operational draught restrictions. Other modern techniques for reducing hull drag 

include anti-fouling paints and gel coats; high quality hull welding, retro-fitted propeller pods etc. 

A detailed study of vessel design is beyond the scope of this project, however a recently 

commissioned ALSF study (MEPF REF 09/P133) will review vessel design and identify key 

issues including optimum operation (e.g. optimum trim, optimum power setting in restricted 

waters), maintenance (engine room tuning, polishing of propellers) and refits (application of 

Energy Savings Devices, improved propeller design). It will also quantify potential reductions in 

environmental impacts for any future new build vessels. 

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�� 

�� 

Report No: 10/J/1/06/1309/0996 

Data on crew numbers are more restricted than vessel age data and the accuracy of the data are 

unknown, however the mean crew on TSHDs worldwide is 27. The largest reported crew 

numbers are 120 crew members on the US Army Corps of Engineers operated TSHD “Esayons”, 

and 92 crew on the Iraqi Ports operated TSHDs “Al Najaf” and “Alkhalij Alarabi”. These contrast 

with the 3 crew members of the TSHD “Toste” operated by the Danish Coastal Authority, and 

which is used for beach recharge operations. Again these numbers are skewed by practices on 

vessels worldwide being radically different from those in the English industry. 

A trend within European dredging companies, particularly DEME, is the development of oneman-operated 

dredgers which have been claimed to be safer and less prone to 

misunderstandings between operators (IHC, 2007). Typically dredging vessels, including all 

English dredging vessels have had at least two crew members on duty on the bridge - one 

handles the ship, navigation and lookout while the other controls the dredging equipment. 

The most recent DEME vessels including the TSHDs "Marieke", "Brabo" and "Breydel” have 

been built so that a single crew member can operate the whole process. The wheelhouse is 

ergonomically designed for a one-man-operation with information supplied through 6 displays 

and alarm systems (Figure 4-3). Since the wheelhouse is positioned in front of the vessel, and 

the dredging equipment aft, the lone operator needs CCTV-systems to check the equipment aft. 

�� 

As with crew numbers, there is a wide variety of TSHD size operating worldwide - from the 

smallest vessels up to the megadreders, which are discussed as a case study below. Table 4-2 

below summarizes the available data (Clarkson Research Services Ltd, 2009). 

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� �� 

Mean 96 17 6.1 

Maximum 231 

Minimum 

TSHD “Queen of the 

Netherlands” 

35 

TSHD“Ria de Navia” 

76 

41 

TSHD “Christobal 

Colon” 

7.4 

TSHD “Oosterschelde” 

15.2 TSHD “Christobal 

Colon” 

1.4 

TSHD “New Era” 

�� 

�� 

The size of the vessel is partially governed by the type of dredging activity undertaken i.e. the 

capital and maintenance dredging industries have very different drivers from the marine 

aggregate industry. Capital dredging involves the creation of new or improved facilities e.g. 

harbour basins, navigational channels etc. while maintenance dredging is the removal of siltation 

from channels in order to maintain the design depths of navigation channels and ports (Van 

Raalte, 2006). Capital dredging typically requires the relocation of large quantities of sediment as 

quickly as possible, since additional dredging time equates to additional development costs while 

maintenance dredging in artificially deepened navigation areas needs to occur quickly so as to 

not compromise the operation of a facility. As a consequence this has driven the development of 

larger vessels, able to rapidly dredge large volumes of sediment at a time, and with high loading 

and transit speeds. Chapter 3.1.5 showed that the driver on vessel size for the English fleet was 

their ability to enter the ports they supply and thus the vessel size is constrained by these 

parameters 

� �� 

Due to its small size, high population density and fast industrial development Singapore has 

constantly reclaimed land for development purposes. Planned construction and land reclamation 

led Moch and Tiarma (2002) to estimate that 1.8 billion m 3 of sand would be needed between 

2001 and 2010. To serve this market a range of larger Jumbo dredging vessels with hopper 

capacities between 17,000 and 25,000m 3 were built. The first example of a jumbo dredger was 

TSHD “Pearl River” (17,000m 3 ) followed later by vessels such as “Queen of the Netherlands”, 

“Volvox Terranova” and others. 

A new phase of major land reclamation projects in the Persian Gulf (e.g. Palm Island, The World) 

led the major world dredging companies to plan and commission even larger dredging vessels – 

the so called Megadredgers. The first of these to be commissioned was the Jan de Nul dredger 

“Vasco Da Gama” (33,000 m 3 ) in 2000. The vessel has twin 1400mm diameter dredge pipes and 

can dredge to a maximum depth of 125m with a loading time for the hopper of just one hour. 

“Vasco Da Gama” has mainly been used for land reclamation. It has also been used for diamond


Report No: 10/J/1/06/1309/0996 

mining in Southern Africa and deep offshore dredging of iceberg protection wells for sub-sea 

production systems off Newfoundland. 

A number of further Megadredgers are in the building or procurement phases. “Queen of the 

Netherlands is scheduled to be lengthened in 2009, while DEME ordered a vessel from IHC in 

early 2009 for delivery in 2011. Jan de Nul has two sisterships – the “Cristobal Colon” and the 

“Leiv Eriksson” – being built and these will both have 46,000m 3 hoppers. “Vox Maxima” is Van 

Oord’s new megatrailer with a 31,000m 3 capacity and was launched in 2009. Table 4-3 below 

gives comparisons for all the Newbuild (N) or Lengthened (L) mega dredgers. 

�� 

�� 

�� 

�� 

�� 

Owner VOA JDN BOKA JDN VOA DEME JDN 

N or L L N L N N N N 

Hoper (m³) 37,293 33,000 35,500 46,000 31,136 30,000 30,500 

Installed 

power (MW) 

Pump ashore 

(MW) 

Dredge power 

(MW) 

Dredging 

depth (m) 

8.6 37.1 27.6 41.5 31.3 ??? 23.6 

11 16 12 16 13.3 ??? 15 

2 x 2.5 2 x 4.5 2 x 6 2 x 6.5 1 x 6 ??? 2 x 3.4 

101 140 115 2 x 155 125 ??? 93.5 

Draught (m) 13.37 14.6 11.49 15.15 14.5 max. 12 

Speed (kn) 15.5 16.3 ??? 18.0 17.0 ??? 16.0 

Commissioned 2008 2000 2009 2009 2009 2011 2010 

�� 

The global financial crisis and the curtailment or cancellation of many of the building projects in 

the Persian Gulf means the markets that these Megadredgers were designed to serve are now 

uncertain. Despite this, the major dredging companies are completing the building programmes 

for these ships. It may be that projections of global recovery indicate that major building projects 

are likely to recommence before delivery of the ships – or that the ships themselves can create 

their own markets by offering previously impossible technical solutions. 

77 

m 

11.0


�� 

Report No: 10/J/1/06/1309/0996 

As with crew numbers and size there is an extremely wide variety of engine types, engine power, 

generators, dredge pumps and dredge powers on vessels of the world fleet. Data are not 

available for all vessels but Table 4-4 below summarizes the available data (Clarkson Research 

Services Ltd, 2009). 

� �� 

�� 

Maximum 

reported 

Minimum 

reported 

38,600 (i.e. 2 x 19,300) 

TSHD “Christobal 

Colon” 

320 

TSHD “Westerschelde” 

78 

�� 

�� 

10,000 

TSHD “Vasco de 

Gama” 

100 

TSHD “Bir Anzarane” 

�� 

�� 

13,100 (i.e. 2 x 6550) 

TSHD “Xin Hai Long” 

119 

TSHD “Currituck” 

�� 

�� 

There is very little data available on general productivity rates for TSHDs. Some examples of 

specialized vessels with high productivities can, however, be derived from literature (Table 4-5) 

�� 

“Brisbane” 2900 25 116 

“Wheeler” 6120 11 556 

�� 

�� 

�� 

At the lower end of the dredge pipe a draghead is installed - the function of which is to excavate 

the sediment and mix it with water for hydraulic transport. It has been shown that dragheads 

used by the UK dredging fleet are older-style dragheads (like the California type dragheads) 

where the primary source of production is erosion of sediment around the edges of the draghead 

caused purely by the water being sucked into the system by the dredge pump. 

One of the keys to maximizing the efficiency of dredging is to minimize the dredging cycle time 

and hence the time taken to load the hopper. This is particularly important for the maintenance 

and capital dredging sectors where there is a key financial driver to minimise dredging time. This 

is of less importance for the English aggregate dredging industry where tidal constraints limit 

many of the cycle times. 

Hopper loading time is strongly controlled by the performance of the draghead. Much research 

has taken place, mainly by the Dutch dredging industry, to maximize the performance of the 

draghead, and many dredging applications around the world now use more active draghead


Report No: 10/J/1/06/1309/0996 

systems. In many situations around the world, modern dragheads (Figure 4-4) use cutting teeth 

or knives, jetting (loosening the soil by the use of waterjets), or a combination of both to mobilise 

the dredged sediment. 

� �� 

�� 

One simple method of loosening the seabed sediment and increasing the erosion rate is the use 

of cutting teeth or knives on the draghead of the TSHD. The teeth act to cut the sediment and 

move the sediment particles into the water flow to form a slurry, which is then sucked up the 

dredge pipe. It has been reported (Brogdon Jr et al., 1994) that knives tend to increase 

production at high travel speeds, since they act as sediment displacement devices. 

Despite the fact that adding cutting teeth to a draghead seems a simple solution to increasing the 

erodibility of the sediment, it may have other operational consequences that are not immediately 

apparent, for instance Herbich (2000) reports that teeth can impart high and variable stresses 

onto the dredge pipe, particularly in the transition of the draghead from unconsolidated to 

cohesive sediments. There is therefore a risk of damage to the dredge pipe. In addition the teeth 

need to be sharp for optimum cutting of bed sediment and therefore need to be replaced on a 

regular basis and this may result in an increased down time in port. 

TSHDs with cutting teeth on the draghead are being commonly used in major projects worldwide 

– examples include “Queen of the Netherlands” which has been used to deepen the shipping 

channel for Melbourne Port; and the “Vasco de Gama” which was fitted with a specially modified 

draghead with an adaptable number of cutting teeth for dredging in Port Sudan (Malherbe and 

De Pooter, 2008). 

Figure 4-5 shows a schematic overview of the cutting process in saturated granular sediment. A 

blade is moving from left to right and cuts a layer with thickness ��. A shear zone develops in 

front of the blade and within the shear zone the soil is dilating and the pore volume increases due 

to shear which is a typical behaviour for densely packed sands. 

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Report No: 10/J/1/06/1309/0996 

�� 

The phenomenon of dilation during cutting causes the development of negative pore pressure 

(under pressure) in the soil, because water needs to flow to the area where dilation occurs. The 

negative pore pressure increases the effective pressure which can result in very high cutting 

forces under water. The magnitude of the decrease in pore pressure is limited since the pressure 

cannot be lower than zero (vacuum) and in that case cavitation of the pore water will occur (Van 

Os & Van Leusen, 1987). 

In calculating the cutting forces under water, two different regimes can be distinguished – the 

drained regime and the cavitating regime. Miedema (1994) defines the cutting force in the 

drained regime as: 

� �� 

� 

� � � � 

� � � � � � � �� 

In which: 

� � Constant [-] 

� � Horizontal cutting force [N] 

� Width of the cutting tool [m] 

� Cutting depth [m] 

� � Relative increase in pore volume, defined as: 

80


Report No: 10/J/1/06/1309/0996 

� � � 

�� 

� 

�� 

�� 

�� 

In the cavitating regime the pore pressure in the shear zone is constant and equal to the vapour 

pressure. The pressure difference, which determines the effective pressure, is therefore 

proportional to the water pressure outside of the sediment, hence the water depth. 

�� 

� � 

In which: 

� � Constant [-] 

� water depth [m] 

The specific energy is therefore the amount of energy needed to excavate a certain volume of 

sediment and is equal to the ratio between cutting power and production. In the cavitating regime 

this quantity is independent of the operational conditions, except the water depth. 

� � 

�� 

� � � � 

�� 

81 

� �� 

� 

�� 

� � � � � � �� 

The coefficient �� depends on the cutting angle � and internal friction angle �� of the soil 

(Miedema, 1994): 

�� 

� � 

� � �� 

� �� 

During dredging the weight of the draghead and dredge pipe penetrates the upper layer of the 

sediment. However when the sediment is compacted or armoured the weight may not be 

sufficient to penetrate the sediment and erosion of the sediments will be reduced. Reduced 

erosion results in low mixture densities and a reduced production speed. 

The theory behind incorporating jet nozzles is that by combining sufficient surface contact 

pressure with the sea bed and injection of jetwater the draghead can liquefy densely compacted 

sediments and increase loading speeds. To maximize efficiency of the jetting Vandyke (2002) 

suggests that the nozzles have to be integrated into the points of the draghead so that the 

waterjets cut the sediment only moments before the draghead penetrates. Brogdon Jr et al. 

(1994) report that water jet erosion benefits tend to increase as the travel speed of the dredging 

vessel decreases, since the jetting forces have a longer time to attack individual particles. The jet 

momentum available for the erosion process can be defined as:


� � 

Where � 

Report No: 10/J/1/06/1309/0996 

� � � 

� �� 

� � � � � �� 

� 

� 

� is the total jet discharge and � � the jet pressure drop over the nozzles in the 

draghead. So jet production can be increased by increasing jet discharge and/or jet pressure. 

The amount of hydraulic power � � (and likewise fuel consumption) used for the jetting system is 

equal to: 

� � 

�� 

Combination the above two equations gives: 

� � 

� � 

� 

� � 

�� 

� � � � � � � � �� 

� � 

From this equation it can be concluded that for a certain level of installed jet power, the 

momentum and therefore jet production will decrease with the jet pressure. In other words a 

combination of low jet pressure and high jet discharge is more efficient than the other way 

around. This however is not entirely true since the jet discharge will increase with decreasing 

pressure for a certain value of jet power. This will dilute the sand water mixture created on the 

seabed. The diluting effect can be shown using a simplified approach where we assume that the 

original jet water volume mixes with the excavated soil volume. The volume of excavated 

sediment (including the pore volume) � � is: 

� 

� 

� 

� � 

� 

�� 

� 

�� 

The sediment concentration � � in a mixture of sediment and water created by the jets is: 

� � 

� 

� 

� 

� � � 

� �� 

�� 

The density � � of the jet mixture follows from: 

� �� 

� � 

� � � � � � �� 

� � � � � 

This effect is illustrated in Figure 4-6 where jet production and the density of the mixture created 

by the jet is shown as a function of the jet pressure for a certain value of jet power. In the lower 

82


Report No: 10/J/1/06/1309/0996 

panel we see the jet production decreases with applied pressure. In the upper panel it is shown 

that density increases. This simple approach yields the upper limit of the jet density. In reality 

density will be lower because a water jet entrains surrounding water and therefore the excavated 

volume of sediment will be diluted with a larger volume of water than leaving the nozzle. 

�� 

The sand – water mixture created by the jets is mixed with the water entering the draghead and 

sucked into the suction pipe. Therefore the mixture density transported to the hopper will always 

be lower than created by the jets. The combination of jet pressure and discharge determines the 

upper limit of the mixture density. This explains the tendency to increase jet pressure in modern 

dragheads. The specific energy of mechanical excavation (cutting) can be compared with the 

specific energy for hydraulic excavation. 

� � 

� 

�� 

�� 

� � 

� � � � � � �� 

�� 

� � � � 

� 

� 

� �� 

�� 

� � � 

�� 

� 

� 

� 

The value �� is the excavation constant specific for jetting. The specific energy for cutting is 

compared with the specific energy for jetting in Figure 4-7 as a function of the jet pressure and 

different water depths. The figure shows that in the jetting process the specific energy increases 

with jet pressure, which is in agreement with Figure 4-6 where jet production decreases with 

pressure. The specific energy for cutting is higher than for jetting and the difference increases 

83


Report No: 10/J/1/06/1309/0996 

with water depth. The actual difference is even larger since here the hydraulic and mechanical 

power consumptions are compared. The cutting force is a result of the ships propulsion system 

and the jet power originates from a centrifugal pump. The efficiency of a jet pump is higher than 

the efficiency of a ships propeller, so when the comparison is made taking these efficiencies into 

account the difference between the two specific energies becomes larger. 

�� 

So for dredging of granular sediment jetting is more efficient compared with mechanical 

excavation. This explains the fact that many modern TSHDs are nowadays equipped with a 

considerable amount of jet power as standard. 

84


� �� 

Report No: 10/J/1/06/1309/0996 

One system that incorporates high pressure jetting nozzles in the draghead of a conventional 

TSHD is the DEME patented DRACULA draghead system (DRedging And Cutting Using Liquid 

Action) (Vandycke, 2002) and was initially designed for dredging stiff clays. The DRACULA 

system was successfully used in a dredging operation to deepen the access channel to the port 

of Antwerp. 

Model tests indicated that in order to dredge stiff clays water pressures of between 300-350 bar 

were needed and therefore the DRACULA water jets generated 380 bar of pressure. This was 

deemed sufficient to erode the seabed sediment at trailing speeds of 2-2.5 knots. 

In order to drive water through draghead nozzles high pressure pumps must be installed. For the 

DRACULA system the pumps were driven by diesel engines, and any pumping system would 

therefore have an additional fuel implication if used on the English fleet. Vandyke (2002) 

suggests, as a rule-of-thumb, that one pump can feed ca. 20 nozzles of 2.0 mm diameter. 

Although the sediment type dredged was different from that dredged by the English marine 

aggregate fleet it is still useful to consider the results of using the jetting nozzles. It was found 

that production of the dredger increased between 15% and 27% and that when the system was 

active, the average fuel rack was 5% less compared to dredging without DRACULA. It was also 

observed that the maximum increase in production (27%) occurred when the jets in the visor 

together with the jets on the heel were active (Vandyke, 2002). 

� �� 

The DRACULA case study showed how introducing water jets to the draghead can increase the 

efficiency of production, while chapter 4.1.1 described how the addition of knives or teeth to a 

draghead can also improve the rate of sediment loading. An obvious further experimental step 

was therefore to test a combination of knives and jetting as a means to improve dredging 

efficiency. The US Army Corps of Engineers first investigated how combinations of knives and 

jets affected dredging performance as long ago as 1994. These tests showed that the addition of 

knives, placed in front of water jets increased the efficiency of dredging (Brogdon Jr et al., 1994). 

� �� 

IHC Parts & Services from the Netherlands designed and patented the Wild Dragon draghead, 

which is characterized by a double row of cutting teeth with water jets contained within the teeth 

themselves. IHC claims that “the dredger production is increased, the dredging process produces 

less wear on components, the hopper mixture’s velocity is much lower, spoil settles sooner and 

the hopper is loaded faster. Lower emissions and less turbidity thanks to lower overflow from the 

hopper make the ‘Wild Dragon’, much better for the environment” (IHC, 2005). The draghead has 

been trialled on the TSHD “Xin Hai Long” in China (Figure 4-8). 

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Report No: 10/J/1/06/1309/0996 

�� 

�� 

The area where the “Xin Hai Long” normally operates, the Shanghai estuary, is known for its 

layers of extremely hard packed sand and has proved to be impossible to dredge by TSHD’s with 

acceptable production rates. Cutter suction dredgers could do the job but their deployment is 

impossible because of the intensive vessel traffic and wave conditions. IHC (2005) reports that 

the use of the Wild Dragon draghead on the “Xin Hai Long” was very successful. During the 

trials, dredging operations were carried out with two types of dragheads simultaneously; a normal 

one at port side and a Wild Dragon draghead at starboard side and the claimed improvement in 

production was up to 100% (Figure 4-9). 

�� 

�� 

�� 

�� 

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Report No: 10/J/1/06/1309/0996 

Similar combinations of teeth and water jets are also in operation in dredgers operated in 

different industrial sectors e.g. in diamond mining off the coast of Namibia dragheads on the 

TSHD “Vasco da Gama” are fitted with teeth and water jets to break up cohesive muds and clays 

and allow diamondiferous sediments to be loaded into the vessel for subsequent processing 

(Pisces Environmental Services (Pty) Ltd, 2008). 

Numerical tests on draghead efficiency with the addition of cutting knives and/or water jets were 

conducted by Brogdon Jr et al. (1994). Table 4-6 (from Brogdon Jr et al., 1994) summarizes the 

results, and it was found that water jets could significantly increase dredging production when a 

dredge was operating at slow speeds (less than 0.9 ms-1 in the tests conducted by Brogdon Jr et 

al. (1994)). 

�� 

�� 

�� 

�� 

87 

�� 

Without knife With knife 

No jet n/a 0.03 0.05 

19 275 0.05 0.07 

19 483 0.1 0.07 

25 138 0.04 0.06 

25 275 0.05 0.07 

��


� �� 

� �� 

Report No: 10/J/1/06/1309/0996 

A technical modification that allows an increased area of seabed to be dredged on each pass 

(and hence reduce dredging time) is to increase the width of the draghead. The Japanese TSHD 

“Seiryu-maru" claims to have the world's largest-class wide span drag head which permits high 

accuracy levelled dredging without leaving excessive ridges (Yano et al., 2006). 

The ship has a 7.2m wide monobloc draghead, which is divided into four sections (Figure 4-10). 

These sections increase the ability of the draghead to follow undulations in the seabed and help 

prevent water suction caused by undulations which is a drawback with rigid monobloc 

dragheads. This draghead also incorporates a recycling system that returns the hold water to the 

drag head. 

Adoption of the wide span drag head has however resulted in a hull layout that has an aft centre 

dredge pipe (which is discussed in more detail in section 4.2). 

�� 

Dredgers with wide dragheads have also been used in other sectors e.g. the “Vasco de Gama” 

was fitted with a 7m wide draghead, capable of excavating 0.5m depth at a pass, for diamond 

mining off the coast of Namibia (Pisces Environmental Services (Pty) Ltd, 2008). 

88


� 

� �� 

Report No: 10/J/1/06/1309/0996 

The Vic Vac dredge head was designed by a US company (J.F. Brennan Co., Inc.) to target 

removal of thin layers of soft sediment above a firm substrate. The Vic Vac dredge does not use 

a conventional draghead, but instead consists of a unit that focuses suction directly below the 

assembly. Figure 4-11 shows the Vic Vac dredge head while Figure 4-12 show an isometric view 

of the bottom of the assembly (Green, ��). 

�� 

�� 

�� 

The Vic Vac was designed to allow precise removal of sediment without resuspending large 

amounts of sediment and hence minimizing the plume caused by dredging. The Vic Vac has 

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Report No: 10/J/1/06/1309/0996 

been successfully used to dredge contaminated sediments with minimal resuspension in the 

Ashtabula River, near Lake Erie, Ohio, USA (USEPA, 2008). 

A second version of a focussed suction system, also developed in the United States, is the 

Tornado Motion dredging system. This is based on an eddy pump (Figure 4-13) which consists of 

an energy generating rotor attached to the end of a drive shaft. As the rotor begins to spin, it sets 

into motion the ambient fluid present and this spinning fluid is forced down into the hollow centre 

of the intake chamber. Within the intake chamber it creates a high speed, swirling synchronized 

column of fluid which agitates the sediment to be pumped and the agitated material travels by 

reverse flow along the sides of the intake chamber, into a volute. Here the sediment, under 

pressure from below, is forced into the discharge pipe (Weinrib, pers comm.). 

�� 

�� 

The eddy pump was first used in a demonstration project in 1994 and it was found that there was 

little resuspension of sediment at the suction head. Creek and Sagraves (1995) reported that, 

average turbidity around the suction head exceeded background levels by 1.2 Nephelometric 

Turbidity Units (NTU). The maximum turbidity increase at the head was 1.9 NTU and maximum 

suspended solids measurement was 9.0 mgl-1. The maximum turbidity of the discharge plume 

was only 12 NTU over ambient. Figure 4-14 shows the Tornado Motion suction head. 

�� 

�� 

90


� 

� �� 

Report No: 10/J/1/06/1309/0996 

91 

� 

An extreme way of controlling the draghead is to make it semi-autonomous, and the Penta- 

Ocean Construction Co. Ltd. in Japan has done that in developing a Submersible Dredging 

Robot for highly accurate seabed dredging. This has been named the “Futaba” and is an 8legged 

robot, which walks on the seabed with automatic movement and posture control, and can 

be controlled by a single operator. The dredge sediment is pumped back along a pipe to shore or 

hopper. “Futaba” has a 320kW dredge pump, however its rate of production is low at only 

70m 3 /hr (Hisao and Sadayoshi, 1999). It is claimed, however, that it is capable of generating 

accurate dredging profiles since it transmits real-time positions and status back to the operator. 

In addition since it operates on the seabed it is relatively insensitive to wave action even under 

severe sea conditions (Penta-Ocean Construction Co., 2009). 

� �� 

The low turbidity draghead is a modified draghead of a TSHD with a specially designed suction 

box. A moveable visor or valve makes it possible to use the draghead in a sweeping motion in 

two different directions. The cutting height can be adjusted to dredge layers of different 

thicknesses with the lower cutting edge shaving the sediment layer and the upper visor adjusting 

to the bottom profile in order to prevent excess water flowing into the draghead. The system 

includes a degassing system which prevents cavitation in the suction pump (Bray, 2008). 

�� 

The efficiency of the dredge pump and impeller to move sediment from the seabed within the 

dredging area and into the hopper of a dredger is fundamental to controlling the loading time of 

the vessel. Therefore, the better the efficiency of the pump and impeller the less time the vessel 

is directly impacting the environment by loading. 

The pressure difference generated between the suction and discharge sides of the dredge pump 

is called the manometric head. The pump must energize the sediment/water mixture, move it 

through the dredge pipe, and overcome the effects of friction and gravity. Conventional dredge 

pumps (Figure 4-15) have an efficiency of approximately 80% at their Best Efficiency 

Point however research by Dutch dredging scientists has led to the development of high 

efficiency pumps (Figure 4-16) which, it is claimed, have an efficiency of about 90% at the BEP 

(IHC, 2004).


Report No: 10/J/1/06/1309/0996 

�� 

�� 

Van den Berg (2001) reports that conventional dredge pumps have impellers with single curved 

blades, a suction shield with a small radius at the entrance and a casing with a large throat area 

while high efficiency pumps have blades that are curved in three dimensions, an increased radius 

of the suction shields, a decrease of the inlet angles of the blades, a change in the design of the 

inlet edge of the blades and optimisation of the blade angle. This has resulted in pumps which 

generate less turbulence, fewer impact losses and less friction. Figure 4-17 shows the increased 

efficiency when using high efficiency pumps and impellers in coarse sand and gravel and 

medium fine sand substrates. 

92


� 

Report No: 10/J/1/06/1309/0996 

�� 

�� 

These dredge pumps are designed to allow them to be retro-fitted to existing dredgers and not 

just new builds. 

There is a wide range of dredge pipe diameters on the TSHDs of the world fleet. The largest 

dredge pipes currently in service (1400 mm diameter) are on the TSHDs “Vasco de Gama” and 

“VolvoxTerranova”, however it is reported that a currently unnamed dredger being built by 

Guangzhou Wenchong in China will have a 9000 mm diameter pipe. These large pipes can be 

contrasted with the USACE TSHD “Currituck” which has a pipe of only 254 mm diameter. 

Another development in dredge pipe technology is IHC’s patented TelePipe design which 

consists of an extendable, synthetic suction tube enclosed in a reinforced synthetic lattice system 

with special gliders that are operated by a remotely controlled winch. The system is still being 

developed by IHC but they report that feasibility studies are promising (IHC, 2009b). The goal is 

to make deeper dredging possible using existing TSHDs, without costly ship extensions or major 

alterations to the gantry arrangement. The system will replace the lower part of conventional 

suction pipes. A typical TelePipe measuring 30 metres extends to about 50 metres and IHC 

claims that the weight, strength and reliability will match the replaced pipe. 

93


� �� 

Report No: 10/J/1/06/1309/0996 

High efficiency pumps have been installed by Warman International on the TSHD “Brisbane” 

which is one of Austalia’s largest dredging vessels at 85-metres long and 5000 tonnes gross 

weight (Ferret, 2001). The pumps weigh 36 tonnes each and can pump directly into the ship’s 

hopper or directly ashore. They are built with specially designed frames and cover plates to 

withstand the high pressures generated by the pumps which can reach up to 1500 kPa. On 

conventional pumps the frame and coverplates are generally cast however in these high 

efficiency pumps they are fabricated to meet the higher pressure requirements. The vessel is 

used to dredge the Brisbane River estuary at depths of up to 25 metres, and the pumps have the 

capacity to pump at 2500 m 3 /s. Complete loading of the 2900 m 3 hopper can be completed in 25 

minutes. 

� �� 

The Dutch dredging company Pinpoint Dredging has constructed and successfully tested and 

operated the Punaise (Dutch for "thumbtack") system which is a remotely operated pumping 

system. The Punaise sits on the seabed, is watertight and contains the dredge pump, motor and 

ballast tanks (Figure 4-18). It is connected to the shore via an umbilical that consists of control 

connections, the power supply and the sediment discharge pipe (Van Rijn et al., 2005). 

�� 

Since it sits on the seabed it can remove bottom sediments without impacting navigation and 

without being affected by storms. 

The first stage of operation consists of positioning the Punaise in an appropriate location and 

sinking the housing by filling the ballast tanks. Fluidizers are activated, which allows the supports 

to settle into the seabed and dredging can begin. As sediment is dredged and pumped ashore a 

94


� 

Report No: 10/J/1/06/1309/0996 

pit is formed with the Punaise at the lowest point. It dredges by means of hydraulic erosion where 

sediment flows down the slopes of the pit and into the suction entrance. It is only suitable, 

therefore, for dredging thick layers of sediments that are non-cohesive and can flow down slope. 

The Punaise system is not new, and the first prototype was developed in 1990. It was used for 

two years to dredge 1.2 million m 3 of silt from Flushing Harbour in The Netherlands. It was sited 

in the centre of the turning basin and pumped the silt through a submerged pipeline directly to 

the Schelde estuary which ends in the North Sea (Williams and Visser, 1997). 

� �� 

A technical modification that allows an increased area of seabed to be dredged on each pass 

(and hence reduce dredging time) is to increase the width of the draghead and the use of this 

technique on the Japanese TSHD “Seiryu-maru" has been discussed in Chapter 4.1.4 above. A 

further modification necessitated by the use of the wide draghead was the hull layout of the 

“Seiryu-maru” that has an aft centre dredge pipe (Figure 4-19). 

�� 

An environmental benefit that may be afforded by an aft centre system is that it allows the 

dredging line to be shown by the ship’s wake. This may improve dredging accuracy and 

efficiency without the need for a Dredge Track Presentation System – if there are other visual 

reference points available. The presence of an aft centre pipe does, however, potentially 

compromise the volume of the hold – the 104m-long “Seriyu-maru” has a hopper capacity of only 

1700m 3 (although it also contains 1500m 3 of oil recovery tanks) (Yano et al., 2006). 

Another vessel with a central pipe is the US Army Corps of Engineers vessel TSHD “Wheeler”, 

built as long ago as 1983. Unlike the “Seriyu-maru” which only has a central pipe the “Wheeler” 

has a central 1000mm diameter dredge pipe in addition to two 660mm diameter side pipes. The 

vessel is solely a maintenance dredger and can dredge at depths of up to 29m for the side pipes 

but the central pipe can only reach a depth of 16.8m. When all three pipes are able to be 

deployed the vessel loads rapidly – the 6120m 3 hopper can be filled in as little as 11 minutes 

(USACE, 2006). 

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Similar to the UK dredging industry, two screening systems are commonly found in dredgers 

worldwide – these are the central screening tower or the revolving system. The central screening 

tower with midships loading channel is the simplest but must be designed to minimise 

interference with the loading and unloading process. The revolving system involves the 

installation of a rotatable screening tower to the side of the hopper. Because it can turn almost 

300º it can evenly spread material over the hopper thus improving the hopper fill ratio. Existing 

hopper dredgers can also be effectively retrofitted with screening hardware. 

�� 

During dredging a mixture of sediment is discharged into the hopper of the dredger. The intention 

is that the sediment settles in the hopper and the excess water flows overboard. Generally, 

however, a proportion of the incoming sediment does not settle in the hopper but instead flows 

overboard with the excess water. Depending on the particle size distribution (PSD) of the 

sediment, the hopper geometry and other process parameters this overflow loss can reach 

values of up to 30-40 % of the total volume of sediment pumped into the hopper. It is important to 

quantify the overflow loss for three reasons: 

� The loading time, and therefore the amount of time the environment is directly affected 

by dredging (as well as the economic cost price of the sediment dredged) increases due 

to the overflow losses. 

� In particular the finer fractions of the particle size distribution (PSD) are present in the 

overflow mixture, implying that sediment contained in the overflow will settle slowly and 

can create a turbid plume around the dredger. 

� For environmental modelling reasons it is important to quantify the quantity and the PSD 

of the overflow sediments since these are needed as inputs to the dispersion models. 

The function of the overflow system is to discharge the excess water and to control the water 

(and therefore sediment) level in the hopper. The excess water is released overboard through the 

overflow structure, which acts as a weir. In modern aggregate dredgers these structures are 

mostly adjustable in vertical position to regulate the overflow level in the hopper. A telescopic 

overflow structure in the cross section of a hopper is shown in Figure 4-20. 

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�� 

The operational conditions which have an influence on loading production and hence overflow 

losses are: 

� Discharge 

� Loading Concentration 

� Loading Time 

� Loading procedure 

� Water Temperature 

The discharge Q, together with the hopper area has the largest influence on the overflow losses. 

The ratio between the discharge and hopper area is called the overflow rate and can be regarded 

as an average vertical flow velocity in the hopper. The sediment concentration of the mixture 

discharged in the hopper has also an important influence on loading and losses. 

The sediment production sucked from the seabed is the product of discharge and concentration 

and a larger concentration means a larger suction and loading production. 

The settling velocity of the sediment particles decreases with sediment concentration - this is 

called hindered settling (Richardson & Zaki, 1954). The hindered settling effect will increase the 

cumulative overflow losses. The gradient of the loading production versus concentration will 

therefore decrease with concentration and Figure 4-21 shows the loading production as a 

function of the inflow concentration for three different grain sizes while inflow discharge is 

constant. In this example overflow losses for the coarsest sediment type (250 micron) are low 

and loading production increases linearly with the inflow concentration. For the finer sands 

hindered settling plays a role in this example. This effect increases with inflow concentration and 

loading production decreases relatively. The difference between the straight line and the curved 

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lines is the overflow loss. For example, with an inflow concentration of 0.3 and particle size of 

150 microns loading production is 6 ton/s. The suction production is 8 ton/s. The difference is the 

overflow loss (2 ton/s). The overflow losses will increase normally with the time elapsed since the 

start of loading. This is mainly caused by the increasing sediment concentration above the settled 

sediment bed in the hopper. 

�� 

The water temperature influences the viscosity of water. For the finer fractions of the particles 

size distribution the flow around the particles is in the laminar of transitional regime where 

viscosity of the fluid influences settling velocity. This is shown in Figure 4-22 where the settling 

velocity for two different values (10 and 40 o C) of water temperature is shown. For coarser 

sediment the flow pattern around the particles becomes turbulent and so viscosity will play not an 

important role anymore. 

�� 

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� �� 

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The rationale behind anti-turbidity systems is to prevent air entrainment in the overflow mixture. If 

air is entrained then bubbles are dragged down with the flow through the overflow pipe and are 

released below the ships hull. The bubbles then rise to the water surface and create an upwards 

flow (air-lift effect) from the overflow outflow location towards the water surface. Sediment 

particles are transported to the surface with this flow and form a plume. Depending on the 

location of the overflow and the flow along the ships hull the air-water flow can even guide the 

sediment particles towards the propeller wake behind the ship. The high turbulence level at this 

position will effectively mix the sediment through the water column. Hence the presence of air 

bubbles will stimulate the forming of a passive plume, especially at the water surface, where very 

limited suspended sediment concentrations are clearly visible. Due to the very low settling 

velocity of the particles in this passive plume, measured concentrations will be larger around the 

dredger compared with situations where air entrainment is prohibited and a dynamic plume is 

present. In that case the trajectory of the plume will be directed straight towards the seabed and 

often no turbidity at the water surface will be visible. 

Designs for overflow svstems that will reduce the surface plume have been proposed for over 30 

years. Ofuji and Ishimatsu (1976) reported on an anti-turbidity overflow system (Figure 4-23) that 

had proven effective in reducing the surface turbidity generated by water overflowing from a 

TSHD. Their system could be incorporated in the existing dredgers through simple modifications 

of existing overflow systems and suppressed the generation of air bubbles in the overflow water 

by inserting an inclined baffle plate in the overflow chute. The chute was modified so the overflow 

water was retained long enough for any contained air bubbles to rise and vanish and the 

overflow water was subsequently discharged through a submerged outlet. 

�� 

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The combination of overflow ports on the bottom of the hull and reduced air bubbles in the water 

leads to the overflow descending rapidly to the seabed with a minimum amount of dispersion in 

the water column. Herbich and Brahme (1991) reported that the Ishikawajima-Harima Heavy 

Industries Company Ltd., Japan, further developed the system and incorporated it in three 

Japanese TSHDs with capacities ranging from 2,000 to 3,975 m 3 . Herbich and Brahme (1991) 

also report that tests were carried out on the dredger “Kairyu Maru” and showed considerable 

reductions in the turbidity at the surface and at 1 m below the surface, both by the side and aft of 

the ship. These data are shown in Table 4-7 below: 

� �� 

Average 

At surface 1m below At surface 1m below 

Concentration of 

Suspended 

Solids (ppm) 

surface 

surface 

At side of ship 627 272 6.0 8.2 

Aft of the ship 119 110 6.5 8.9 

�� 

�� 

No data are available on sediment concentrations at the bottom of the water column by the side 

and behind the dredger. 

� �� 

In a standard overflow system a large volume of air is mixed with the water and sediment due to 

the high fall height and this causes turbulence and increased spreading of the dredge plume 

(Minerals Management Service, 2004). Jan de Nul (2003) has developed a low turbidity valve 

which is an adjustable valve in the overflow funnel which chokes the flow such that no air is taken 

down with the suspension leaving the hopper (Figure 4-24). 

�� 

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Minerals Management Service (2004) notes that data supplied by the manufacturer indicate that 

the size of the plume and the total amount of sediment in the plume are reduced through using 

the low turbidity valve (Figure 4-27). From 1995-2000 a monitoring project for the maintenance 

dredging operations in Belgian coastal harbours and waterways was carried out under the name 

MOBAG 2000 (Van Parys et al., 2001). Turbidity was measured during dredging with a TSHD 

using an environmental valve in the overflow and a standard overflow without environmental 

valve. It was found that dredging with the standard method gave the highest turbidities and the 

natural background situation returned after 30-45 minutes. The environmental valve reduced the 

turbidity to 40% of the standard method and background situation returned after 20-30 minutes. 

The overflow mixture leaves the hopper through the overflow arrangement, which is normally a 

telescopic pipe. Depending on: 

� The draught of the vessel, 

� The mixture level in the hopper, 

� The overflow discharge, 

� The diameter of the overflow pipe and overflow crest, and 

� The relative opening of the green valve, 

The following two situations can occur as shown in Figure 4-25. In the left panel of the figure the 

water level in the overflow pipe is lover than in the hopper. The mixture flows over the crest of the 

overflow weir and plunges into the overflow pipe. Hence the mixture enters the pipe as a 

plunging jet. It is known that above a certain critical speed a plunging jet entrains air at the 

interface between the impinging jet and the stagnant water (Ervine (1976); Bins (1988)). The 

entrained air bubbles will be transported downward and leave the vessel with the sediment 

mixture at the bottom of the ship. The only method to avoid entrainment of air is taking care that 

the plunging jet situation does not occur. This can be achieved by increasing the water level in 

the overflow pipe by increasing the hydraulic resistance of the overflow pipe. In that case the 

situation as shown in the right panel of Figure 4-25 will occur. The water level must be high 

enough relative to the overflow level to avoid air entrainment, but should not be too high 

otherwise the maximum draught of the vessel will be exceeded. The mixture level should stay 

between these two limits. This optimal level will depend on the influences mentioned above, 

therefore the (extra) hydraulic resistance due to the green valve must be variable. The variable 

resistance is mostly created by placement of a butterfly valve in the overflow pipe. A control unit 

must be present to adjust the valve position to keep the level between the limits without 

exceeding maximum draught. 

� 

�� 

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� �� 

A system, sometimes called the “Green Pipe”, has been developed by both IHC, Holland and 

Krupp Foerdertechnik of Germany to reduce plumes and increase dredging efficiency. This 

system recirculates the overflow waters back to the draghead in a closed system and reduces 

the surface turbidity plumes. Instead of allowing the overflow (composed of fine organics and 

transport water) to go directly back into the water, the new system pumps the overflow along the 

dragarm to the draghead to assist in suction operations (McLellan and Hopman, 2000). Reusing 

overflow water in this manner maintains a closed system so that only a minimal turbidity plume is 

produced at the sea surface and potential contaminated sediment is not discharged. This is 

sometimes called the “Green Pipe” system. 

Report No: 10/J/1/06/1309/0996 

� 

� 

�� 

This technique has three main benefits: (a) minimization of sediment/turbidity plumes at the sea 

surface by avoiding transport water overflow and a possible related water pollution problem 

(Figure 4-27); (b) increased material flow in relation to transport water, which in turn increases 

dredge efficiency by reducing loading time, and (c) decreased pressure drop inside the 

draghead, which reduces dredge pulling force. This reduces energy needed for propulsion and 

lowers fuel consumption. Some manufacturers claim a 20% increase in efficiency for dredging 

silty sand with the recirculating system (Francingues et al., 2000). 

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�� 

Minerals Management Service (2004) states that sediment balance considerations suggest that 

the same amount of sediment would be “overflowed” with the “Green Pipe” - the only difference 

being its release close to the bed. Minerals Management Service (2004) also states that there is 

a concern that this could promote the development of a turbidity current near the bed (i.e., due to 

the high and concentrated sediment loading at the drag head with this approach) and this if such 

a turbidity current were to develop at the bed, the sedimentation footprint may extend much 

further from the dredge site than normally expected. 

�� 

Discharging sediment from the dredger can have potential environmental impacts – particularly 

where the sediment is contaminated with pollutants or fine grained material. Indeed many 

contaminated cargos are simply dumped, either at licensed dumping areas at sea, or in landfill 

sites. 

One answer to the problem of discharging contaminated sediment is the Mobile Soil Washing 

Plant (MSWP) which was developed by Boskalis Dolman and has been used on dredging 

projects on the Miami River (Taylor et al., 2006). The MSWP is “portable” – prior to its use in 

Miami it was in use in south-eastern England – although it did take three weeks to disassemble, 

transport and reassemble at site. The MSWP is a high capacity plant, processing up to 150 

tonnes of sediment per hour and combines coarse fraction separation via a stationary grizzly 

screen, rotating sieve drums, vibrating shaker screens and hydrocyclones, and mechanical dewatering 

of the fine fraction through a pre-thickener tank and a series of belt filter presses. 

Taylor et al. (2006) describe the operation of the MSWP - coarse material with a particle size of 

from 1 m to 3 cm pass the grizzly screen and are placed in a rotating wash and sieve drum. The 

finer sediment is placed through vibrating shaker screens before the sand separation process 

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begins using hydrocyclones and countercurrent washing. The de-sanded sediment is then put 

through a mechanical de-watering process where flow and density measurements are conducted 

and automated polymer dosing occurs. This results in the pre-conditioning of fines of 20 percent 

dry solids in a pre-thickener tank. The mechanical de-watering table uses belt filter presses which 

reduces the remaining dry solids content by about 50 percent. More than 50 percent of the 

dredged sediment was processed clean (coarse) fractions which could be beneficially used in the 

community at little to no cost to the project; and around one quarter was dewatered filtered cake 

which was transported by truck for ultimate disposal at one of three licenced landfills. 

�� 

�� 

� �� 

While it is not possible to state definitively, it is likely that navigation on the majority (if not all of 

the world’s TSHDs) utilizes Differential GPS. 

� �� 

In addition the International Maritime Organization (IMO) requires all vessels over 299 Gross 

Tonnes to be fitted with an Automatic Identification System (AIS). Only one TSHD has a weight 

less than this – the Spanish vessel “Ria de Navia” (reported 298 Gross Tonnes (Clarkson 

Research Services Ltd, 2009)). The AIS transponder transmits the vessel name, position, speed 

and course is intended to help ships avoid collisions, as well as assisting port authorities to better 

control sea traffic. 

A refinement of navigation technology and electronic charting is the development of draghead 

steering for dredging management. This has a number of advantages both for production and for 

environmental efficiency. The ability to accurately steer the draghead means that the offtake of 

resource within a dredging area can be maximised. Ridges of sediment between dredging lanes 

that were previously left undredged can be dredges, thus helping to minimise the overall area of 

seabed dredged. 

IHC has developed a system that can be used on TSHDs called the Dredge Track Presentation 

System (DTPS) (Mallee, 2000). This is a software package, written in C++ and running on a 

standard Windows PC, that allows the dredge operator to see in real-time the position of the 

draghead and an updated bathymetry (Figure 4-28). The DTPS has been designed to operate as 

either a stand-alone system or be integrated with other electronic systems on a dredger, such as 

the ECDIS, any dredge pipe position monitoring system and dynamic positioning and dynamic 

tracking (DP/DT) systems. 

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�� 

The DTPS was developed by IHC Systems in close co-operation with DEME and the system 

underwent extensive testing on DEME vessels. The system has been in use since 1999 when it 

was trialled on the DEME vessel “Lange Wapper” and used for dredging in the Western Schelde 

(IHC, 2001). 

� �� 

A number of recent dredging operations include almost real time management of dredging 

efficiency by using multi-beam sonar to provide instantaneous data to the bridge of the TSHD 

and interfacing these data with the navigation systems. One example is a trenching and 

backfilling operation for a pipeline running from Yung-An to Tung-Hsiao in Taiwan and reported 

by Van Melkebeek (2002). During this campaign the TSHD “Gerardus Mercator” installed a multibeam 

echosounding device in the moon pool of the dredger. This provided on-line, day-to-day, 

survey information for the project team and client representatives on board the vessel. Similar 

multi-beam survey equipment was installed on the TSHD “Oranje” for use in a dredging 

campaign for the Balgzand-Bacton pipeline. This allowed the vessel to survey the efficiency of its 

operations in dredging 381 sand ridges in water depths of up to 50 m (Boskalis Offshore bv, 

2009). 

In 2003 the TSHD “Vasco da Gama” was employed by Husky Energy, a Canadian oil company, 

to excavate a Glory Hole approximately 200 nautical miles south-east of Newfoundland on the 

Grand Banks in the Atlantic Ocean. A Glory Hole is a depression, up to 9 m below seabed level, 

which protects wellheads against iceberg scouring. During the dredging process, progress was 

monitored in real-time on board the dredger by means of the dredging control systems. The 

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actual draghead depth and the target depth per layer were compared online and any difference 

displayed. The position of the draghead was visualised on screen on a background of 

bathymetric data by means of a plan view with a differential colour chart showing the amount still 

to be dredged together with a longitudinal and cross profile of the Glory Hole, marking seabed 

level and target level. Progress was checked using a multibeam system installed in the moon 

pool and the system continuously updated the dredge levels achieved (Jan de Nul, 2009). 

In Belgium, the Maritime Schelde Department has standardized a management system that 

records real-time data on dredging operations including: location, depth of cut, sediment mixture 

concentration, and several other parameters that help determine the performance of the 

operation (Francingues et al., 2000). 

� �� 

A further refinement of on-board real-time management is a system that extends to enabling 

shore-based offices to access real-time data from a dredging vessel. In 2005, IHC Systems was 

awarded a contract by the Sri Lanka Port Authority to develop, build and install such a system on 

the TSHD “Hansakawa” (IHC, 2006). The system consisted of: 

� Real Time Kinematic (RTK) DGPS positioning system 

� Suction Tube Position Monitor (STPM) 

� Dredge Track Presentation System (DTPS) for the vessel and a second for an onshore 

office 

� Interfaces for existing instrumentation systems. 

The RTK system allowed the position of the ship to be determined with an accuracy of a few 

centimetres and the set up included a dual RTK DGPS receiver on board the ship and an RTK 

DGPS base to serve as a shore reference station. The shore-based DTPS system interfaced with 

the “Hansakawa” using a full-duplex radio link. An interface was created between the DTPS and 

the dredger permitting online presentation of production both on board and at the SLPA offices 

and the system was fully operational by July 2006 (IHC, 2006). 

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� �� 

� �� 

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Chapter 3.2.5 described how the UK aggregates industry uses an Electronic Monitoring System 

(EMS) to monitoring the activities of dredgers on English licence areas. The use of similar 

systems on board aggregate dredging vessels is now common practice among vessels from 

other European countries including the Netherlands (Figure 4-29), Denmark, Germany, Spain 

and Belgium. 

�� 

�� 

�� 

�� 

�� 

�� 

Report: ��/��108 

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� �� 

Minerals Management Service (2004) report that the US Army Corps of Engineers uses an 

automated monitoring system referred to as “Silent Inspector” (SI). SI consists of a Dredge 

Specific System (DSS), a Ship Server, and a Shore Server. The DSS collects and displays 

standard information on dredging operations that is then transmitted to the Ship Server. Figure 

4-31 shows the DSS dredge display which gives real time sensor data including production rate, 

mixture velocity and density, pumping effort etc. 

Report No: 10/J/1/06/1309/0996 

�� 

�� 

The Ship Server acts as the dredge based data archive and report creation centre as well as 

performs automated reviews of the data. The Shore Server is a larger system operated and 

maintained by the US Army Corps of Engineers. “Silent Inspector” provides information on 

dredge location history, quantity history, and status of a given project. It monitors all aspects of 

operations from contract compliance to assurance that the operation is being performed in an 

environmentally safe manner (Rosati, 1999). 

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�� 

� �� 

Spatial mitigation techniques are common in dredging projects worldwide, and work on similar 

principles to spatial mitigation in English areas – i.e. potential impacts are mitigated by reducing 

the total area of seabed dredged. Reducing the area dredged potentially mitigates the effects of 

the dredging in a number of ways including: 

Report No: 10/J/1/06/1309/0996 

� reducing the area of direct removal of biomass 

� allows the majority of an area to be undredged, or undergoing recolonisation, at any 

particular time 

� reduces the area over which sediments are returned to the seabed 

� targeting sediments with low silt and clay reduces plumes 

� reduces conflict with other users of an area 

It is beyond the scope of this report to discuss all dredging projects worldwide where spatial 

mitigation has been used, however four recent case studies where spatial mitigation techniques 

were successfully used will be described. 

� � 

� �� 

The Port of Melbourne is Australia’s largest container and general cargo port, and aims to 

expand its operations. The port’s expansion plan depends upon deepening the entrance to Port 

Phillip Bay, which also acts as a social, cultural and recreational asset (Bradford and Siebinga, 

2009). Port Phillip Bay includes two Marine National Parks, a Ramsar wetland and is a habitat for 

multiple fish species, little penguins, whales, dolphins, seals, various coldwater coral species and 

natural seagrass habitats and is used extensively for swimming, diving and boating. 

Bradford and Siebinga (2009) report that the main environmental impacts associated with 

dredging the 400,000 m 3 required related to turbidity and contaminated material. The concern 

was that dredging induced plumes within the Bay could potentially harm benthic organisms, 

seagrasses and fauna that depend on these habitats for food and protection. Trial dredging was 

conducted using the TSHD “Queen of the Netherlands” and showed that although the draghead 

was dredging efficiently, large amounts of sediment and rubble were being left on the seabed 

and were subsequently remobilized by currents and waves and deposited on flora and fauna 

living on a nearby deep canyon reef. 

To mitigate against these impacts a number of measures were put in place to minimise the 

possibility of sediments and rubble being transported into the canyon and impacting the marine 

life. These included spatial mitigations including: 

� When dredging towards the canyon the draghead had to be lifted so that no dredging 

occurred within 5 metres of the canyon edge. 

� When dredging the canyon edge itself only dredging towards the plateau was allowed. 

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� When dredging the area closest closest to the Port Phillip Heads Marine National Park, 

a dredge zone was implemented so that a ridge was left in place until the remaining area 

had been dredged to design level. This ridge was then removed separately, after 

additional clean up of the area behind the ridge. 

Post-dredging video surveys demonstrated that the spatial mitigation measures put in place were 

effective in reducing the amounts of sediment and rubble deposited on the deep water reef 

(Bradford and Siebinga, 2009). 

� �� 

Sand dredging was proposed for the Gulf of Mexico and the potential impact of operations on 

sea turtles was a significant concern. The US National Marine Fisheries Service published a 

Regional Biological Opinion for dredging in the Gulf of Mexico (NMFS, 2004) which designated a 

number of Hardground Buffer Zones. This stipulated that all dredging in potential aggregate 

areas should be designed to ensure that dredging did not occur within a minimum of 400 feet 

from any significant hardground areas or bottom structures that served as attractants to sea 

turtles for foraging or shelter (NMFS, 2004). Minerals Management Service (2004) reports that 

for the purposes of this spatial mitigation strategy only, significant hardgrounds in dredging areas 

were defined as areas that over a horizontal distance of 150 feet had an average elevation above 

the sand of 1.5 feet or greater, and had algae growing on them. It also required that pre-dredge 

seabed surveys were carried out at a sufficient scale to allow the hardgrounds to be mapped. 

� �� 

The island of Vilufushi in the Maldives was partially destroyed by the tsunami of 2004, however a 

decision was taken by the government of the Maldives to reconstruct and increase the size of the 

island. This would be done through dredging coral sand from an area of the northern reef edge of 

Vilufushi and pumping it directly into the area to be filled. This involved dredging and reclamation 

of 1,000,000 m 3 of coral sand from the reef to increase the surface and raise the height of the 

island (Bosschieter, 2007). 

The very rich biodiversity required protecting through monitoring and spatial mitigation 

procedures. To ensure that adverse effects were minimized environmental monitoring measured 

a range of parameters including turbidity, suspended solids concentrations (SSC) and dissolved 

oxygen levels in the water at 8 coral reef locations; sedimentation rates at two seagrass locations 

and potential erosion. 

The spatial mitigation involved the pre-dredging construction of bunds around the active dredging 

area, isolating it from the ocean. The dredge plume was therefore confined only to a minimised 

spatial area. The active dredge zone was subdivided into four compartments and the plume 

waters were made to flow through the four compartments before being discharged to the ocean. 

This allowed the suspended sediment to settle as much as possible, thus mitigating the effect of 

releasing the plume into the ocean. 

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� �� 

The previous three case studies have described spatial mitigation through zoning (similar to the 

English practice) and bunding (where physical barriers are built). A further method of spatially 

mitigating the impact of overflow on the environment is to use a temporary physical barrier within 

the water column e.g. the use of silt curtains, silt screens and bubble curtains. Silt curtains and 

silt screens are flexible barriers that hang down from the water surface and both systems use a 

series of floats on the surface and a ballast chain or anchors along the bottom. Their use is 

generally limited to areas where maximum wave height is less than 1.5m and current speeds are 

less than 0.5 ms -1 . 

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Silt curtains are made of impervious materials, such as coated nylon, while screens are made 

from synthetic geotextile fabrics, which allow water to flow through, but retain a large fraction of 

the suspended solids inside the screened area (Averett et al., 1990) (Figure 4-32). Both systems 

can be kilometres in length and can be used to screen environmentally sensitive areas from the 

effects of plumes (USEPA, 1994). 

�� 

�� 

Francingues and Palermo (2005) suggest that at depths greater than approximately 4-5 m loads 

and pressures on curtains and mooring systems become excessive and can result in failure of 

standard construction materials. Because these barriers contain the sediment plume, they may 

temporarily increase sediment concentrations within the barrier compared with the concentrations 

that would occur if the barrier was absent (USEPA, 2005; Francingues and Thompson, 2006). 

Silt curtains have been used in dredging projects worldwide to protect the environment, with 

varying degrees of success. For example, silt curtains were ineffective in limiting sediment plume 

migration during dredging operations at New Bedford Harbour, USA, primarily because of tidal 

fluctuations and wind (Averett et al. 1990). Conversely a silt curtain was found to reduce 

suspended solids from approximately 400 mg/l (inside) to 5 mg/l (outside) during dredging in 

Halifax Harbour, Canada (USEPA 1994). Sydney Ports Corporation, Australia, has planned the 

construction and operation of a new container terminal, the Port Botany Expansion, where the 

dredging will take place both within, and outside, a range of silt curtains (Sydney Ports, 2009). 

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� �� 

Temporal mitigation techniques have been used in dredging projects worldwide, and are based 

on similar principles to temporal mitigation in English areas – i.e. potential impacts are mitigated 

by restricting dredging or minimising sediment entering the water column during specific 

seasons, time frames or tidal states. Three case studies describing temporal mitigation measures 

(often described as Environmental Windows) will be described below. 

� �� 

The Øresund Fixed Link is a bridge linking Denmark and Sweden, and was one of the largest 

infrastructure projects built in Europe and involved dredging in excess of 7 million m 3 of flint, 

limestone and clay till. The water in the Øresund has a low natural turbidity, resulting in good 

underwater visibility and allows species such as eelgrass to flourish (Jensen, 2006). Introduction 

of turbid plumes to the water column would therefore have a potentially major environmental 

impact. 

Report No: 10/J/1/06/1309/0996 

The overspill sediment during the project was limited to a maximum of 5% of the dredged 

quantity. In addition there were additional temporal limitations on the daily and weekly tonnage 

allowed as overspill in defined environmental areas during prescribed seasonal windows which 

were linked to the eelgrass growing season (March to November). 

� �� 

The idea of dredging during Environmental Windows was introduced in the USA in the National 

Environmental Policy Act in 1969 (Burt, 2002; Randall, 2006). This was reinforced by a US 

National Academy of Sciences (NAS) workshop in 2001, which resulted in a guidance document 

- “A Process for Setting, Managing and Monitoring Environmental Windows for Dredging 

Projects” (NAS, 2001). The environmental window is a period set aside when no dredging is 

permitted to protect biological resources and habitats from the potential harmful effects of the 

dredging. 

The environmental window concept has been widely applied to a number of dredging projects in 

the United States, with about 90% of civil and maintenance dredging confined to specific times of 

the year (Burt and Hayes, 2005). Randall (2006) reports that physical disturbance to habitat and 

nesting is the justification for more than 75% of the environmental windows in the US. Reine et 

al. (1998) suggested that potential detrimental impact to either individual or groups of sport and 

anadromous fishes and the protection of threatened and/or endangered species (e.g. sea turtles, 

marine mammals) were the justification for the majority of environmental windows affecting the 

USACE. 

Randall (2006) also indicated that the use of environmental windows increases the costs of 

dredging and since the majority of environmental windows constrain dredging operations during 

spring and summer months (March-September) to avoid potential conflicts with biological 

activities such as migration, spawning, and nesting, often restrict dredging to winter months when 

weather conditions are most hazardous for dredging operations. 

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Report No: 10/J/1/06/1309/0996 

Concerns were raised by the USACE (Reine at al., 1998) about the use of environmental 

windows in the US suggesting that the technical justification for a window was often anecdotal. In 

addition, in some districts, environmental windows are in place during all seasons making it 

difficult to fulfil dredging requirements. An example is given in Figure 4-33 which depicts a 

“typical” profile taken from a dredging project file in the New England District. In this example the 

dredging operation could not be completed within the remaining unrestricted period, 

necessitating an “exemption” during September through mid-November and the latter half of 

January (Reine et al., 1998). 

�� 

�� 

Burt (2002) concludes therefore that the severe concept of environmental windows as applied in 

the United States would unreasonably restrict dredging operations if applied to Europe, where 

the majority of dredging operations take place all year round. 

113


� �� 

� �� 

One means of preventing the sediment plume associated with overflow is to prevent any overflow 

water from the dredging vessel entering the environment. One way of doing this is to contain all 

the dredged sand-water mixture within a vessel’s hopper. An example of this in practice is a 

dredging programme that took place at the Ketelmeer, in the Netherlands, in 2005 (Van der 

Linde et al., 2005). During this project overflowing was prohibited, therefore all the water-sand 

mixtures dredged were discharged into hopper barges with a capacity of approximately 800 m 3 . 

To ensure that no overflow occurred the barges were actually underfilled – containing 

approximately 600 m 3 (150 m 3 sand and 450 m 3 water) – before being transported to the 

discharging site in the mouth of the River Ijssel where the mixtures were pumped into a settling 

basin (Van der Linde et al., 2005). A similar method of preventing overflow from the TSHD 

entering the environment is to decant the overflow water to barges alongside the dredging 

vessel. These overflow waters can then be conducted to settling ponds and floculant added to 

remove the fine grained sediment. 

� �� 

The Øresund Fixed Link is a road crossing between Denmark and Sweden, and the 

environmental concerns associated with its construction focused on the effects of blocking water 

flow between the Baltic and the North Sea, and the effect of the dredging and landfill activities on 

the local environment (Jensen, 2006). The EIA process indicated that the largest environmental 

impacts would be associated with dispersal of overspill sediment and a feedback monitoring 

programme was implemented as part of the environmental management system. Feedback 

monitoring is a form of surveillance monitoring where a few fast reacting environmental variables 

are forecast by modelling and then monitored continuously while the dredging is occurring 

(Netzband and Adnitt, 2009). The principles of feedback monitoring are illustrated in Figure 4-34. 

Report No: 10/J/1/06/1309/0996 

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NO 

NO 

New work plan from the contractor 

including a spill scenario 

Environmental impact assessment of the 

work plan based on numerical modelling of 

the spill scenario 

Are the operational criteria fulfilled? 

The dredging work starts 

Monitoring of selected variables 

Are the operational criteria exceeded? 

Intensified monitoring 

Numerical modelling 

Are the authorities criteria exceeded? 

�� 

The purpose of the feedback monitoring is to ensure that possible exceedence of environmental 

criteria can be forecast in such good time that dredging plans can be altered accordingly and 

costly down-time avoided (Netzband and Adnitt, 2009). 

In the Øresund example, integrated into the feedback monitoring programme were the outputs of 

computer models which forecasted sediment dispersion, sedimentation, impact on water quality 

and impact on key habitats (such as eelgrass beds). Jensen (2006) states that for feedback 

monitoring it is essential that the variables measured meet certain demands: 

115 

YES 

NO 

YES


Report No: 10/J/1/06/1309/0996 

� Must have an unambiguous, easily measurable relationship to the organisms 

representing the ecosystem concerned; 

� Measurement result must be available in a short timescale (few days maximum); 

� Background data for determination of statistically relaiable limit values 

� Impact of various conditions should be calculable in advance. 

Therefore for monitoring of eelgrass in the Øresund case, shoot density, leaf and root biomass 

and carbohydrates in the rhizomes were the monitored variables and sediment concentration and 

sedimentation rates and total were measured weekly to give early warning data and validation for 

the computer model (Jensen, 2006). The use of indicators is a growing element of marine 

management and regulation (Gubbay, 2004). Bayer et al. (2008) report that their use is often 

masked by a range of synonymous terms e.g. standards, targets, thresholds, criteria—against 

which specific variables (i.e. indicators) are assessed . 

�� 

A complete review of how dredging is regulated worldwide is beyond the scope of this report, 

however a comprehensive summary can be found in Sutton and Boyd (2009). Instead this report 

will examine four case studies, looking at transnational regulation (e.g. across Europe) and 

national regulations in the Netherlands, the USA and Australia. 

� �� 

In the case of environmental law, countries of the European Union have agreed to transfer 

legislative and executive competancies to a transnational level. EU laws have three levels: 

� Framework Directives (Directive is equivalent to a law in national legislation) define a 

general approach which sets a number of boundary conditions and constraints and have 

to be implemented by each member state in accordance with its specific circumstances. 

� Directives are equivalent to laws and are binding on the member states, except for the 

fact that they first have to be “transposed” into national law. 

� Regulations are legal decisions taken at EU level that are binding as such for the member 

states and do not need transposition. 

(Mink et al., 2006) 

In addition to EU law there are also International Conventions and Treaties which may influence 

regulation and take priority of EU laws e.g. the London Convention and the Oslo-Paris (OSPAR) 

Convention for the Atlantic and the North Sea. The London Convention is a global convention 

and member countries agree to introduce legislation in their own countries to implement the 

Convention. Its aim is to protect the marine environment and although it is limited to nonterritorial 

waters many signatories choose to apply it to their territorial waters, including estuaries 

(Burt and Hayes, 2005). It defines the nature, temporal and spatial scales and duration of 

expected impacts based on reasonably conservative assumptions. OSPAR functions in a similar 

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Report No: 10/J/1/06/1309/0996 

way to the London Convention but only applies to countries bordering the North Sea and North 

East Atlantic. 

EU law does not deal specifically with dredged material, however a number of Directives have an 

impact on dredging. For maintenance and capital dredging the Waste Framework Directive is 

highly important since the dredged sediment may potentially be defined as waste. “Waste” is 

defined as “any substance or object which the holder discards or intends to discard” and the EC 

argues that dredged sediment is “waste” since the holder needs to dispose of it in some way. 

This obviously has less impact on the marine aggregate dredging industry where the dredged 

sediment is a commercially valuable resource. The EuDA Environment Committee concludes 

that for dredging in marine waters current EU law may only be relevant, and applicable to marine 

aggregate dredging where there is a requirement to dispose of dredged material on land when 

the Landfill Directive may come into effect. 

An EU Directive that may eventually impact all dredging operations, including marine aggregate 

dredging, is the Water Framework Directive. Its goal is to gradually improve the quality of 

European waters to some standard which may be called “good”. This is a long-term goal, and it is 

recognised that water quality varies considerably over time and as a result of many factors. How 

severely the Directive will impact dredging will depend on the consideration of this temporal 

variability – for instance will short term increases in suspended sediment due to dredging matter 

if average values are within defined limits? 

The Habitats and Birds Directives aim to protect biodiversity and rare biotopes and species and 

their implementation process has led to the establishment of Natura 2000 sites across Europe. 

These consist of Special Areas of Conservation (SACs) under the Habitats Directive and Special 

Protection Areas (SPAs) under the Birds Directive. These may impact marine aggregate 

dredging since dredging areas may be located close to or in these Natura 2000 sites which 

imposes restrictions on operation or obstacles to permitting of new Licences. 

Finally the EC-published Thematic Strategy on the Protection and Conservation of the Marine 

Environment (see http://ec.europa.eu/environment/water/marine.htm) may have implications for 

dredging in the future since it aims to achieve “good environmental status” of European marine 

waters by 2021 and also claims competence to regulate the status of the seabed and its subsoils. 

Mink et al. (2006) suggest that by analogy with the Water Framework Directive “good 

environmental status” is likely to be defined on the basis of parameters including physical and 

chemical conditions, biological and ecological processes and physiographic and geographic 

factors. 

� �� 

Marine aggregate dredging in the Netherlands is regulated according to the Sediment Extraction 

Law, amended in 1997, and Licences are granted by Ministry of Transport, Public Works, and 

Water Management; the Directorate�General of Public Works and Water Management, and the 

North Sea Directorate. Sutton and Boyd (2009) indicate that from 2004 a distinction was made by 

the Dutch Government between licences that allowed extraction of < 10 million m 3 per licence 

and those that allowed extraction of > 10 million m 3 . For the smaller-scale extractions maximum 

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Report No: 10/J/1/06/1309/0996 

extraction depth was set at 2 m while larger�scale extractions allowed an extraction depth of 

more than 2 m if this was a preferred option in an EIA. 

Not all dredging applications in require an EIA – an EIA only has to be produced when an 

extraction exceeds an area of 500 ha (EEZ) or 100 ha (territorial sea) and/or exceeds a volume 

of 10 million m 3 . In addition the landward limit for extraction of marine sediments is the 

established NAP 20 m depth contour, defined by the NAP (Dutch Ordnance Level ~ Mean Sea 

Level). Seaward of this contour, extraction is allowed in principle (Sutton and Boyd, 2009). 

� �� 

The major US statutes and laws that relate to dredging activities are mainly concerned with 

disposal of dredged material within United States waters, rather than dredging for marine 

aggregates (Randall, 2006) and regulation of disposals is a shared responsibility between the US 

Army Corps of Engineers (USACE) and the Environmental Protection Agency. The Outer 

Continental Shelf Act 1983 (amended in 1994), however, allows for the leasing of areas of the 

shelf for sand and gravel extraction. The agencies responsible for these activities would be the 

USACE and the Minerals Management Service. Any extraction of marine sand and gravel would 

need to take into account the Sustainable Fisheries Act (1996), which requires the National 

Marine Fisheries Service to define essential habitats for various commercial species and all 

federal agencies must consult the National Marine Fisheries Service on any action that may 

adversely affect essential fish habitats. 

Dredging of offshore sediment for beach nourishment is relatively common in the United States 

however the Minerals Management Service has decided not to proceed with the designation and 

leasing of offshore areas for marine aggregate mining. Thus the US currently has only two 

commercial marine aggregate dredging operations, neither of which is offshore. Since 1985 there 

has been a licence to dredge aggregates from the entrance to New York Harbour, producing 

between 1.2 and 1.9 million m 3 of sand and gravel per year. The company responsible has also 

attempted to gain a licence to dredge a large sand and gravel deposit located in Federal waters 

offshore of northern New Jersey, however public opposition to the project has prevented the 

Minerals Management Service from allowing this. The only other aggregates mining is on Lake 

Erie and produces about 306,000 m 3 of aggregates annually, using a TSHD. These small 

volumes of aggregate compare with a total volume dredged in the US of between 200 and 250 

million m 3 a year. 

A further legislative influence on dredging in the US is the “Jones Act” (the 1920 Merchant Marine 

Act). Dredgers working in the US must comply with the “Jones Act” which is a cabotage law 

(cabotage is the transport of goods or passengers between two points in the same country). The 

“Jones Act” requires that all goods transported by water between US ports be carried in USflagged 

ships, constructed in the US, owned by US citizens, and at least 75% of the crew must 

be US citizens. Operating under the “Jones Act” increases the costs associated with shipping the 

product carried. 

Every dredger operating within the US is therefore subject to the Jones Act and this has led to 

the US being a closed market. It is not subject to foreign competition, and approximately 300 

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dredging companies operate with an outdated fleet of mainly small vessels (the largest TSHD is 

the USACE’s vessel "Wheeler", which has a capacity of 8000 m 3 ). 

� �� 

Unlike the English, Dutch and (to a lesser extent) American examples, the Australian marine 

aggregate dredging industry is in its infancy. However the sector is likely to expand in the near 

future due to diminishing land-based aggregate resources close to the major conurbations 

(Littleboy and Boughen, 2007; Johns, 2008). Because of this anticipated expansion of the sector 

CSIRO has undertaken a review of potential operations as part of National Research Flagships 

Wealth From the Oceans program (Johns, 2008). 

A number of different acts, guidelines and policies comprise Australian legislation affecting 

mining of offshore minerals: 

� Seas and Submerged Lands Act 1973, as amended by the Maritime Legislation 

Amendment Act 1994 

� Commonwealth Offshore Minerals Act 1994 

� State/Northern Territory Offshore Minerals Acts 1998, 1999, 2000, and 2003 

� Native Title Act 1993 

� Environmental Protection and Biodiversity Conservation Act 1999 

� Guidelines on the Application of the Environment Protection and Biodiversity Conservation 

Act 1999 to Interactions Between Offshore Seismic Operations and Larger Cetaceans 

October 2001 

The Seas and Submerged Lands Act 1973 (SSLA) provides legislation which relates to 

Australia’s sovereignty over defined waters of the sea, airspace over, and the seabed and subsoil 

beneath those waters, and to sovereign rights relating to the continental shelf and the recovery of 

minerals (other than petroleum) from the above listed areas. Defined waters of the sea involve 

those of the state/territory coastal waters, territorial sea, contiguous zone, exclusive economic 

zone, and continental shelf. 

The Commonwealth of Australia Offshore Minerals Act 1994 (OMA), along with six other 

associated Acts provide the legal framework for the exploration and recovery of minerals, other 

than petroleum, on Australia’s continental shelf, including the payment of royalties, annual fees 

and user charges for licences. The OMA establishes a code for the development of offshore 

minerals in the area under the Australian Government (Commonwealth) jurisdiction beyond the 

outer limit of the States and Northern Territory coastal waters (i.e. > 3 nm). There is no 

Commonwealth involvement in the administration of offshore mineral activities within the State or 

Northern Territory jurisdiction (i.e. < 3 nm). Under the OMA, a mineral is defined as a naturally 

occurring substance or mixture of substances, which may be in the form of sand, gravel, clay, 

limestone, rock, evaporates, shale, oil-shale, or coal; but does not include petroleum. 

The Act covers 5 types of licences: Exploration, Retention, Mining, Works, and Special Purpose 

Consent Licences. An Exploration Licence (4 years) enables the holder to explore for all minerals 

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and may be renewed for up to 10 years, but the licence area must be reduced by 50% on each 

renewal. A Retention Licence (up to 5 years) allows an Exploration Licence holder to retain rights 

where a significant mineral deposit exists but is not currently commercially viable. A Mining 

Licence (up to 21 years) enables the holder to explore for and recover minerals over an area 

where a significant mineral deposit has been identified and evaluated. Before a Mining Licence is 

granted, the applicant must lodge a mining development plan and environmental impact 

statement for assessment and approval. A Works Licence (up to 5 years) enables a licence 

holder to carry out major licence-related operations outside the licence area of his exploration, 

retention or mining licence. A Special Purpose Consent title (up to 1 year) allows the holder to 

conduct scientific investigation, reconnaissance surveys, or small mineral sample collection. 

The State/Northern Territory Offshore Minerals Acts 1998, 1999, 2000, and 2003 allow each 

State and the Northern Terrirtory to have its own Offshore Minerals Act which provide legislation 

for the exploration and mining of minerals (other than petroleum) within the first 3 nm of the 

territorial sea. 

The Native Title Act 1993 (NTA) provides the legal basis for the recognition and protection of 

native title. Native title represents the rights and interests (i.e. hunting, gathering, or fishing) of 

Aboriginal and Torres Strait Islander people in relation to land and waters according to their 

traditional laws and customs, which are recognised under Australian law. Under the NTA, 

exploration and mining companies are required to negotiate access arrangements for land on 

which native title has been granted or is subject to claim. During this process many Indigenous 

communities have become important stakeholders in mineral resource evaluation and 

development. The NTA requires that when the Government intends to grant an Exploration or 

Mining Licence, it must provide proper notification including advertising its intention to grant the 

licence in a major newspaper that circulates in the area to which the notice relates and an 

Aboriginal newspaper. 

The Environmental Protection and Biodiversity Act 1999 (EPBC) provides the legal basis for the 

protection of the environment and conservation of heritage and aspects of national environmental 

significance. The EPBC specifies the process for environmental assessment and approvals, 

environmental conservation and management. The EPBC also promotes ecologically sustainable 

development; conservation of biodiversity; cooperation between governments, communities, 

land-holders, and indigenous peoples; and recognition of the conservation role and knowledge of 

indigenous peoples to the sustainable use of Australia’s biodiversity. 

In all Australian waters, including state and territory waters, the EPBC regulates actions that will 

have, or are likely to have, a significant impact on the environment of any listed threatened and 

migratory species (including cetaceans and other marine species) in a Commonwealth marine 

area. Marine species, not including whales and other cetaceans, include: sea-snakes, seals, 

crocodiles, dugong, turtles, seahorses, sea-dragons and pipefish, and birds which occur naturally 

in Commonwealth marine areas. 

Guidelines on the Application of the Environment Protection and Biodiversity Conservation Act 

1999 to Interactions Between Offshore Seismic Operations and Larger Cetaceans October 2001 

were drawn up to ensure potential impacts on whales from industry-related activities (particularly 

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� 

Report No: 10/J/1/06/1309/0996 

seismic surveys) are avoided. A proposed seismic operation would therefore require the approval 

of the Minister for Environment and Water Resources if it is regarded as being likely to have a 

significant impact on cetacean/whale species (including threatened and migratory cetacean 

species). 

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�� 

Report No: 10/J/1/06/1309/0996 

Benchmarking is a qualitative process looking at an objective and comparing alternative ways of 

achieving it measured in differing ways e.g. cost, efficiency, environmental impact, practicality, 

etc. The following chapter will benchmark the technologies and practices identified in Chapter 4 

against the English example. It will compare the technologies and practices in use in English 

aggregate dredging to those used elsewhere in the world. It will rate each option under the 

categories ‘Best Practice’, ‘Fit for Purpose’ and ‘Sub-optimal’ as they apply to the English 

situation. 

122

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123 

�� 

�� 

Dragheads � Single visor 

Dredge 

pumps and 

impellers 

� California 

� No jetting or 

cutting 

� Standard 

pump / 

impeller 

� Cutting teeth 

and jetting 

common 

� High 

efficiency 

pump / 

impeller 

� Punaise 

system 

Fit for 

purpose 

Fit for 

purpose 

�� 

�� 

� � English draghead technology is unadvanced compared with 

worldwide technologies, however it is rated as being fit for 

purpose. 

� Cutting teeth and jetting are common in dragheads on most new 

build TSHDs however many of these vessels are used in capital 

or maintenance dredging where soils are often consolidated, 

unlike English aggregate deposits. 

� Other speciality dragheads identified as part of this project (wide 

dragheads, focussed suction, semi autonomous) are rated as 

sub-optimal and inapplicable to the English aggregate dredging 

industry. 

� � Many TSHDs worldwide use high efficiency pumps and 

impellers compared with standard pumps and impellers at use in 

the English fleet. 

� High efficiency pumps and impellers could improve loading rates 

by 10% or more, however since much of the English industry is 

governed by tidal turnaround the time saved would not allow 

increased cargo collection. 

� Faster loading could however reduce the environmental impacts 

of dredging by reducing the amount of time the vessel was 

actively impacting the dredging area 

� The Punaise system is rated sub-optimal and inapplicable to the 

Benchmarking Equipment, Practices and Technologies

Report No: 10/J/1/06/1309/0996 

124 

Dredge pipe � Single pipe � Single pipe 

Screening � Fixed screens 

Overflow 

control 

� Screening 

towers 

� Generally 

none 

� Moveable 

spillways 

� Twin pipe 

� Central pipe 

� Fixed 

screens 

� Screening 

towers 

� Moveable 

spillways 

� Anti-turbidity 

valve 

Best 

practice 

Best 

practice 

Sub 

optimal / 

fit for 

purpose 

English aggregate industry. 

X � While single pipes are the most common arrangement in the 

world fleet a number of vessels have twin pipes. This allows 

faster loading of the hopper. 

� Since much of the English industry is governed by tidal 

turnaround the time saved would not allow increased cargo 

collection. 

� Faster loading could however reduce the environmental impacts 

of dredging by reducing the amount of time the vessel was 

actively impacting the dredging area 

� Central pipes would be sub-optimal for the English industry 

since they impact on hopper capacity 

� � Where screening is carried out the screen arrangement of fixed 

screens or screening towers are used. No novel technologies 

with regards screening were identified. 

� The use of fixed screens is fit for purpose while rotating screen 

towers are considered best practice. 

� Screening technology is in excess of 20 years old – 

investigation of latest land based screening techniques is 

recommended 

� � The majority of English aggregate vessels have no overflow 

control while a small number of vessels have moveable 

spillways 

� Anti-turbidity valve technology has been available for a number 


Report No: 10/J/1/06/1309/0996 

125 

Vessel 

design 

� Aft or forward 

config. 

� Typically no 

bulb on bow. 

� Relatively 

large crew 

numbers 

� Green pipe 

recirc. 

system 

� Aft or 

forward 

config. 

� New hulls 

have bulb 

on bow 

� Move 

towards 

smaller crew 

numbers 

� New 

designs 

include 

reduced 

drag 

measures 

including 

Sub 

optimal / 

Fit for 

purpose 

of years and is efficient, having been shown to reduce turbidity 

to 40% of the standard method. Anti-turbidity valves are rated as 

Best Practice for overflow control. 

� The green pipe recirculating system has also been shown to 

reduce surface turbidity and hence the sediment plume. 

However the same amount of sediment is returned to the 

environment and concerns have been expressed that it may 

increase bed turbidity currents and the size of the sediment 

footprint and is hence not applicable to the English industry. 

� � Vessels in the English aggregate fleet are old compared with the 

fleets of the major European dredging contractors. 

� Designs of English ships do not incorporate the latest 

technologies to reduce drag and improve hull efficiency and fuel 

comsumption. 

� There is a move on the newest European dredging vessels to 

reduce crew numbers 

� The design of vessels in the English fleet is considered sub- 

optimal when compared with the design of the newest TSHDs 

being built, but is fit for purpose for dredging in English licence 

areas. 

� There is, however, major scope for development in the design of 

English aggregate vessels when new builds are commissioned. 

A recently commissioned ALSF study (MEPF REF 09/P133) will 

review vessel design and identify key issues including optimum 

operation and quantify potential reductions in environmental 


Report No: 10/J/1/06/1309/0996 

126 

Dry 

Discharge 

� Shore 

discharge 

� Drag scrapers 

� Bucket wheel 

system 

� Grab 

discharge 

anti-fouling 

paints; gel 

coats; high 

quality hull 

welding, 

propeller 

pods etc. 

� Pump out Best 

practice 

�� 

� �� 

�� 

Positioning � DGPS 

� DGPS 

Best 

practice 

impacts for any future new build vessels. 

� � No novel technologies with regards to dry discharging were 

identified. 

� Shore discharge is considered sub-optimal, drag scrapers are fit 

for purpose while bucket wheel systems / grab discharge are 

considered best practice 

� There may be further opportunities for development by 

examining land-based technologies and adapting suitable ones 

for TSHDs 

�� 

X � DGPS navigation systems are common to most modern 

dredging projects worldwide. 


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127 

Navigation 

and 

electronic 

sensors 

Electronic 

Monitoring 

Spatial 

mitigation 

� AIS / ECDIS 

� Draghead 

offset from 

DGPS 

position 

� Pipe angle 

indicator 

� AIS / ECDIS 

� Draghead 

position 

� Draghead 

angle and 

cutting 

depth 

� Mixture 

density 

� Flow 

velocity 

� Vacuum 

� Production 

rate 

� Draghead 

steering 

� Real time 

bathy 

Sub 

optimal / 

fit for 

purpose 

� EMS � EMS Best 

practice 

Best 

practice 

� � Linkage of navigation data to an electronic display 

information system is common to most modern dredging 

projects worldwide 

� The English dredging industry is sub-optimal with regards 

to dredging sensor information and draghead steering 

� Many modern TSHDs are fitted with a much wider range of 

electronic sensors and information displays than vessels in 

the English fleet 

X � The use of a tamper proof electronic monitoring system is 

common, particularly in Europe and the United States. 

� � There is a commitment by English dredging companies to 

relinquish unproductive license areas. 

� Dredging within English licence areas is controlled through 

voluntary zoning to minimize the area dredged at any one 


Report No: 10/J/1/06/1309/0996 

128 

Temporal 

mitigation 

Best 

practice 

time. 

� Communication with other users and voluntary exclusion 

zones to minimize conflict with fishermen are routine within 

the English dredging industry. 

� Similar means of spatial mitigation are also used on 

dredging projects worldwide. 

� The use of spatial mitigation measures by the English 

industry is considered best practice when compared with 

world wide practices, however there is continued 

opportunity for development if knowledge and 

understanding continues to grow. 

� � English dredging industry mitigates against environmental 

impacts on vulnerable species by reducing or avoiding 

screening; or prohibiting dredging entirely, on certain 

licence areas during particular time frames. 

� Similar temporal mitigation has been used worldwide to 

protect species or habitats vulnerable at certain times of 

the year. 

� Environmental Windows as widely used in the USA 

severely restrict dredging operations and would 

unreasonably restrict aggregate dredging operations if 

applied in English waters. US style Environmental 

Windows are therefore considered sub-optimal in the 

English setting. 

� The use of temporal mitigation measures by the English 


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129 

Knowledge 

and 

understandi 

ng 

Best 

practice 

Permitting Best 

practice 

industry is considered best practice when compared with 

world wide practices, however there is continued 

opportunity for development if knowledge and 

understanding continues to grow. 

� � Marine aggregate industry in the UK has invested 

significant sums of money in increasing knowledge and of 

the marine environment and enhancing understanding of 

the impacts of aggregate dredging. 

� It has done this through internal research projects, the 

Environmental Impact Assessment process and most 

significantly through the Aggregate Levy Sustainability 

Fund (ALSF). 

� Worldwide knowledge is not so advanced, although other 

research programmes exist such as the DOER programme 

in the US and Australian CSIRO research projects 

� It is considered that the knowledge and understanding 

developed within the English system is world-leading, 

although further opportunities to develop and maintain this 

position do exist. 

X � The English regulatory process is transparent, puts the 

environment first, and involves significant consultation. 

� An environmental statement and supporting studies are 

required in al cases and unless the applicant can 

demonstrate that the environmental impacts are 

acceptable then a dredging permission will not be granted. 


Report No: 10/J/1/06/1309/0996 

130 

� Consents are accompanied by a schedule of legally 

enforceable conditions 

� Regulation worldwide differs between countries, however 

examples from Europe (Netherlands) showed that, unlike 

the UK, not all dredging applications required an EIA 

� US regulations are concerned mainly with disposal of 

dredged material rather than aggregate dredging. 

Responsibilities for regulation are split between the 

USACE and the Environmental Protection Agency. 

� Australian industry is evolving and the regulatory 

framework is similarly evolving. Currently regulation is 

complex with 6 Acts, Guidelines and Policies comprising 

the Australian legislation. 

� The English regulatory process is considered best practice 

when compared with examples worldwide. 



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Within the English fleet there are a number of old vessels which are reaching the end of their 

serviceable lives, which is determined by a combination of the ability to maintain the machinery in 

a reliable condition and the cost of maintaining the vessel’s structure which escalates significantly 

beyond 30 years old. Operators are therefore going to be faced the dilemma of developing cost 

effective vessel designs that can continue to serve the existing wharves or alternatively find new 

ways of supplying the markets that the wharves served. There is may be pressure to move 

operations into deeper, more exposed waters, particularly in the North East and the West Coast. 

There will also be continued environmental demands imposed on the industry, some of which will 

provide economic benefits. 

From the benchmarking process it is concluded that in terms of technology the English fleet is 

generally fit for purpose when compared with the world fleet: 

� Dragheads – fit for purpose 

� Dredge pumps and impellers – fit for purpose 

� Dredge pipes – best practice 

� Screening – best practice 

� Overflow control – sub-optimal / fit for purpose 

� Vessel design – sub-optimal / fit for purpose 

� Dry discharge – best practice. 

The English fleet is older than comparable fleets operated by the major European dredging 

contractors and, therefore, it is concluded that while many of the technologies are fit for purpose, 

or even current best practice, there are opportunities for development in the fields of dragheads; 

dredge pumps and impellers; screening, overflow control, vessel design and discharge. 

Compared with the technological side the benchmarking process has shown that in terms of 

dredging practices, mitigation and Regulatory Framework the English example is generally worldleading: 

� Positioning – best practice 

� Navigation and electronic sensors – sub-optimal / fit for purpose 

� Electronic monitoring – best practice 

� Spatial mitigation – best practice 

� Temporal mitigation – best practice 

� Knowledge and understanding – best practice 

� Permitting – best practice 

There is a distinct difference between the navigation information relating to drag head position, 

steering and electronic dredging sensors employed in the English fleet and those used on the 

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newest build comparable TSHDs. However in all other regards the benchmarking process 

considers the English examples to be best practice. There are, however, further opportunities for 

development in the fields of navigation and electronic sensors and knowledge and 

understanding; and should knowledge and understanding of impacts continue to grow then there 

would be opportunity to further enhance spatial and temporal mitigation practices. 

The overall conclusion of the benchmarking exercise is that having surveyed the world fleet, 

technology and practices it is not possible to identify major improvements that are applicable to 

the English industry and that would significantly enhance the environmental performance of the 

English fleet. The English industry is, however, open with regards to its environmental reporting 

and willing to develop knowledge on enhancing environmental performance. 

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The sedimentation process in a hopper has a strong resemblance to the sedimentation process 

in secondary sedimentation tanks used in the field of sewage and water treatment. It is therefore 

no coincidence that some models used in the dredging industry are based on work done in 

wastewater treatment. In this respect the work of Camp (1946) must be mentioned since this 

forms the basis of many subsequent models. 

The method of Camp is based on the supposed trajectories of particles during settling (see 

Figure A-1). The mixture enters the hopper in the inlet zone and is transported towards the outlet 

zone. The trajectories of particles are observed. A particle with a settling velocity �� reaches the 

bed regardless of the vertical position leaving the inlet. Particles with lower settling velocity only 

reach the bed in cases where the vertical starting position is not too high. 

� 

�� 

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This leads to the settling efficiency for a certain fraction of the particle size distribution. The 

settling efficiency (or removal ratio) for a certain fraction is: 

� 

� 

� 

� �� 

� 

� 

The removal ratio is the ratio of the incoming particles of a certain fraction that settle in the 

hopper. With v as settling velocity of that fraction and �� defined as overflow rate: 

Where: 

� 

� 

� 

� �� 

�� 

� Mixture discharge into hopper [m 3 /s] 

� Length of the hopper [m] 

� Width of the hopper [m] 

In Figure A-1 the particles follow a straight line during settling. In reality this will not be the case 

since flow will be turbulent. To take the influence of turbulence into account Camp solved an 

advection / diffusion equation with a constant diffusion coefficient. The result of this analysis is 

shown in Figure A-2. 

�� 

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Report No: 10/J/1/06/1309/0996 

Lines are drawn for different values of�� and the position on the horizontal axis includes the 

influence of turbulence which creates mixing. The mixing, or diffusion coefficient is � hence high 

values of turbulence are shown on the left side of the graph. 

�� 

The Vlasblom and Miedema (1995) model is an refinement of the Camp model. The Camp model 

uses a constant mixture level in the basin. The sludge that settles on the bottom is scraped to a 

central location and sucked away. In a hopper of a TSHD the mixture level decreases in time due 

to the rising bed level and this effect is taken into account in the Vlasblom & Miedema model. 

Furthermore the effect of hindered settling is also taken into account. Hindered settling is the 

decrease of settling velocity with increasing concentration. 

�� 

Van Rhee (2001) published a one dimensional model based on the observation that the flow in 

the hopper is dominated by density effects. In the inlet zone the flow is directed vertically towards 

the bed due to buoyancy. At this location settling occurs and on average the flow is vertical in the 

area above the density current. This vertical flow is modelled using an advection / diffusion 

equation for different fractions of the PSD. 

�� 

The flow in a hopper can be regarded as two dimensional in the horizontal (length) and vertical 

direction over the largest part in a hopper. Based on this observation Van Rhee (2002a, 2002b) 

developed a 2DV model based on the Reynolds Averaged Navier Stokes (RANS) equations with 

a �-� turbulence model (Rodi (1993)). In the model the momentum equations and sediment 

transport equations are coupled. The model has a movable bed and water level (the latter for 

changing overflow levels during time). Inflow and overflow location, inflow concentration and 

velocity and the inflow PSD can all be varied in time. 

This model is the most sophisticated model available. In Figure A-3 a result of this model is 

shown. In the upper panel the flow velocity is shown while the lower panel shows the 

concentration. The inflow is at the left side of the hopper, the overflow at the right side. A settled 

bed can be seen in both panels. Where the mixture enters the hopper at the left side an erosion 

crater develops. The mixture flows out of this crater as a heavy fluid – a density current. Due to 

this phenomenon the velocity is not uniformly distributed over height. 

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Report No: 10/J/1/06/1309/0996 

�� 

�� 

For a first quick estimate of the cumulative overflow loss one might expect during loading of 

sediment with a certain PSD when the hopper size and discharge is known Camp’s (1946) 

method provides good results. 

To get more insight on the development during (loading) time of losses the Vlasblom and 

Miedema (1995) model can be used. This model predicts the particle size distribution of the 

overflow mixture during time. Calculation effort is limited therefore a large number of calculations 

can be performed in a restricted time. If more results are needed, for instance to investigate 

loading and overflow strategy, the effect of the water level in the hopper at the start of the loading 

process and the influence of the hopper shape, the 

. 

Vlasblom and Miedema (1995) model is too 

restricted and a more sophisticated model is needed 

147


�� 

�� 

Report No: 10/J/1/06/1309/0996 

�� 

IMO ship pollution rules are contained in the “International Convention on the Prevention of 

th 

Pollution from Ships”, known as MARPOL 73/78. On 27 September 1997, the MARPOL 

Convention 

was been amended by the “1997 Protocol”, which includes Annex VI, 

“Regulations for the Prevention of Air Pollution from Ships”. MARPOL Annex VI sets limits on 

NOx and SOx emissions from ship exhausts, and prohibits deliberate emissions of ozone 

depleting substances. The IMO emission standards are commonly referred to as Tier I - III 

standards. The Tier I standards were defined in the 1997 version of Annex VI, while the Tier II/III 

standards were introduced by Annex VI amendments adopted in 2008. 

�� 

The 1997 Protocol to MARPOL, which includes Annex VI, became effective 12 months after 

being accepted by 15 States with not less than 50% of world merchant shipping tonnage. On 18 th 

May 2004, Samoa deposited its ratification as the 15th State (joining Bahamas, Bangladesh, 

Barbados, Denmark, Germany, Greece, Liberia, Marshal Islands, Norway, Panama, Singapore, 

Spain, Sweden, and Vanuatu). At that date, Annex VI was ratified by States with 54.57% of world 

merchant shipping tonnage. 

Accordingly, Annex VI entered into force on 19 th May 2005. It applies retroactively to new 

engines greater than 130 kW installed on vessels constructed on or after January 1 st , 2000, or 

which undergo a major conversion after that date. The regulation also applies to fixed and 

floating rigs and to drilling platforms (except for emissions associated directly with exploration 

and/or handling of sea-bed minerals). In anticipation of the Annex VI ratification, most marine 

engine manufacturers have been building engines compliant with the above standards 

since 2000. 

�� 

Annex VI amendments adopted in October 2008 introduced: 

� New fuel quality requirements beginning from July 2010. 

� Tier II and III NOx emission standards for new engines. 

� Tier I NOx requirements for existing pre-2000 engines. 

The revised Annex VI enters into force on 1 st July 2010. By October 2008, Annex VI was ratified 

by 53 countries (including the Unites States), representing 81.88% of tonnage. 

Emission Control Areas 

Two sets of emission and fuel quality requirements are defined by Annex VI: 

� Global requirements 

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Report No: 10/J/1/06/1309/0996 

� More stringent requirements applicable to ships in Emission Control Areas (ECA). 

An Emission Control Area can be designated for SOx and PM, or NOx, or all three types 

of emissions from ships, subject to a proposal from a Party to Annex VI. Existing SOx 

Emission Control Areas include the Baltic Sea (adopted: 1997 / entered into force: 2005) 

and the North Sea (2005/2006). Future Emission Control Areas could also include zones 

around pollution sensitive ports. 

�� 

NOx emission limits are set for diesel engines depending on the engine maximum operating 

speed (n, rpm) as indicated in Figure B-1. Tier I and Tier II limits are global, while the Tier III 

standards apply only in NOx Emission Control Areas. 

�� 

Tier II standards are expected to be met by combustion process optimization. The parameters 

examined by engine manufacturers include fuel injection timing, pressure, and rate (rate 

shaping), fuel nozzle flow area, exhaust valve timing, and cylinder compression volume. 

Tier III standards are expected to require dedicated NOx emission control technologies such as 

various forms of water induction into the combustion process (with fuel, scavenging air, or incylinder), 

exhaust gas recirculation, or selective catalytic reduction. 

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Report No: 10/J/1/06/1309/0996 

The main changes to MARPOL Annex VI will see a progressive reduction in sulphur oxide (SOx) 

emissions from ships, with the global sulphur cap reduced initially to 3.50% (from the current 

4.50%), effective from 1 st January 2012; then progressively to 0.50 %, effective from 1 st January 

2020, subject to a feasibility review to be completed no later than 2018. 

The limits applicable in Sulphur Emission Control Areas (SECAs) will be reduced to 1.00%, 

beginning on 1 st July 2010 (from the current 1.50 %); being further reduced to 0.10 %, effective 

from 1 st January 2015. 

Progressive reductions in nitrogen oxide (NOx) emissions from marine engines were also 

agreed, with the most stringent controls on so-called “ Tier III ” 

engines, i.e. those installed on 

st 

ships constructed on or after 1 January 2016, operating in Emission Control Areas. 

(IMO, 2008). The consequence of the new IMO legislation on fuel oil is that from 2015 only light 

distillates can be used in SECA’s and from 2020 only light distillates can be used worldwide. 

�� 

The Kyoto Protocol is an international agreement linked to the United Nations Framework 

Convention on Climate Change. The major feature of the Kyoto Protocol is that it sets binding 

targets for 37 industrialized countries and the European community for reducing greenhouse gas 

(GHG) emissions.These amount to an average of five per cent against 1990 levels over the fiveyear 

period 2008-2012. Shipping and aircraft were however excluded from these agreements. At 

the Bali summit of 2007 governments agreed to include both shipping and aircraft during the 

Copenhagen summit in December 2009 (COP15 conference). This forced the IMO into action 

and it started procedures to come up with an Energy Efficiency Design Index (CO 2 index) for 

ships. Such an index will probably be used by (port) authorities in the tender process for 

maintenance and land reclamation. Irrespective of such an index and the consequences it may 

have for ship’s operations, the upcoming agreements from the COP15 conference will result in 

limited CO2 emission rights for ship owners, and significant costs for excess of CO 2 emission 

limits. These emission rights will be limited progressively in time. CO 2 emissions must be 

reduced by 20% in 2020 with respect to the emissions in 1990. It is expected that emissions 

reduction targets for 2050 will amount to 50 %. It is therefore of the utmost importance that the 

CO2 index calculated according to IMO norms, correctly reflect the energy consumption and 

energy efficiency of dredging equipment (Van de Ketterij et al. 2009). . 

It is to be expected that the agreements on CO2 reduction in the future will lead to increased 

costs for emission and limited emission rights for ship owners. Apart from this cost factor it could 

lead to the decision of authorities not to allow the use of ships (dredgers) with insufficient 

energy efficiency. 

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To compare the energy efficiency of vessels IMO proposed an energy index during the MEPC58 

conference (IMO, 2008). The essence of the index is that the ratio between the CO2 emission 

from the installed power sources and the product of tonnage and speed is computed. The 

product of tonnage and speed can be regarded as a benefit for society, while the CO2 emission is 

the price that has to be paid. The index in simplified shape reads: 

Where : 

�� 

�� 

� 

�� 

� � 

� �� 

�� 

�� 

�� 

�� Conversion factor between fuel consumption and CO2 emission. [-] 

�� Specific Fuel consumption of a main engine 

(at 75 % of the rated installed power (MCR)) [g/kWh] 

�� Specific Fuel consumption of the auxiliary engines [kW] 

� �� 

75 % of the MCR of the main engine nr. i [kW] 

� �� 

Power of the auxiliary engines [kW 

�� Number of main engines [-] 

�� Deadweight [ton] 

� �� 

Ships speed on deep water at max load [kn] 

151 

�� 

(B-1) 

The CO2 index for ships generally is based on the energy index. This index has been introduced 

by Gabrielli and Von Karman (1950) in a famous paper. In this paper they tried to compare 

energy consumption of various means of transportation, making them dimensionless by division 

by a transport capacity. 

� 

� � (B-2) 

��


Report No: 10/J/1/06/1309/0996 

�� 

An updated version, with modern transport means, of this figure is shown in Figure B-3. Here it is 

shown that transport by shipping is very energy efficient compared with the other transport 

methods. 

For a Trailing Suction Hopper Dredge the energy index cannot be simply captured with the 

definition of equation B-2, 

and likewise the definition of a CO2 index in the shape of equation 

B-1 cannot be used because the power installed on board of a TSHD is not primarily used for 

horizontal transportation of sediment. For the loading phase and the discharge phase a 

considerable amount of energy is needed as well. Therefore Van de Ketterij et al. (2009) 

proposed an alternative method to compute the energy efficiency of a TSHD taken the complete 

dredge cycle into account. 

152


� 

Report No: 10/J/1/06/1309/0996 

�� 

The total amount of energy needed for the complete dredge cycle reads: 

�� B-3 

Where: 

� �� = total energy needed for the dredge cycle, �� = the power used during sailing empty, � �� = 

the power used for sailing fully loaded, � � = the power used during dredging, � �� = the power 

used for discharging the load, �� and � �� the time needed for sailing loaded, sailing full, 

dredging and discharging respectively. 

By taking the average energy consumption over the total cycle the following index is found: 

� � � � � 

� �� 

� � � � � 

� � � 

�� 

� �� 

�� 

�� 

153 

B-4


Report No: 10/J/1/06/1309/0996 

The gravitational acceleration was introduced in the denominator to get a dimensionless index. 

Velocity � is defined as the sailing distance full divided by the cycle time: 

B-5 

� 

� � 

� 

�� 

�� 

This energy index takes all phases of the dredge cycle into account and is therefore a much 

more objective measurement to compare the energy efficiency (and CO2 efficiency since CO2 is 

directly coupled to the energy efficiency). Using this method the expression in eq. B-1 can be 

extended to an EEDI index for a TSHD by splitting up the first term in the nominator in the power 

contributions over the different phases and using definition B-5 for the sailing speed. 

A complication of the method is that a standard dredging cycle must be defined to compare 

different dredgers. This involves a certain selection of sediment properties, sailing distance, 

dredging depth and discharge method. The selection itself can have a large influence. If for 

instance the particle size is very coarse efficiency improvement of the loading system will not be 

rewarded. If a long sailing distance is chosen the sailing phase dominates over the other phases 

and improvements of the dredging equipment will not be measured. In Van de Ketterij et al. (2009) 

a certain standard cycle was proposed, but discussions on this topic are ongoing. 

154


Aggregate Levy 

Sustainability Fund 

MALSF 

© Crown Copyright 2010 

Published by the Marine Aggregate Levy Sustainability Fund (MALSF). 

For more information, please visit: 

http://www.alsf-mepf.org.uk/ 

http://www.english-heritage.org.uk/ALSF 

http://alsf.defra.gov.uk 

Cover Image Credits 

Top Row: Left to Right Bottom Row: Left to Right 

BMAPA Crown Copyright, Courtesy of Cefas 

Crown Copyright, Courtesy of Cefas BMAPA 

St Andrews University & English Heritage Wessex Archaeology & English Heritage

Mitigation of Marine Aggregate Dredging Impacts ... - Cefas - Defra

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