HA 118/06 - Standards for Highways

1 May 2006 design MANUAL FOR ROADS AND BRIDGESVOLUME 4 GEOTECHNICS ANDDRAINAGESECTION 2 DRAINAGEPART 8HA 118/06 design OF SOAKAWAYSSUMMARYThis advice note gives design guidance on howsoakaways may be incorporated into systems used totreat and store road runoff prior to discharging toground. It describes the steps needed to protectreceiving groundwater and the constraints these mayplace on soakaway design and construction. Thisdocument provides general design advice, howeverthere are a number of design procedures describedherein which must be adopted as mandatory practice,for which reference is made to HD 33 (DMRB ) FOR USEThis is a new document to be incorporated into Contents pages for Volume new Contents pages for Volume 4 datedMay HA 118/06 into Volume 4, Section 2,Part archive this sheet as : A quarterly index with a full set of VolumeContents Pages is available separately from TheStationery Office 118/06 design of SoakawaysSummary.

2 This advice note gives design guidance on how soakaways may beincorporated into systems used to treat and store road runoff prior todischarging to ground. It describes the steps needed to protect receivinggroundwater and the constraints these may place on soakaway design andconstruction. This document provides general design advice, however there area number of design procedures described herein which must be adopted asmandatory practice, for which reference is made to HD 33 (DMRB ) MANUAL FOR ROADS AND BRIDGESTHE Highways AGENCYTRANSPORT SCOTLANDWELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRUTHE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELANDV olume 4 Section 2 Part 8 HA 118/06 May 2006 REGISTRATION OF AMENDMENTSA mendPage NoSignature & Date ofAmendPage NoSignature & Date ofNoincorporation ofNoincorporation ofamendmentsamendmentsRegistration of AmendmentsVolume 4 Section 2 Part 8 HA 118/06 May 2006 REGISTRATION OF AMENDMENTSA mendPage NoSignature & Date ofAmendPage NoSignature & Date ofNoincorporation ofNoincorporation ofamendmentsamendmentsRegistration of AmendmentsVOLUME 4 GEOTECHNICS

3 ANDDRAINAGESECTION 2 DRAINAGEPART 8HA 118/06 design OF of Soakaway and Construction of Soakaway and Management of MANUAL FOR ROADS AND BRIDGESMay 2006 Volume 4 Section 2 Part 8 HA 118/06 May 20061/1 Chapter 1 Introduction1. This Advice Note gives guidance on howsoakaways may be incorporated into systems used totreat and store road runoff prior to discharging toground. This Advice Note has been compiled tosummarise and review the current available designguidance in use within the UK on the selection andconstruction of soakaways . The design guidanceprovided in this document focuses on point dischargesto the ground from single soakaways (or combinationsof these).

4 These are defined as drainage structures (pits,chambers and trenches) that allow infiltration to theground through their base and sides and that incorporatebelow ground The Advice Note originates from a review ofavailable information on the fate and transport of roadcontaminants and the design of existing road soakawaysystems. The review focussed primarily on anunderstanding of the processes operating in theunsaturated zone ( above the water table) and howthese may influence soakaway design and preventgroundwater In most of the UK the term Highways isequivalent to Scottish roads . In this document, as withthe guidance provided in HA 216: Road Drainage andthe Water Environment (DMRB ), the term roads will be used as standard This document provides general design advice,however there are a number of design proceduresdescribed herein which must be adopted as mandatorypractice, for which reference is made to HD 33: Surfaceand Sub-Surface Drainage Systems for Highways (DMRB ) as There is growing awareness by theEnvironmental Protection Agencies (EPAs) that roadrunoff may, under certain circumstances, have anadverse effect on receiving waters, includinggroundwater.

5 This has arisen both because of improvingknowledge of the polluting content of road runoff andbecause of improved treatment of other sources ofpollution. Contact details for the various EPAs can befound on their respective web- Overseeing Organisations have a duty underpollution protection legislation to ensure that roadrunoff does not pollute receiving waters. This can ariseboth from the effects of routine runoff and fromspillages on the road. For discharges to ground, TheGroundwater Regulations 1998 (SI 1998 No 2746) inEngland, Scotland and Wales or The GroundwaterRegulations (Northern Ireland) 1998 (Statutory Rule NI1998 No 401) include lists of substances whose entry togroundwater must be prevented or controlled.

6 Thesesubstances are commonly referred to as List I and ListII substances. Details of these substances are set out inHA 216 (DMRB ). Because consents are notrequired for the discharge of road runoff, theresponsibility is with the Overseeing Organisations todischarge their duty not to pollute receiving waters byensuring that systems for the treatment of road runoffare installed when necessary. EPAs should, however, beconsulted about proposals for discharging road runoff,as they have the power to serve prohibition notices inrespect of discharges that are in breach of pollutionlegislation. Implementation into UK law of the WaterFramework Directive (2000/60/EC) ( The WaterEnvironment (Water Framework Directive) (Englandand Wales) Regulations 2003 (SI 2003 No 3245);(similar regulations pertain to Scotland and NorthernIreland) has given added weight to the need to protectexisting water The disposal of road runoff through soakawaysystems is reasonably well established within the , the attenuation mechanisms operating on roadrunoff to reduce pollutant loadings at the dischargepoint below the soakaway, and within the unsaturatedzone above the water table, are currently poorlyunderstood.)

7 Changes in design practice, as well aslegislation controlling the discharge of pollutants tocontrolled waters and the results of future research intothe fate of pollutants in soakaways and the unsaturatedzone may cause current guidance to be design of other drainage structures such as lineardrains is discussed in HD 33 (DMRB ). The use ofvegetated systems to treat road runoff is discussed inHA 103: Vegetated Drainage Systems for HighwayVolume 4 Section 2 Part 8 HA 118/06 May 20061/2 Chapter 1 IntroductionRunoff (DMRB ). HA 216 (DMRB )provides guidance on assessing the risks to groundwaterfrom road There are three major objectives in the drainageof roads: the speedy removal of surface water to providesafety and minimum nuisance for the road user; provision of effective sub-surface drainage tomaximise longevity of the pavement and itsassociated earthworks; minimisation of the impact of road runoff on thereceiving These design objectives require that to operatesuccessfully, soakaways should be sited in porous andpermeable ground of sufficient depth and lateral extentto be able to accommodate potential maximumdischarges under storm conditions.

8 Furthermore, whatis good for the physical disposal of road drainage ( dispersal of flow) is the exact opposite of what isrequired with respect to the in-ground attenuation ofpollutants. Where slow infiltration rates are utilised tooptimise these in-ground processes, the use ofbalancing ponds or other flow attenuation mechanismsmay be required to accept the quantity of runoffgenerated under peak flow conditions by the This design guidance allows the designer toconsider those processes that may occur in the passageof road runoff through the soakaway system andthrough the unsaturated zone, prior to its ultimatedischarge into groundwater. However, application of therisk assessment methodology described in HA 216( ) may demonstrate the requirement forpollution prevention measures to be incorporated in thedesign of the drainage system.

9 These should beincorporated upstream of the soakaway discharge. Suchmeasures could include structures for the control andcontainment of accidental spillages, or introducemethods for the treatment of pollution derived fromroutine road runoff. Guidance for these measures maybe found in HD 33 (DMRB ) and HA 103 ( ). Recently much attention has been paid to thedesign of drainage systems with more emphasis oncontrolled transmittal of runoff to discharge points(notably to surface waters). Such systems place agreater reliance on retaining water in storage ponds andallowing slow recharge to groundwater and have inparticular been applied to the reduction of the flashynature of runoff from urban drainage.

10 Guidance on theuse of these systems for roads is provided in HA 103and there is also published guidance available from theEnvironment Agency (A Guidance Manual forConstructed Wetlands), and additional information ontheir The design of soakaways , of any description,should also take into account constraints arising fromthe possible impact on landscape and to groundwater may potentially affectbiodiversity ( through impacts on wetland habitats)and any such effects should be considered both in thelocation and the function of the In spite of the promotion and introduction ofmeasures that allow for the slow infiltration of roaddrainage into the ground, there is little data availablethat provides a sufficient basis to be able to quantifythese effects, either in the short or longer term.