Granular Filter Design for Scour Protection at Offshore Structures

For more information contact:James Bernegger, DirectorCorporate Marketing and Communications863-644-2431 x112E-mail: news@sun-n-fun.orgFOR IMMEDIATE RELEASESUN `N FUN RECOGNIZES 20 TH ANNIVERSARY OFOPERATION DESERT STORMSUN ’n FUN CAMPUS, LAKELAND, FL. – (February 21, 2011) – The 20 th anniversary of “OperationDesert Storm” will be recognized during the 37 th annual SUN ’n FUN International Fly-In and Expo.This year’s Fly-In will take place March 29 – April 3 at Lakeland Linder Regional Airport in Lakeland,Fla.United States Army Brigadier General Rhonda Cornum and her husband, United States AirForce Brig. Gen. Kory Cornum, both served in Desert Storm and will provide their unique perspectiveson the military strategies and their successful conclusion during the First Persian Gulf War.“It’s hard to believe that it’s been 20 years since Desert Storm,” said SUN ’n FUN President andConvention Chairman John Burton. “We are honored to have these two high-ranking military veteranswho so honorably served in Desert Storm and who remain in service to our country to this day. Theirprogram will be one that SUN ’n FUN participants and guests will be talking about for a long time.”The two Generals (who are also medical doctors) will offer their insights and experiencesrelating to the conflict from the Army (Rhonda) and Air Force (Kory) perspectives and provideadditional information on the critical roles played by other branches of the military. Both served inDesert Storm: Rhonda as a Flight Surgeon with the Army’s 2/229 Attack Helicopter Battalion flyingAH-64 Apaches and UH-60 Blackhawks and Kory as a Flight Surgeon with the Air Force’s 58 th TacticalFighter Squadron flying F-15s.During the last week of February 1991, while performing a search and rescue mission for adowned Air Force F-16 pilot, Rhonda’s Blackhawk helicopter was shot down. Five of the eight-personcrew were killed. The three survivors, including Gen. Cornum, were captured by Iraqi forces and heldfor eight days before being repatriated.- MORE -

ICSE6 Paris - August 27-31, 2012David Schürenkamp, Matthias Bleck and Hocine Oumeraci - Advanced Filter DesignIn the following, a scour protection made of a rockarmour with a multi-layer granular filter is exemplarilyconsidered to address the afore mentioned issues. Sucha scour protection has been tested in the ‘Großen Wellenkanal’(GWK, Hannover) as shown in Figure 1(Oumeraci et al., 2007).The failure of marine structures is often due to thefailure of their granular filters as reliable filter designformulae are still lacking (Oumeraci, 1996). Therefore,a brief critical review of the literature of the presentknowledge will be conducted below to show that theknowledge associated with the hydro-geotechnicalprocesses involved in the flow-filter-interaction is stillnot sufficient for the development of such formulae.Moreover, this review will enable through a combinationof the existing knowledge and formulae to proposeand critically discuss a tentative approach for the designof granular filters for a scour protection around amonopile foundation of offshore wind turbines whichFigure 1: Scour protection around a monopile foundation,‘Großer Wellenkanal’ Hannover (Oumeraciet al., 2007)fulfils the afore mentioned requirements with respect to hydraulic stability, constructability and cost effectiveness.Finally some recommendations for further research will be given.IIPRESENT KNOWLEDGE FOR THE DESIGN OF GRANULAR FILTERSII.1Types of flow conditions within a granular filterThe primary function of a granular filter of marine structures is to prevent the underlying soil layers frombeing washed outthrough the pore structurea) Parallel unidirectional flow b) Parallel oscillatory flowof the protectivelayers. In addition,Hsufficiently thickgranular filters can alsosubstantially contributeto dissipate pressureArmour LayerFilter Layeru wArmour LayerFilter Layer(e.g. pore pressure inthe soil, uplift pressureon the marine structure).Because of thisSubsoilc) Perpendicular unidirectional flow d) Perpendicular oscillatory flowSubsoilsecondary function(particularly in the caseHof high interfacial pressurezgradients), butalso in view of furtherArmour LayerArmour Layeradvantages such asFilter LayerFilter Layerdurability and interlockingSubsoilSubsoilwith the pro-tective layer, granularfilters are often favouredin comparison Figure 2: Types of flow conditions within a scour protection of a monopile foundationto geotextile filtersfor offshore structures(CEM, 2008).As a result of waves, currents and their combined action near shore and offshore, the sea bed and granularfilters are subject to four basic types of flow conditions, depending on the flow direction relative to the interfacesof the different layers (indicated in red colour in Figure 2 a-d): (i) parallel unidirectional flow, (ii) paralleloscillatory flow, (iii) perpendicular unidirectional flow and (iv) perpendicular oscillatory flow.1028

ICSE6 Paris - August 27-31, 2012David Schürenkamp, Matthias Bleck and Hocine Oumeraci - Advanced Filter DesignFor parallel flow conditions, the interface between armour and filter layer is more relevant, whereas the interfacebetween filter layer and sea bed is more relevant for perpendicular flow conditions. The actual flowconditions may also result from the combination of any of these four types of flow conditions (de Graauw etal., 1983). The loading conditions in a marine environment with waves and tides for a scour protection aregenerally the result of a combination of types a, b and d in Figure 2. Types c in Figure 2 may occur for instanceas a result of ship-induced wave.II.2Filter design approachThere are three basic concepts for the design of granular filters: (i) geometrical filter criteria, (ii) hydraulicfilter criteria and (iii) dynamic filter for broadly graded filter (CUR161, 1993; Schiereck, 2001). Differentdesign approaches can be applied: stable filter, semi-stable and dynamically stable filter concepts (CUR161,1993). Depending on the design approach selected, different filter criteria must be fulfilled. These includesamongst others (i) retention criterion to ensure stability against contact erosion and thus prevent leaching offiner fractions through the adjacent layer, (ii) internal stability criterion to prevent suffusion, (iii) permeabilitycriterion to minimise hydraulic pressure gradient through the layer (iv) filter thickness criterion to dampsevere hydrodynamic loading. Further considerations such as covering sea bottom irregularities, compensationof differential settlements, construction method and tolerances, exposure during construction, constructionin deeper water may dictate much thicker filters than required from the hydraulic stability criteria alone(de Graauw et al., 1983; Wörman, 1989). Moreover, stability against segregation during construction mayalso be an important issue depending on the water depth and the construction methods. To reduce segregationgeometrical criteria are primarily applied. For a filter construction under water, a narrowly graded andpossibly coarser filter material with a uniformity coefficient Cu ≤ 5 is recommended by the BAW (1989).II.3Geometrically closed filterHitherto, geometrical filter criteria based on characteristic grain sizes of the filter layers and adjacent soillayers are mostly applied in engineering practice. Such an approach implicitly requires that, irrespective ofthe hydraulic loading conditions, the filter material should have openings (constrictions) sufficiently small toretain the finer particles of the soil material likely to migrate from being washing out. If the constrictions aresmaller than the characteristic grain size of the base material, the filter is called geometrically closed. However,depending on the function of the filter, its application area and maintenance, the required retention mustnot be total. Generally, the primary aim of a filter is to retain the soil as a whole but not necessarily all particles.The challenge of any filter design is therefore to ensure a proper balance between soil retention andpermeability (optimum retention).Particularly when subject to highly turbulent, multidirectional and oscillatory flow, such as filter applicationsfor coastal and offshore structures, this requirement can only be fulfilled by using a multi-layer filter(generally two or three filter layers between the base material and the armour layer). The construction ofsuch filter layers is however associated with considerable difficulties, including the risk of drift and segregationof the granular material during placement, imprecise distribution in thin multi-layers, increasederodibility around the pile structure to be protected and thus erosion of the unprotected filter layers beforecompletion of the cover layer, etc. A further drawback of multi-layer filters in deeper water is the minimumlayer thickness requirement for an adequate construction (≥ 0.50 m) which together with all difficulties associatedwith their construction would result in excessively high costs (Schürenkamp, 2011).Most of the geometrically closed filter criteria originate from dam engineering where the filter and the basematerial are subject to non-turbulent and unidirectional flow and generally a total retention is required. Basedon the results of the laboratory experiments of de Graauw et al. (1983) who investigated systematically thestability of granular filter under both unidirectional and oscillatory flow, Oumeraci (1996) concluded that theapplication of these criteria to filters subject to sea waves is questionable.1029

ICSE6 Paris - August 27-31, 2012David Schürenkamp, Matthias Bleck and Hocine Oumeraci - Advanced Filter DesignII.4Geometrically open filterII.4.1 GeneralIn contrast to the geometrically closed filter, the hydraulic loading conditions are taken into account in orderto achieve a more economic design. The filter should sufficiently reduce the hydraulic loads at the basematerial, so that the base material is not moved. Such a filter is called a geometrically open filter. Its applicationrequires however knowledge of both actual and critical hydraulic loads (critical hydraulic gradient orcritical flow velocity). The critical hydraulic loads causing failure may occur as a result of flow perpendicularor parallel to the interface between the layers of the granular material as illustrated in Figure 3 showingtwo types of failure mechanisms.To assess such failure mechanisms andto properly design the filter, a detailedknowledge of the hydraulic processesinvolved within the filter and the basematerial is needed. The failure andtransport mechanism in both filter andbase material will depend on the type anddirection of the internal flow. As alreadyillustrated in Figure 2 four basic flow conditionsin the porous media may be distinguished.Perpendicular flow directed upwardswill lift the soil particles (liquefaction)while parallel flow will cause shearfailure, but both effects actually occur incombination (Zichel & Oumeraci, 2012).II.4.2 Parallel flowIt is generally assumed that mobilizing forces on a single particle of the granular material are drag, lift andinertia forces while the resisting forces are provided by gravity, friction and interlocking (Raudkivi, 1982;Hansen, 1985). A particulardifficulty is the determinationof the flow velocity and shearstress inside the filter andcover layer (see Figure 4).In order to investigate thedamping of the flow inducedby a layer of granular material,laboratory experimentswere performed by Booij(1998) for uni-directionalflow and by Wenka andKöhler (2007) for oscillatoryflow conditions. Several casestudies for the filter stabilityof different types of bedU (z)u fLiquefactiona) Flow profile b) Shear stress distribution1,5 ∙ D 50unidirectional flowFilter layerBase Layeri krit perpendicularscour protection and slope revetments were conducted for parallel and unidirectional flow conditions. Manyof the hydraulic filter criteria applied are based on a critical shear stress according to the Shields concept.The approaches proposed by Bakker et al. (1994), Sumer and Fredsøe (2002), Hoffmans et al. (2008) arerelated to scour protection of a horizontal sand bed under turbulent flow conditions. The only design formulayet available and applicable for a scour protection around a cylindrical monopile subject to parallel and unidirectionalflow has been proposed by Wörman (1989) on the basis of hydraulic model tests. A single protectivelayer of rock embedded with the top at the bed level without any further layer was investigated by usingtwo cylinder diameters, five types of rock material and three types of sand. The primary function of the protectiverock layer is to achieve a sufficient damping of the hydraulic gradient i within the rock layer, and thusto prevent bed scour. For this purpose, the required rock layer thickness D F depends on the velocity of theD Fτ (z)τ b1,5 ∙ D 50Shear failurei krit parallelFigure 3: Failure mechanisms induced by perpendicular and parallelflow to the filterunidirectional flowFilter Filter layer layerBase LayerFigure 4: Flow profile and shear stress distribution in a filter (Booij, 1998;Zichel & Oumeraci, 2012)D F1030

ICSE6 Paris - August 27-31, 2012David Schürenkamp, Matthias Bleck and Hocine Oumeraci - Advanced Filter Designpore water flow, the properties of the pore fluid, the pile diameter as well as on the size and porosity of therock material. The primary objective of the analysis of the results was to obtain the relationship between thepressure gradient U²/(g · D F ) within the rock layer with thickness D F and the ratio (d 85B /d 15F ) of the grain sizeof the base material d 85B to that of the rock layer d 15F . U represents the mean velocity of a the undisturbedflow (assumed as fully developed, turbulent boundary layer over the rock layer) and g gravity acceleration,Such a relationship is intended to represent a criterion for incipient erosion-transport of the base materialthrough the rock layer which essentially depends on the undisturbed mean flow velocity U, the thickness D Fof the rock layer and the grain size ratio d 15F /d 85B . The obtained relationship shows that thickness D F increaseswith increasing ratio d 15F /d 85B (given same flow conditions) and with increasing flow velocity U. Moreover,it is shown that for extremely high pressure gradients, ratio d 85B /d 15F asymptotically approaches the conditionsof geometrically closed filters.Madsen and Grant (1976) have experimentally demonstrated that the Shields concept for incipient motionof granular material, initially developed for unidirectional flow, can also be used for oscillatory near bottomflow induced by sea waves. Similarly, it is assumed that the concept proposed by Wörman (1989) for incipienterosion-transport of the base material through the rock layer under unidirectional flow can also be appliedfor oscillatory flow in the marine environment. In the following section, the additional flow component perpendicularto the interface between the protective coarser layer and base material is considered.II.4.3 Perpendicular flowAs shown in Figures 2c & 2d, both unidirectional and oscillatory flow perpendicular to the interface betweenfilter and base material may principally occur. Actually, however, for a bed protection subject to seawaves perpendicular oscillatory flow represent the most relevant case as compared to the perpendicular unidirectionalflow. According to the results of de Graauw et al. (1983), a smaller grain size ratio of the granularfilter and base material will be required for oscillatory flow than for unidirectional flow. This is particularlydue to stabilizing mechanism such as “arching effect” which may occur and sustained for unidirectional flowbut not for oscillatory flow. This effect is illustrated in Figure 5a for a unidirectional flow where the formationof an arch prevents the finer particles of the base material from passing through the filter. Under oscillatoryflow conditions (Figure 5b), a smaller grain size ratio of the filter and base material is required to ensurea) Arching b) Geometrically closedFilterstability. Moreover, theload exerted by the weightof the filter on the basematerial is also importantfor stability. As illustratedin Figure 5c, the unloadedarea may be subject tolocal soil liquefactionunder high pressure gradients.The processes illustratedin Figure 5 highlightthe relevance of the oscillatoryflow in normal directionfor stability analysis.Its improper considerationmay result in failure and thus in unsafe filter design. There the flow components in both parallel andorthogonal direction must be properly considered. The still existing knowledge gaps related to this issue andto the processes in Figure 4 may explain why process-based generic filter design formulae are still lacking.The most urgent tasks are therefore to determine: (i) reliable prediction formulae for the actual hydraulicgradients in the different layers, (ii) the effect of the thickness and weight of the filter and cover layers on thestability of the base material and (iii) the development of pore pressure within the base material and its explicitconsideration in the final stability formulae for the design of filter and cover layers.FiltersuperimposedloadBase Base Baseunidirectional flowoscillatory flowc) Local liquefactionFilternon-loadzoneloadedzoneFigure 5: Arching effect and further processes (modified from de Graauw et al.(1983); Zichel and Oumeraci (2012))IIISUGGESTION OF A “HYBRID” FILTER DESIGN APPROACHAt present, geometrically based criteria are generally favoured due to the uncertainties associated with theknowledge particularly related to the oscillatory flow conditions in orthogonal direction. Based on theknowledge readily available, the most reasonable alternative for filter design seems to be a proper combinationof geometrically and hydraulically based filter criteria. For oscillatory flow in parallel direction a filter1031

ICSE6 Paris - August 27-31, 2012David Schürenkamp, Matthias Bleck and Hocine Oumeraci - Advanced Filter Designdesign based on the incipient motion of granular material according to the Shields concept is theoreticallywell-founded, but the empirical relation derived by Wörman (1989) using a similar approach to assess incipienterosion-transport offers substantially more advantages:Implicit account of hydraulic conditions, so that only material parameters of the basis and filter arerequired as inputsExplicit consideration of the thickness of the cover and filter layers which damps the hydraulic loadsand thus strongly affects the stability of the filter and the base material Implicit account of the turbulent flow and vortex structure (e.g. horse shoe vortex) around the pilestructure.A combination of geometrically and hydraulically based filter criteria is proposed, in which the stability atthe interface between Filter and base material can be achieved through geometrical filter criteria while that atthe interface between cover and filter layer through hydraulic filter criteria. This “hybrid filter design approach”may however result in a progressive erosion of the base material under very extreme hydraulic conditionsif the commonly applied geometrical filter criteria are used. This can be slowed down by using improvedcriteria; otherwise periodic monitoring and maintenance will be required. Further uncertainties willarise if the actual conditions significantly differ from those tested by Wörman (1989). For instance, the embeddedcover layer with the top at the sea bed level which was tested offers a substantial advantage comparedto non-embedded scour protections (e.g. Figure 1) as the flow in both horizontal and vertical directionsis significantly reduced. An example application of the suggested “hybrid” approach will be given for illustrationin the next section.IVEXAMPLE APPLICATION OF THE SUGGESTED APPROACHThe scour protection of the seabed around a slender monopile asshown in Figure 6 is considered.The armour layer has been designedaccording to the commonly usedhydraulic stability formulae. As aresult an armour with a mean stonediameter d 50D = 0,42 m, a densityρ s,D = 2900 kg/m³ and a thicknessD D = 0,90 m is obtained (seeSchürenkamp, 2011). For the exampleapplication, this armour willtherefore be assumed as given in afirst step. Further given input conditionsare summarized in Table 1.hDLH Su cArmour Layeru wFilter LayerSubsoilFigure 6: Scour Protection for Monopile Foundation (Sketch)Description Symbol Unit Valuewater depth h m 20,0wave height H s m 15,0dry densitiy ρ W kg/m³ 1025stone diameter d 15D m 0,33stone diameter d 50D m 0,42stone diameter d 85D m 0,50porosity of armour layer n D - 0,40thickness of armour layer D D m 0,90dry density of armour material ρ s,D kg/m³ 2900dry density of filter material ρ s,F kg/m³ 2650Table 1: Input conditions for the example application1032

ICSE6 Paris - August 27-31, 2012David Schürenkamp, Matthias Bleck and Hocine Oumeraci - Advanced Filter DesignFirst step: Applying the geometrical filter criterion according to CEM (2008) yields:d15F 4to5·d85B(1)With a grain size of the base material d 85B = 0,35 mm (sand), typical for the sea bed material in the NorthSea, the required grain size of the filter material is obtained from equation 1: d 15F = 1,40 mm to 1,75 mm. Inthis example d 15F = 1,75 mm is selected with a uniformity coefficient Cu = 5 to reduce segregation effectsand a filter layer thickness D F = 0,50 m (see BAW, 1989). The grading of the armour and filter layer (simplifiedrepresentation) and that of the base material are given in Figure 6, resulting in d 85F = 17 mm for the filtermaterial.Second step: The thickness of the armour layer is now recalculated as a function of the properties of bothfilter material (d 85F and relative density of rock ρ s,F / ρ w to that of water) and armour material (d 15D, d 85D andrelative density ρ s,D / ρ w ) according to the formula derived by Wörman (1989): sD,/ w1 nDd85D·d15DDD 0,16·· ·(2) 1sF ,/ w1 nDd85FConsidering (ρ s,D = 2900 kg/m³, ρ s,F = 2650 kg/m³ and ρ w = 1025 kg/m³), a layer thickness of the armourlayer D D = 1.2 m (instead of 0.9 m) will be required.Silt Sand Gravelfine medium coarse fine medium coarse fine medium coarseCobblesBouldersd 85B = 0,35 mmd 85F = 17 mmMass distribution in %d 15F = 5 · d 85Bd 15F = 1,75 mmSieve sizeGrain diameter in mmFigure 7: Grading of base, filter and armour materialThird step: For considering the accuracy of underwater placement in offshore environment a minimumthickness of the filter layer should be met. The thickness of the filter layer D F can be chosen by the commendationof CEM (2008) with D F ≥ 0.50 m.Among others the stability against erosion of the uncovered filter layer during the different constructionphases should be analysed and a monitoring and maintenance programme should be elaborated.VCONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCHA comprehensive review of the available filter criteria was performed and those particularly suitable forscour protection of offshore structures such as monopiles were comparatively analysed (see also details inSchürenkamp, 2011). Particularly critical is the contact erosion due to the upwards directed oscillatory flowwhich is not or not properly taken into account in the available design formulae. The same is valid for thecombined effect of both oscillatory flow (waves) and unidirectional flow. Solely based on the present knowledgeand the available experience it is hardly possible to reliably design and properly construct a granularfilter for a scour protection of offshore structures such as monopiles. Since considerable research and timewill be required to achieve this goal, a “hybrid” design approach is tentatively proposed and an example applicationis provided for illustration. The “hybrid” approach attempts to combine geometrical filter criteria todetermine such as the criterion recommended by CEM (2008) and hydraulically based criteria such as thosewhich are summarized by the formula by Wörman (1989). Application of the former allows to design the1033

ICSE6 Paris - August 27-31, 2012David Schürenkamp, Matthias Bleck and Hocine Oumeraci - Advanced Filter Designfilter according to the base material while the application of the later allows to adjust the thickness of thecover layer according to the properties of the filter and cover layer.One of the greatest difficulties for the development of design formulae is the consideration of the oscillatoryflow in orthogonal direction. Further important issues which urgently require further research is associatedwith the application of broadly graded material, including the development of reliable formulae for thedynamic stability (semi-stable filters), segregation behaviour in large water depths (fall pipes), etc.Both basic and applied research are needed to enhance the process understanding, and based on this improvedunderstanding to develop design formulae and guidance for the engineering practice. Candidate topicsare for instance: Flow velocity and shear stress distribution in both filter and armour layer. Pore pressure distribution in the base material (hydraulic gradients) Incipient motion/transport of granular material under combined waves and currents for different filterand cover layer configurations, including internal erosion Dynamic design concept/formulae for semi-stable filtersVIREFERENCESBakker, K.J.;Verheij, H.J.; de Groot, M.B. (1994): Design Relationsship for Filters in Bed Protection.Journal of Hydraulic Engineering, Vol. 120.BAW (1989): Merkblatt: Anwendung von Kornfiltern an Wasserstraßen (MAK). Karlsruhe: Bundesanstaltfür Wasserbau.Booij, R. (1998): Erosie onder en geometrisch open filter. TU Delft, Hydraulic Engineering, Report 1998-2,Delft, 25 S.CEM (2008): Coastal Engineering Manual. United States, A. and Knovel, eds., U.S. Army Corps ofEngineers, [Washington, D.C], Online-Ressource.CUR161 (1993): Filters in de waterbouw. Rapport / CUR 161, Civieltechnisch Centrum Uitvoering Researchen Regelgeving, CUR, Gouda, 212 S.de Graauw, A.;van der Meulen, T.; van der Does de Bye, M. (1983): Design criteria for granular filters.Delft: Delft Hydraulics, 25 Bl.Hansen, U.A. (1985): Wasserbausteine im Deckwerksbau - Bemessung und Konstruktion. Heide i. Holstein:Westholsteinische Verlagsanstalt Boyens & Co.Hoffmans, G.;Den Adel, H.; Verheij, H. (2008): Horizontal Granular Filters. 4rd Intern.Conference on Scourand Erosion ICSE-4, C-16, Tokyo, Japan, 480-485.Madsen, O.S.; Grant, W.D. (1976): Sediment transport in the coastal environment. Research Report.Department of Civil Engineering, School of Engineering, MIT, Cambridge, MA, USA, 105 p.Oumeraci, H. (1996): Filters in coastal structures. Proceedings of 2nd International Conference onGeofilters, Geofilters '96, Montreal, Canada, pp. 337-347.Oumeraci, H.;Grüne, J.;Sparboom, H.;Schmidt- Koppenhagen, R.; Wang, Z. (2007): Investigations on scourand scour protection for monopile foundation of wind offshore turbines. Forschungszentrum Küste(FZK), Hannover, pp. 79.Raudkivi, A.J. (1982): Grundlagen des Sedimenttransports. Berlin [u.a.]: Springer, 255 S.Schiereck, G.J. (2001): Introduction to bed, bank and shore protection. Delft, The Netherlands: DelftUniversity Press, 397 p.Schürenkamp, D. (2011): Filteraufbauten an Sohlsicherungen von Offshore-Windkraftanlagen. Leichtweiß-Institut für Wasserbau, Fachbereich Bauingenieurwesen, Technische Universität Braunschweig,Braunschweig, Germany, 183 S., M-1/2011.Sumer, B.M.; Fredsøe, J. (2002): The mechanics of scour in the marine environment. Advanced series onocean engineering, River Edge, NJ, USA: World Scientific Publishing, 552 p.Wenka, T.; Köhler, H.-J. (2007): Simultane Druck- und 3D-Geschwindigkeitsmessungen im Porenraumeiner Kiessohle. Bundesanstalt für Wasserbau, Mitteilungsblatt Nr. 90, Juli 2007, WasserbaulichesVersuchswesen, Karlsruhe, 119-136.Wörman, A. (1989): Riprap Protection without Filter Layers. Journal of Hydraulic Engineering, Vol. 115.Zichel, B.; Oumeraci, H. (2012): Kornfilter mariner Bauwerke. Berichte Leichtweiß-Institut für Wasserbau,Technische Universität Braunschweig, Nr. 1014, Braunschweig, Germany, 120.1034