Sustainable Drainage Systems (SUDS)

a guide for developers

www.environment-agency.gov.uk

www.environment-agency.gov.uk

The Environment Agency is the leading public bodyprotecting and improving the environment in Englandand Wales.

It’s our job to make sure that air, land and water arelooked after by everyone in today’s society, so thattomorrow’s generations inherit a cleaner, healthier world.

Our work includes tackling flooding and pollutionincidents, reducing industry’s impacts on theenvironment, cleaning up rivers, coastal waters andcontaminated land, and improving wildlife habitats.

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© Environment Agency March 2003

All rights reserved. This document may be reproduced with prior permission of the Environment Agency.

This report is printed on Cyclus Print, a 100% recycled stock, which is 100%post consumer waste and is totally chlorine free. Water used is treated and inmost cases returned to source in better condition than removed.

Development can harm our water resources if a traditionalapproach to drainage is adopted. Removing water from thesite as quickly as possible causes a range of impacts:

• increased run-off resulting from more extensive hard paving androofing can increase the risk of flooding downstream, as well as causingsudden rises in water levels and flow rates as the water is dischargedinto watercourses;

• surface water run-off cancontain contaminants such asoil, organic matter and toxicmetals. Although often at lowlevels, cumulatively they canresult in poor water quality inrivers and streams, adverselyaffecting biodiversity andamenity value. After heavy rain,the first flush is often highlypolluting;

• by diverting rainfall to piped systems, the amount of water infiltratingthe ground is reduced, depleting ground water and reducing flows inwatercourses in dry weather.

As a result, many urban watercourses are lifeless and unattractive, and areoften hidden in culverts under the ground. Some pollution arising fromurban run-off may be unavoidable, and water treatment at every outfallmay be impractical. However, by moderating flows and filtering run-off,Sustainable Drainage Systems (SUDS) can deliver not only significantreductions in impacts on our water resources, but also improvements inthe quality of our built environment.

Surface water run-off

The problem

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Towards sustainable development

SUDS (Sustainable Drainage Systems) are designed with three objectives inmind:

• to control the quantity of run-off from a development;

• to improve the quality of the run-off ;

• to enhance the nature conservation, landscape and amenity value ofthe site and its surroundings.

SUDS deal with run-off as close to its source as possible and balance allthree objectives, rather than focussing only on flood prevention.

Implementing SUDS contributes significantly towards achieving sustainabledevelopment. In recognition of this, Local Plans increasingly state that allapplicants should, in the first instance, aim to incorporate SUDS intodevelopment proposals. SUDS are also considered suitable for mitigatingadverse impacts and supporting water-conservation objectives.

SUDS - the benefits

Implementing SUDS may lead to cost savings, for example, by avoiding orreducing the need for:

• gully pots;

• constructing or requisitioning surface water sewers;

• piped connections to distant outfalls.

SUDS can be cost-effectively designed to work with retained naturalfeatures such as ditches or ponds, and to form an integral part of hard andsoft landscaped areas. In this way, they can contribute towards anattractive scheme that enhances the nature conservation and amenityvalue of the development, while also recycling the valuable water resource.

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SUDS and the planning process

SUDS include tried-and-tested techniques that are already beingimplemented on a range of projects in England and Wales as well aselsewhere. They incorporate cost-effective techniques that are applicable toa wide range of schemes, from small developments to major residential,leisure, commercial or industrial operations with large areas ofhardstanding and roof. They can also be successfully retro-fitted to existingdevelopments. Planning Policy Guidance Note 25 for England ondevelopment and flood risk emphasises the role of SUDS and introduces ageneral presumption that they will be used. SUDS will probably featureincreasingly in such guidance documents as they are revised.

As with other key considerations in the planning process — transport,landscape, heritage and nature conservation — incorporating SUDS needsto be considered early in the site evaluation and planning process, as wellas at the detailed design stage.

Many planning authorities will expect planningapplications, whether outline or detailed, to demonstratehow a more sustainable approach to drainage is to beincorporated into development proposals, and for detaileddesign information to be submitted at the appropriatestage. Planning authorities may use planning conditionsto secure the implementation of SUDS.

Adoption and future maintenance

In the early stages of design, consideration should be given to thearrangements for adoption and future maintenance of the system. This islikely to influence the design just as much as technical considerations. It isrecommended that maintenance should be the responsibility of a publiclyaccountable body, which will often necessitate the payment of acommuted sum or a legal agreement, possibly backed up by the deposit ofa financial bond. The adopting organisation should approve the designbefore construction.

Permeable pavements

The need for surface water drainsand off-site sewers can be reducedor eliminated where run-off isencouraged to permeate througha porous pavement, such aspermeable concrete blocks,crushed stone and porous asphalt.Depending on the groundconditions, the water may infiltratedirectly into the subsoil or bestored in an underground reservoir(for example, a crushed stonelayer) before slowly soaking intothe ground. If infiltration is notpossible or appropriate (forexample, because of groundcontamination), an impermeablemembrane can be used with anoverflow to keep the pavementfree from water in all conditions.Pollutant removal occurs eitherwithin the surfacing or sub-basematerial itself, or by the filteringaction of the reservoir or subsoil.

Sustainable Drainage Systems (SUDS)

Sustainable drainage is a design philosophy that uses arange of techniques to manage surface water as close toits source as possible. To produce a workable andeffective scheme, SUDS must be incorporated intodevelopments at the earliest site-planning stage.

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Permeable pavement

Infiltration

Overflow to disposal or further treatmentif required

Permeable sub-base

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Swales and basins

These can be created as featureswithin the landscaped areas of thesite, or they can be incorporatedinto ornamental, amenity andscreen-planted areas where theywould be looked after as part ofthe normal maintenance contract.They provide temporary storagefor storm water, reduce peak flowsto receiving waters, facilitate thefiltration of pollutants (depositedand incorporated into thesubstrate) and encouragemicrobial decomposition, as wellas allowing water infiltrationdirectly into the ground. Swalesand basins are often installed aspart of a drainage networkconnecting to a pond or wetland,prior to discharge to a naturalwatercourse. They may be installedalongside roads to replaceconventional kerbs, thereforesaving construction andmaintenance costs.

Inflow Inflow

Infiltration

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Green roofs and rainwater reuse

Other techniques which reduce flow rates and improve water qualityinclude green roofs and rainwater reuse. Green roofs can reduce the peakflow and the total volume discharged and improve water quality. Inaddition, they can improve insulation and increase the lifespan of the roof.Rainwater reuse (or harvesting) involves the collection of the rainwater onsite and its use as a substitute for mains water, for example in watering agarden or for flushing toilets.

Infiltration trenches and filter drains

Infiltration trenches comprisestone-filled reservoirs to whichstormwater run-off is diverted, andfrom which the water graduallyinfiltrates into the ground. Theirlongevity is enhanced byincorporating a filter strip, gully orsump pit to remove excessivesolids at the inflow. Widely used byhighway authorities for drainingroads, filter drains are similarstructures through which aperforated pipe runs. Thisfacilitates the storage, filtering andsome infiltration of water passingfrom the source to the dischargepoint. Pollutants are removed byabsorption, filtering and microbialdecomposition in the surroundingsoil. Systems can be designed tosuccessfully incorporate bothinfiltration and filter systems.

Infiltration

Inflow

Stone fill

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Ponds and wetlands

Although these can be designed aswet or dry ponds, or wetlands,they are most likely to contributeto visual amenity and biodiversitywhere they include a permanentwater body. Ponds or wetlands canbe designed to accommodateconsiderable variations in waterlevels during storms, therebyenhancing flood-storage capacity.By allowing adequate detentiontime, the level of solids removalcan be significant. The algae andplants of wetlands provide aparticularly good level of filteringand nutrient removal, as well asbeing able to recycle grey water.Ponds and wetlands can be fed byswales, filter drains or pipedsystems, and the use of silt traps isrecommended to reducesedimentation.

SUDS on brownfield sites

Where there is a risk of environmental damage from land contamination,the use of infiltration-based solutions requires careful thought. The focusmust be to avoid mobilisation of contaminants.

Inflow via a pipe or controlled surface flow

Outflow and overflow in a pipe or channel

Water level varies in a pond

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SUDS during construction

The early implementation of appropriate SUDStechniques can prevent the pollution of watercoursesduring construction. However, it is essential that suitablemeasures are taken to either protect SUDS systems fromsiltation during construction or to restore them beforethe site is occupied.

Choosing the right SUDS system

The choice of SUDS system will depend on a number of factors such as:

• the pollutants present in run-off;

• the size of and drainage strategy for the catchment area;

• the hydrology of the area and infiltration rate of the soil;

• Groundwater Source Protection Zones or contaminated land.

Large-scale ponds and wetlands are generally more appropriate for siteslarger than 5ha. Infiltration trenches, swales, filter strips and porouspavements are suitable for both large and small sites. The best drainagesolution for a site will often incorporate a mix of mechanisms.

Soil permeability and hydrology

Soil permeability can have a significant effect on selecting SUDSmechanisms. Infiltration techniques may not be effective if the infiltrationrate is below 10mm/hr for the upper soil layers. Swales and ponds, workingby a combination of filtration and infiltration, are more tolerant of poorsoils. In highly permeable soils, wet ponds need to be lined. SUDS must bedesigned to avoid discharge to old mine workings where they exist.

It is important for developers to establish the soil conditions and hydrologyof their site at an early stage in the planning process. The results of suchinvestigations should be provided to the planning authority with theproposals for a drainage system included with the planning application.

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Acknowledgements

CIRIA (Construction Industry Research and Information Association) forimages taken from the SUDS best practice manual (C535) and the SUDSdesign manual (C522)

Photos

Page 6: Permeable pavements, Formpave Ltd, GloucestershirePage 7: Swale, K McDonald, University of Abertay, Dundee

Further information

Sustainable Drainage Systems – an introduction, EnvironmentAgency/SEPA/EHS

Designs that hold water – Sustainable Urban Drainage Systemsexplained, a 25-minute video or DVD, Environment Agency/SEPA

Both available through the Environment Agency National Call Centre,telephone 08708 506506, email: environment.agency@dmsltd.co.uk

Sustainable Urban Drainage Systems – design manual for England andWales, CIRIA ISBN 0 86017 522 7

Scope for Control of Urban Run-off, CIRIA Report 124,

Both available from CIRIA, Tel: 020 7549 3300, www.ciria.org.uk

See also the CIRIA SUDS website, www.ciria.org.uk/suds

For further information about Groundwater Source Protection Zones,contact your local Agency office

We welcome feedback including comments about the content andpresentation of this guide.

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For further copies of this guide or other reports published by theEnvironment Agency, contact general enquiries on 08708 506 506or email us on enquiries@environment-agency.gov.uk

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