the hydraulic and water quality performance of sustainable drainage ...

THE HYDRAULIC AND WATER QUALITY PERFORMANCE OF SUSTAINABLE
DRAINAGE SYSTEMS, AND THE APPLICATION FOR NEW DEVELOPMENTS AND
URBAN RIVER REHABILITATION
Michelle Malcolm, Bridget Woods–Ballard, Alain Weisgerber, Jeremy Biggs, Stella Apostilaki
HRWallingford , the DTI and Industry Partners and the EU
INTRODUCTION
The alteration of natural flow patterns (increases in both the total quantity and peak flows of run-off)
by the extension of built development can lead to problems elsewhere within the river catchment,
particularly flooding downstream. Amenity issues, such as water resources, community facilities,
landscaping potential and the provision of wildlife habitats have largely been ignored in past planning
and design of drainage systems. Continuing to drain built up areas without taking these wider issues
into consideration is not a sustainable long-term option.
Flood risk and other environmental damage can be managed by minimising changes in the volume and
rate of surface runoff from development sites through the use of sustainable drainage systems. This
paper will look at the potential benefits of incorporating SUDS components as part of development
drainage infrastructure, together with current perceived challenges to their implementation and
adoption.
The paper will discuss three current research projects being undertaken by HR Wallingford for the
DTI and industry partners and for the EC.
1. The first DTI project is concentrating on monitoring the hydraulic performance of several SUDS
components and their impact on water quality over a three year period, with attempts being made
to identify any influence of maintenance activities.
2. The second DTI project addresses in some detail three associated issues – public perception,
ecological benefits, and whole life costs, all important contributory factors in shaping designs and
in ensuring the long-term success of such systems.
3. The third EC project – URBEM – is investigating the use of SUDS for urban river restoration.
Many urban watercourses have been heavily engineered for flood control purposes. SUDS may be
a solution to allow the management of flood flows, which could in turn reduce the need for
engineered channels, and thus allow for the enhancement of the ecological and amenity functions
of these watercourses.
HYDRAULIC PERFORMANCE OF A TREATMENT TRAIN
For 18 months HR Wallingford has been undertaking monitoring of the hydraulic performance of a
SUDS treatment Train at Hopwood Service Station of the M42 near Birmingham.The treatment train
receives water form a conventionally drained car park, service area and road and treats this water in a
series of small ponds and a swales before it is discharged into the Hopwood stream.
Hydraulic data
We have so far collected, 18 months of rainfall data and flow data from 3 flow meters at the site.
Figure1 below illustrates the reduction of total volume from the inflow to outflow of the treatment
train as a function of season. In summer good reductions are achieved in the winter less, so and in
some cases there is more flow at the outlet in winter, this is because of constant groundwater fed base
flow. Figure2 illustrates the relationship between the size of the peak of the flow and the reduction in
the peak as the flow travels through the treatment train system. There is a significant reduction in peak
for most events, for the very largest events there is a smaller reduction in peak
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Michelle Malcolm et al
Volume reduction as a function of event date
40.0%
20.0%
Volume reduction ST-DS
0.0%
04/04/02 00:00 24/05/02 00:00 13/07/02 00:00 01/09/02 00:00 21/10/02 00:00 10/12/02 00:00 29/01/03 00:00
-20.0%
-40.0%
-60.0%
Very small event. The accuracy
limit of the instruments is
h d
Events following 2 dry months.
The conditions are summer-like
-80.0%
-100.0%
Figure 1
Lag time as a function of upstream peak flow
140
120
100
Peak lag time (min)
80
60
40
20
0
0 10 20 30 40 50 60 70 80 90 100
ST peak flow (l/s)
Figure 2
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TREATMENT TRAIN WATER QUALITY PERFORMANCE
HR Wallingford and the Environment Agency have been taking grab water quality samples at the site
over a 2 year period. These samples have been taken both in wet and dry conditions and illustrate the
effectiveness of the ponds in the removal of pollutants.
25
Coach Park Treatment Train (BOD)
20
15
10
5
0
Silt trap Interceptor Service yard
Total Wetland Wetland Swale Pond 2
08-Feb-00 17-Aug-00pond
13-Nov-00pond
30-Jan-01 24-Apr-01 Inputs 12-Nov-01 04-Dec-01 20-Feb-02 20-Mar-02
01-May-02 23-Oct -02 26-Nov-02
100
Coach Park Treatment Train (Lead)
90
80
70
60
50
40
30
20
10
0
Silt trap Interceptor Service yard
Total Wetland Wetland Swale Pond 2
08-Feb-00 17- Aug- 00pond
13-Nov-00 pond30-Jan-01 24-Apr-01 Inputs 12-Nov-01 04-Dec-01 20-Feb-02 20-Mar-02
01-May-02 23-Oct-02 26-Nov-02
This water quality data was further analysed as part of ecological performance research was
undertaken by Jeremy Biggs from the Ponds Conservation Trust under sub-contract to HR
Wallingford.
A major part of the research was analysing the water quality data collected by HR and the
Environment Agency against ecological indicators. It is valuable to look at water quality data in this
way because often SUDS water quality performance is expressed in percentage removal of
determinands. However a truer indication of performance is not how much of deteminand a SUD
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system can remove, but rather if the SUD system can remove enough of the determinand so as it will
not impact on the resident and downstream ecology.
140
120
Sus
100
pen
ded
soli
ds 80
(mg
/l)
60
TSS average 2000-2002
40
20
EU urban waste water
discharge standards
Mean for natural, minimally
impaired pond
0
Coach Park Silt
Trap
Interception Pond
Interceptor
Wetland
Swale
Balancing Pond
WHOLE LIFE COSTING
An understanding of the potential financial implications of taking responsibility for these systems in
the long-term is a vital consideration for any potential adopting authority, be they property
management companies, local authority service teams, or sewerage undertakers. The tools that are
being developed by the project should allow appropriate comparisons to be made between different
design solutions, and between sustainable systems and more conventional alternatives. Through
identification of the likely expenditure profile for the system over its design life, this should support
future operators in entering into maintenance agreements with increased confidence, and with
appropriate level of funding having been secured.
Whole Life Costing (WLC) is about identifying future costs and referring them back to present day
values using standard accounting techniques (such as Present Value). It is recognised as an
appropriate technique for use in valuing total costs of assets that have regular operating and/or
recurrent maintenance costs, based on formalised maintenance programmes.
The different approaches to WLC are presented below:
WHOLE LIFE COSTING
ECONOMIC APPRAISAL
FINANCIAL APPRAISAL
Non-Monetary Costs +
Monetary Costs
Environmental
Costs
e.g. flood damage,
pollution
Environmental
Benefits
e.g. amenity /
ecological
enhancement
Direct Costs
e.g. capital,
operating, disposal
costs, system
revenue
Indirect Costs
e.g. standby
equipment hire,
compensation
payments
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Selection of the most appropriate discount rate is one of the most contentious issues in whole life
costing, as it can have a significant effect on the outcome of the analysis. As a general guide,
calculations Present Value (PV) is the standard accounting method used, and is defined in MAFF
(1999) as “the value of a stream of benefits or costs when discounted back to the present time”. It can
be thought of as the sum of money that needs to be spent today to meet all future costs as they arise
throughout the life cycle of a scheme or structure. The formula for calculating the present value is
given by the following equation:
t
PV = ∑ = N
Ct
t= 0
r t
(1+
)
100
Where N = Time horizon in years; C t = Total monetary costs in year t; r = Discount rate
Using a high discount rate will make future costs less important, and a lower discount rate will
increase their significance. In the public sector, the discount rate is set by the Treasury. They are
currently recommending a rate of 3.5 % - a recent shift from a long-term historic value, which had
been set at 6 %. The Treasury are also recommending that for projects with long-term impacts, over
thirty years, a declining schedule of discount rates should be used rather than the standard discount
rate, as shown in the following table:
Period of years 0 - 30 31 - 75 76 - 125 126 – 200 201 - 300 300 +
Discount rate 3.5 % 3.0 % 2.5 % 2.0 % 1.5 % 1.0 %
The following graph shows the impact of time on the relative contribution of yearly costs to the total
in a 100-year analysis, using the current Treasury recommendations and the historic 6 % discount rate.
Discount Factor
1.2
1
0.8
0.6
0.4
0.2
0
0 20 40 60 80 100 120
Year
6 % Fixed Long Term Variable (from 3.5 %)
This confirms that, using the new Treasury recommendations, the contribution of operation and
maintenance expenditure will be of increased significance to the WLC of the drainage system.
sustainable drainage systems. In theory, with appropriate maintenance, their life should be unlimited
as there is no risk of structural failures.
A spreadsheet model was developed that allows costs to be built up for systems, depending on their
design criteria. The tool highlights all the important materials and processes, and allows the user to
incorporate additional, site specific costs should they so choose. The model presents a best estimate
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of ‘average’ or ‘likely’ unit costs, but these should be reviewed and adjusted for site-specific
application.
Retention Pond
Development Characteristics
Default
User Entry
Area of development 1 ha
Percentage impermeability 80 %
WRAP Soil Class (1-5) 4
SOIL factor 0.45 0.00
M5-2D rainfall 50 mm
M5-60 as a fraction of M5-2D 0.40
M5-60 20 0 mm
Hydraulic Design: Standard of Service
Assumptions for hydraulic SoS:
1) 80 % impermeability
2) South East England (Wokingham climate)
Return Period (choose from 10, 30, 50, 100 yrs 50 yrs
Throttle Rate (choose from 3,7,10 l/s/ha) 3 l/s/ha
Computed detention rate (using simplified rainfall runoff model) 325 m 3 /ha
Computed detention volume (using simplified rainfall runoff model) 325 m 3
Total Detention Volume 325 m 3
Water Quality: Standard of Service
Treatment volume rate, Vt/ha 152 m 3 /ha
Treatment volume rate, Vt 152
Permenant pond volume factor (should equal 4 X Vt (Ciria C522)) 4 factor
Total Treatment Volume 608 m 3
There are a range of design characteristics that will influence the cost of a sustainable drainage
scheme. The model default assumes characteristics based on current design best practice, as set out in
the CIRIA design manuals. However, the user may also enter site-specific features. The following
figure shows an example of model performance criteria, for a retention pond:
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Retention Pond
Default
User Entry
Geometric Characteristics
Length:Width Ratio (3:1 recommended) 3 X:1
Average Permanent Pool Depth (1-2 m recommended) 1.5 m
Maximum Total Pool Depth (max 3 m and max 2 m above permanent depth) 3 m
Freeboard 0.25 m
Pond Side Slopes (max 1 in 4 recommended) 4 1in X
Shallow Benching
Y
Width of Shallow Bench (model assumes 10% of permanent pond area ) 1
Design Chacteristics: Infrastructure
Forebay / Silt Trap Pond (assume 10% of total pond, Vt) Y Y/N
Engineered Silt Trap Structure N Y/N
Design charactersitics: Excavation
Will excavated material be disposed of on site Y Y/N
Is the soil hard (chalk, stiff clay, soft rock, broken rock) N Y/N
Design Chacteristics: Planting
Aquatic planting (width of marginal vegetation) Y Y/N
Will there be bankside / barrier planting Y Y/N
Bankside / barrier planting (width of bankside vegetation) 5 m
Land allocation
Additional width around pond allocated for essential access / maintenance 5 m
Will there be an area allocated for sediment de-watering? Y Y/N
Additional area allocated for sediment dewatering (% of pond area) 20 %
Will there be an extra area allocated for amenity/recreation? Y Y/N
Additional area allocated for recreation / amenity benefit (% of pond area) 5 %
Easement width
m
Easement length
m
This model will then be validated against the information available from literature, as indicated in the
following figures. Actual cost values cannot currently be published, due to ongoing work in this area.
Texas HA (2001)
Amenity pond,with reedbed
Co
st
(£)
Amenity pond, large concrete
weir outlet
Weigland (1987)
SWRPC (1991)
Schueler (1991)
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000
Volume (m 3 )
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Sustainable drainage schemes require ongoing maintenance in order to ensure short-term operation
and minimise risks to long term performance. Maintenance activities are likely to comprise a major
contribution to total life costs, and the lack of understanding and quantification of these to date is
currently one of the barriers to facilitating adoption agreements.
Operation and maintenance activities can be classified as follows:
• Monitoring
• Regular, planned maintenance (e.g. rodding culverts, clearing debris from manholes, grass-cutting,
vegetation management, sediment removal, jetting of permeable surfaces and silt traps)
• Unplanned maintenance / rehabilitation (e.g. responding to problems e.g. blocked culverts/trashracks,
pollution incident, vegetation death etc)
• Intermittent maintenance (e.g. for major mid-life refurbishment such as geotextile replacement,
vegetation replacement, soakaway replacement etc).
REGULAR MAINTENANCE (SUB-ANNUAL/ANNUAL)
Activity Frequency Frequency Unit rate Unit rate Unit Quantity Quantity Total
(times per year) (times per year) (£) (£) (Nr. of units) (Nr. of units) costs
model User model User Model User per visit (£)
Mow amenity and easement 12 2.67 100m 2 0.35 0.93
Mow pond surrounds and access 12 2.67 100m 2 4.81 12.83
Collect litter from amenity and easement 12 5.00 100m 2 0.35 1.75
Collect litter from around the pond 12 5.00 100m 2 7.64 38.16
Collect litter from pond 12 5.00 100m 2 7.00 35.00
Manage bank vegetation 2 38.50 100m 2 0.96 36.79
Manage aquatic vegetation 2 0.00 100m 2 1.26 0.00
Check inlets and outlets 12 24.30 nr 1 24.30
Rip-rap and weir inspection 12 25.00 nr 1 25.00
Emptying of engineered silt trap 0 0.00 hour 1 0.00
Additional monitoring
IRREGULAR MAINTENANCE (REFURBISHMENT AT > 1 YR RECURRENCE INTERVAL)
Activity Frequency Frequency Unit rate Unit rate Unit Quantity Quantity Total
(Nr of years
between visits)
(Nr of years
between visits) (£) (£) (Nr. of units) (Nr. of units) costs
Model User model User Model User per visit(£)
Planned Refurbishment
De-silting forebay 3 15.44 m 3 16.25 250.92
De-silting pond 0 15.44 m 3 0 0.00
Mobilisation for silt removal 3 5 nr 15 76.03
Landfill charge (special waste) 3 100.00 m 3 15 1521.00
Replace topsoil and planting 25% 0 19.34 m 2 46 887.12
Replace geotextile 0 7.15 m 2 46 327.92
Unplanned Rehabilitation
clearing inlet and outlet blockages. 0 100.00 nr 0 0.00
Construction Silt
Remove construction silt (inc: de-silting and landfill as (special waste)) 0 26.25 m 3 0 0.00
Mobilistation for construction silt removal 0 5 nr 0 0.00
RESULTS AND CONCLUSIONS OF WLC MODEL
Generic profiles similar to the following are currently being developed for different SuD components,
and guidance provided on how to use the simple spreadsheet model, for site-specific application.
The project has demonstrated the relevance of using a Whole Life Costing approach for planning and
implementing sustainable drainage systems on new developments. It is hoped that such a tool will
enhance industry confidence in SUDS, and support the development of long-term operation and
maintenance agreements
Comparison WLC Pond and Storage Tank
3.5% disount rate, 1 ha catchment, 50 yr design event
Retention
Pond
£
Underground
Storage
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
Years
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PUBLIC PERCEPTION OF SUDS PONDS
Stella Apostoklai from Abertay University worked under subcontract to HR Wallingford, to assess the
public’s perception of SUDS ponds A survey was undertaken of residents living close to SUDS ponds.
The purpose of the survey was to determine the level of knowledge people had about the function of
SUDS ponds and to assess what were the perceived advantages and disadvantages with the ponds.
Pond's Perceived Disadvantages
70.0%
70%
Safety Risks
60%
Pe
rce
nta 50%
ge
s
of
Re
40%
sp
on
de 30%
nts
20%
10.0%
Litter pollution
None
Not aesthetically pleasing
Vandalism
10%
5.0%
0%
Percent.
Perceived disadvantages of the new pond
Pond's Perceived Disadvantages
50%
45%
40%
35%
30%
25%
20%
15%
10%
5%
0%
45.5%
18.2%
9.1%
1
Perceived Disadvantages of the
established pond
None
Floods over the
road
Increased traffic
due to visitors
Vandalism
Badly maintained / It
is silted up
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THE USE OF SUDS IN URBAN RIVER REHABILITATION
HR Wallingford is co-ordinating an EC funded project – URBEM – urban river enhancement
methods. The project has 12 partners from 6 European countries and will over the next 2 years. One of
the outputs from the URBEM project is to develop a decision support systems for determining the
urban rivers with the most potential for enhancement.
The potential of urban rivers will be considered under five categories:
Hydraulic functioning
Ecological Status
Amenity
Aesthetics
Social and economic
A range of methods for enhancing rivers will be considered from a catchment to channel scale.
Spatial scale
Catchment
Sub-catchment
Channel
Methods for improving urban rivers
Planning
Land use control
Abstraction planning and consents
Discharge Planning consents
Water framework directive planning
SUDS
Attenuating new development to green field run off rates
Treating storm water so it has no adverse effect on in-stream ecology
Green corridor
Cycle ways and footpaths
Planting, landscape improvements
Bins, streetlights, seat
SUDS
Retrofitting CSO’s
Geomorphological and engineering solutions
Day lighting
Meanders, riffle pool
Fish passes
SUDS can be used at catchment or sub-catchment level to reduce the cumulative impact of new
development on urban rivers. Also be used as engineering solutions at the channel scale with potential
to be used to improve local water quality problems – for example reed-beds to treat CSO discharges.
SUDS also offer a potential solution in improving channels that are heavily engineered for flood
alleviation purposes. Where sufficient land exists SUDS could be used locally to reduce the flood
flows and potentially the needs for such an engineered channel.
REPORTS
More information can be found on the research discussed in this paper from a series of technical
reports available from HR Wallingford.
HR Wallingford 2003: SR622 Assessment of the Social Impacts of Sustainable Drainage
HR Wallingford 2003: SR625 Maximising the Ecological Benefits of Sustainable Drainage Schemes
HR Wallingford 2003: SR626 The Operation and Maintenance of Sustainable Drainage and
Associated Costs
HR Wallingford 2003: SR627 The Whole Life Costing for Sustainable Drainage
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