Sustainable DrainagePresented ByPaul StewartAssociate, Mayer Brown LimitedWoking, Surreypstewart@mayerbrown.co.uk
Sustainable DrainageWhat I will be discussing• Define SuDS• Timeline of Events & Planning Context• Flood & Water Management Act – What it Means for SuDS• Design Principles• Masterplanning• Costs• Research
Sustainable DrainageWhat Do You Think SuDS are?• Sustainable (urban?) Drainage Systems• A sustainable drainage system employs a range oftechniques, which vary depending on a range of variables toachieve 3 main aims: • Control of run-off (both rate and volume considered) • Control & treatment of pollution • Amenity (biodiversity / visual amenity / recreation?)• There is rarely any one appropriate design to a givensituation.•Details discussed later.......
Sustainable DrainageTimeline Publications Events1998 Easter 1998 Floods 5 deaths, £400M damage, 1500 people evacuated19992000 March 2000 First CIRIA SuDS Design Guide Autumn / Winter 2000 Floods £1 Billion damage, 11,0002001 July 2001 – PPG25 people evacuated October 20012002 CIRIA Best Practice Guidance2003
Sustainable DrainagePPG25 - July 2001“Local Planning Authorities should, therefore, workclosely with the Environment Agency, sewerageundertakers, navigation authorities and prospectivedevelopers to enable surface-water run-off to becontrolled as near to the source as possible by theencouragement of sustainable drainage systems”Part H Building Regulations also updated to promoteinfiltration.
Sustainable DrainageTimeline Publications Events2004 July 2004 August 2004 – Boscastle National SuDS Working Party Floods Interim SuDS Code of Practice2005 January 2005 – Carlisle Floods 3 Fatalities2006 December 2006 - PPS 25
Sustainable DrainagePPS25 - December 2006SuDS features heavily:“RPBs and LPAs should further the use of SUDS by:• incorporating favourable policies within Regional Spatial Strategies;• adopting policies for incorporating SUDS requirements in LocalDevelopment Documents;• encouraging developers to utilise SUDS wherever practicable in thedesign of development, if necessary through the use of appropriateplanning conditions or by planning agreements;• developing joint strategies with sewerage undertakers and theEnvironment Agency to further encourage the use of SUDS as an aidto mitigating the rate and volume ofsurface water flows; and• promoting the use of SUDS to achieve wider benefits such assustainable development,water quality, biodiversity and local amenity.”
Sustainable DrainageTimeline Publications Events2004 July 2004 August 2004 – Boscastle Floods National SuDS Working Party Interim SuDS Code of Practice January 2005 – Carlisle Floods2005 3 Fatalities2006 December 2006 - PPS 252007 February 2007 June / July 2007 Floods CIRIA 697 – The SuDS Manual 13 fatalities 600 pages of guidance! Predominantly local flooding of2008 urban areas June 2008 – The Pitt Review November 2009 – Cumbria Floods October 2008 – Planning Regs 316mm in 24 hours – UK Record2009 for driveways changed 1 Fatality – transport links severed
Sustainable DrainageSo, a question to see who’s still awake!What has been the problem over the last 10 years?Almost 10 years on from PPG25 and 4 years on fromPPS25, why has so little progress been made on theimplementation of SuDS? No Legislation compelling the use of SuDS............................................................................................... No body made responsible for SuDS Adoption............................................................................................... Water Company ‘concerns’ about legal definitions in the Water Industry Act preventing connections...............................................................................................Remember Sir Michael Pitt’s Review?Pitt made 92 recommendations to the Goverment. These ledto the Flood & Water Management Act 2010.
Sustainable DrainageRECOMMENDATION 9:Householders should no longer be able to lay impermeablesurfaces as of right on front gardens and the Governmentshould consult on extending this to back gardens andbusiness premises.1st October 2008 – Change to permitted development rightsfor paved surfaces >5m2RECOMMENDATION 10:The automatic right to connect surface water drainage ofnew developments to the sewerage system should beremoved.Flood & Water Management Act 2010RECOMMENDATION 20:The Government should resolve the issue of whichorganisations should be responsible for the ownership andmaintenance of sustainable drainage systems.Flood & Water Management Act 2010
Sustainable DrainageMain SuDS Issues• A body will be created at Local level known as the SuDSApproval Body (SAB).• As drafted, the Act defines the SAB as the County Councilor Unitary Authority. So in your area the SABs will be: • Hampshire County Council • Portsmouth City Council • Southampton City Council • Isle of Wight Council
Sustainable DrainageWhat is the Remit of the SAB?• Approval of ALL drainage works. • “Construction work which has drainage implications may not be commenced unless a drainage system for the work has been approved by the approving body.”How will the planning authority be involved?• If the works require planning approval, the SuDS approvalapplication may either be free standing, direct to the SAB, orcombined with a planning application.• If the latter, the planning authority must consult the SAB (ifdifferent) and inform the SAB of its planning decision.• When informing the applicant of the planningdetermination, the planning authority should also informthem of the SAB’s determination.
Sustainable DrainageDoes the Act sort out adoption?Yes!!The SAB must adopt and maintain an approved drainagesystem, or part of a drainage system which serves morethan one property and meets the criteria. (except PublicHighways)“You also ask whether shared areas of pervious pavement,such as parking courts would be adoptable SuDS. Under theFWMA, permeable surfaces that form part of a SuDS thatserve more than one property will be adoptable in newdevelopments and redevelopments.” - DEFRAAlso, the Sewerage Undertaker must accept water from anapproved (SuDS) drainage system.
Sustainable Drainage SuDS Approval & Adoption Timescales (These are best guesses, as not fixed!)Early 2011 Publish Consultation Draft National SuDS StandardsOctober 2011 Publish National SuDS StandardsApril 2012 SAB Duties Commence?? – May be phased in Another Question..... So what happens to developments between now and then?? a)Continue Ad Hoc private / public adoption & resistance from Southern Water. b)Early negotiations with SAB organisations to agree retrospective adoption.
Sustainable DrainageA couple of loose ends.....• Funding for maintenance is uncertain. The Act makes provision forchecking fees and performance bonds, but is strangely quiet on how theextra burden will be paid for.• DEFRA claim SuDS maintenance will be funded in full, but have todate given no details of this.• In discussion with Southern Water regarding Waterlooville MDA, thereis some uncertainty over the upstream interface of SuDS & Sewers.Southern Water currently claim that all surface water pipework upstreamof the final SuDS feature will be part of the Drainage System andtherefore adoptable by the SAB. (I disagree, but it needs to be clarified)• This means that on the future Waterlooville MDA, there would be noSouthern Water surface water sewers on a development of 2550 (STP)dwellings. They would be SAB maintained.
Sustainable DrainageSuDS ComponentsReference CIRIA C687Planning For SuDS – Making It Happen
Sustainable Drainage Levels of Service Technical definitions Return period – The statistical probability of a particular storm occurring within a particular period. 1 in 100 year storm will statistically occur once every 100 years. Storm duration – The length of a design rainfall event. •BS EN 752 : 2008 / Sewers for Adoption 6th Edition standard return period designs. • Rainfall attenuated on site to the 1 in 100 year RP (Not necessarily all in SUDS features) • Overland flow checked against 1 in 200 year RP (Association of British Insurers / Water Companies) • Climate Change – PPS 25 suggests 10%, 20% or 30% increase in rainfall intensity depending on design horizon.
Sustainable Drainage Infiltration •The Perfect SUDS situation is where 100% of run-off can be infiltrated back to ground. • Consider infiltration first and test if borehole records or site knowledge suggest that there is potential. • Be sensible! Do not insist on infiltration tests of boreholes show significant depths of dense clay. • Groundwater recharge is desirable, however groundwater should be protected from pollution. • Where possible avoid traditional soakaways without prior water quality treatment. • Check with EA for groundwater Source Protection Zones and consult on restrictions.
Sustainable Drainage Infiltration • In Hampshire, much of the potable water is taken from Chalk Aquifers. Revised EA groundwater protection guidance was issued in 2008 - GP03. • EA policy:
Sustainable Drainage Infiltration • So what happens in Flood Zone 1? EA Interpretation of policy in a recent consultation: “In line with policy P4-12 of GP3, we will also object to the use of Sustainable Urban Drainage Systems (SuDS) at this location for the discharge of surface water run-off. Surface water from areas of car parking should be directed to the nearest surface water sewer.” • This is slightly at odds with the normal EA support, which SuDS enjoys. • In chalk areas there are likely to be a lack of surface water sewerage. I understand that the policy may be clarified. in practice, SuDS could be used subject to EA agreement, but would need to prevent direct infiltration and achieve a high level of treatment.
Sustainable DrainagePollution Treatment• A number of methodologies have been proposed byacademics to score the effectiveness of SuDSfeatures. The SuDS Manual (CIRIA C697) contains atable suggesting minimum numbers of treatment traincomponents
Sustainable Drainage Run-off Treatment • Treatment Train – multiple levels of SUDS are best for treatment. • Source Control – Initial treatment should occur as close to the pollution source as possible. Examples: • Pervious paving to car parks and private drives • Filter Strips & Swales (alongside roads) • Bioretention areas and rain gardens • Green / Brown roofs (roofs attract airborne pollutants) • Source control techniques are very effective at reducing the pollutant loading on the downstream features, especially where they trap the most polluted ‘first flush’ of a rainfall event. • Source control features are principally for pollutant removal and initial volume losses. Although they can be designed with additional volume storage if necessary.
Sustainable Drainage Run-off Treatment • Local Treatment or Site Controls - Typically catchments and sub-catchments up to 5 Ha • Basins • Ponds • District Treatment or Regional Controls – Typically final treatment and balancing from catchments over 5Ha. • Lakes • Wetlands • Treatment Volume (Vt) – Permanently wet final treatment is good. The permanent volume should be equivalent to at least 10mm of Rainfall • Conveyance features such as swales & reed channels can provide additional treatment, particularly where they are designed to settle sediments. *See CIRIA 697 Para 1.3.4 for processes and applications
Sustainable Drainage Run-off Volume Reduction (Interception Storage) • In nature, very little of the initial rainfall runs off to watercourses, it soaks into the upper soil. (Even in Clay areas. • Coincidentally in developed areas this initial ‘first flush’ carries the most pollutants, particularly following long dry periods • CIRIA 697 introduces the concept of ‘Interception Storage’. This is the aim to capture at least the first 5mm of rainfall and dispose of via filtration, evaporation or evapotranspiration. Features used include: • Swales (check dams and underdrains assist) • Green roofs • Pervious pavements • Bioretention areas
Sustainable Drainage Environmental Enhancement • Vegetated SUDS systems should usually be given first priority over pure engineering solutions, as their operation is easier to observe and pollution incidents can be easily detected. (Although underground tanks are sometimes appropriate and should be given due consideration) • Environmental Enhancement could be defined in a number of ways. Such as: • Habitat creation promoting ecological benefits • Public space for rest or recreation • Care should be taken to avoid designing unattractive or inaccessible spaces which detract visually or attract antisocial behavior • SUDS designers should include or work closely with Landscape professionals.
Sustainable Drainage Run-off Rate Reduction (Greenfield) • SUDS on greenfield developments should aim to discharge surface water at rates not exceeding the calculated greenfield rate • ‘Sites’ (catchments) under 200Ha – use Institute of Hydrology Report 124 equation to calculate the ‘mean annual flood’ QBARrural. Then apply Flood Studies Report Regional Growth Curves to derive permitted rates for different return periods. • However, if no long term volume storage applied (explained shortly) a flat rate of discharge should be applied at the greater of QBAR or 2l/s/Ha. • Example – Newlands Common, Waterlooville
Sustainable Drainage Run-off Rate Reduction (Greenfield Example) •Example – Newlands Common, WaterloovilleWith urbanisation of less than 5%, the QBARrural equation (equation 7.1) in IOH 124 is appropriate, and can be expressed asfollows:QBARrural = 0.00108 AREA0.89 SAAR1.17 SOIL2.17Where,QBAR = Mean annual floodAREA = Catchment areaSAAR = Standard average annual rainfall (790mm from FEH CD-ROM)SOIL = Soil index (assumed as 0.45, based on FSR WRAP class 4)Using the above formula, a catchment area of 50 Ha results in a QBAR of 250l/s.To produce greenfield run-off rates for a range of return periods the Flood Studies Report regional growth factors are applied asfollows: 2 5 10 20 30 50 100 100 Return +20% Period (1 in x) Growth 0.88 1.28 1.62 2.00 2.26 2.62 3.19 3.828 Factor 50 Ha 220 320 405 500 565 655 798 957 discharge (l/s) Discharge 4.40 6.40 8.10 10.00 11.30 13.10 15.96 19.14 Rate (l/s/Ha)
Sustainable Drainage Run-off Rate Reduction (Previously Developed) • PPS 25 seeks to mimic the pre-development surface water flows and where practicable reduce flood risk. • This is open to a certain amount of interpretation and neither PPS 25 or CIRIA C697 describe how to treat discharges from previously developed areas. Local Planning Policies should consider the brownfield scenario and provide guidance. In practice, the EA seek a (varying) degree of betterment. • My opinion is that the pre-development calculated flows from frequent storms such as the 1 in 2 year should be taken as the limiting discharge factor with SUDS attenuating this rate to the 1 in 100 year return period. This provides betterment for the more severe storm events. • The other SuDS principles still apply!
Sustainable DrainageRun-off Volume Reduction (Long term storage)• If we simply attenuate run-off rates, we are not actually simulatinggreenfield conditions, as the total volume of run-off is not being reduced.• This will lessen the benefits of SUDS to the downstream floodplains oflarge rivers, where flood waters tend to build over extended time periods.• Therefore, CIRIA 697 introduces the concept of long term storage, wherethe aim is to capture the difference in volume between the greenfield anddeveloped situation for the 1 in 100 year 6 hour duration storm (arbitraryfigure).• This volume is segregated and either infiltrated over an extended periodor discharged at a rate < 2l/s/Ha.• If no volume storage is provided, extended attenuation’s required at arate of QBAR or 2l/s/Ha.
Sustainable DrainageMasterplanning• Some controls can be incorporated in any development, regardless ofmasterplanning issues – Pervious Paving / Green Roofs / RainwaterHarvesting• However, for best results SuDS design should be considered at an earlystage to make best use of the topography.• Assess the existing drainage routes, overland flow, conveyance,disposal. The best SuDS mimic this process.• Allow space adjacent to impervious paving for linear SuDS.• Overlook open space SuDS to enable residents to ‘take ownership’• Slow water is best , follow contours with swales where possible, perhapsalign roads to contours. Cascade pools across contours.
Sustainable DrainageCommon Misconceptions• My site’s on Clay, so is unsuitable for SUDS. • Wrong. SUDS is not just soakaways. Applicants making statements like this are ill-informed (or trying it on!)• No space for SUDS, conflict with PPS3 densities. • Wrong. SUDS can be incorporated as Public Open Space, or counted as essential infrastructure and excluded from density calculations.• SUDS is expensive and less houses affects profitability • Debateable. Studies have shown that SUDS can be cheaper to build / maintain and can add a premium to house prices around well designed features• SUDS is experimental. There’s no design guidance. • Where have you been for the last 10 years. Design manuals from CIRIA since 2000 (C522). Current best practice CIRIA C697 (2007) is 604 pages.
Sustainable DrainageLand Take Issues.• Land take for SUDS varies considerably from site to site, anddepends primarily on infiltration potential and prior use.• In addition the wider employment of source control SUDS, suchas green roofs, pervious pavements and bioretention areas willreduce the space requirement for strategic SUDS. “L10 - Housing•The worst case would be for a greenfield site on heavy clay. developments will be permitted if they contribute towards the provision of:• An example of such a site is Newlands at Waterlooville. Original a open space in the area to the National Playing FieldsApplication1550 units + commercial in Hampshire on clay site. Association (NPFA) standards;SUDS is approximately 6% of the red line boundary, and falls b general amenity space as part of the scheme. “within the requirement for Public Open Space.• Increased land values are often reported around well designedand landscaped SUDS features.
Sustainable DrainageSource Control Approximate Costs• Green Roofs (cost of coverings) • Extensive green roof (50mm) - £65/m2 • Welsh slate - £65/m2 • Eternit fibre slate - £45/m2 • Concrete Tiles - £25/m2 (Loadings to consider, 0.7kN/m2 for extensive green roof / 0.4kN/m2 for tiles)• Pervious Pavements light duty (1msa) • Pervious block paving (Clay) - £70/m2 • Porous asphalt (car park) - £40/m2 • Standard block paving (Clay) - £60/m2 • Standard asphalt - £40/m2 •Plus possible savings on drainage?• Bioretention area • Intentional ponding in landscaped area. •Same cost as verge / small landscaped area
Sustainable DrainageCommuted Sums• Newlands agreed rates for 20 year commuted sums (per m2) were as follows(May 2006):• Swales £6.54*•Attenuation Basins £5.70*•Wetlands £10.08 (£12.59)•Lake £6.52** Rates increased to normal Public Open Space rate of £7.43 (£9.28)
Sustainable DrainageWaterlooville SuDS Research• In 2003 the EA, Grainger and the Local Authorities set up a group to studythe effectiveness of SuDS for the Waterlooville MDA.• The aim was that the research would be unique in that it would look at thebaseline conditions, construction phases and post construction and was partfunded by developer contributions and the EA.• The baseline conditions of the River Wallington and the flow from the sitehas been undertaken by the EA.• Due to EA financial pressures Mayer Brown took on greater responsibility forthe research and applied for government funding to recruit a researchassociate in cooperation with the University of Portsmouth.• The continuation of the project will depend on future funding, but we wouldlike to continue with the original aims of the project.
Sustainable DrainageWaterlooville SuDS Research
Sustainable DrainageWaterlooville SuDS Research• Hampshire Example – West of Waterlooville• What has been constructed is the main accessjunction and associated SUDS, including:• Bioretention Area
Sustainable DrainageWaterlooville SuDS Research• Hampshire Example – West of Waterlooville• What has been constructed is the main accessjunction and associated SUDS, including:• Bioretention Area•Swale
Sustainable DrainageWaterlooville SuDS Research• Hampshire Example – West of Waterlooville• What has been constructed is the main accessjunction and associated SUDS, including:• Bioretention Area• Swale• Pond
Sustainable DrainageWaterlooville SuDS Research•Ponds well oxygenated with low BOD – typical of road runoff ponds•Pond outlet NH4+, TON, sPO4 very low and less or same as river
Sustainable DrainageWaterlooville SuDS Research•Heavy Rainfall Event – 22nd January 2010•COD very high in runoff.•Progressively reduced acrosstreatment train.•Pond outlet less than river.
Sustainable DrainageWaterlooville SuDS Research•Heavy Rainfall Event – 22nd January 2010•Ammonia reduced less in the initial stages.•Soluble pollutant.•Good removal efficiency in pond components. Road After Pond River Runoff Swale Outflow Wallington