BUILDING SURVEYING

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1 BUILDING SURVEYING Session topic SUDS - Sustainable Urban Drainage Systems Session objectives To examine various Sustainable Urban Drainage Systems (SUDS) for managing rainwater run-off and discuss associated guidelines and Building Regulations. Intended audience Clients, designers or specifiers wishing to include SUDS techniques in their scheme. Summary Programme Amidst mounting concern over flooding and soil erosion, Sustainable Urban Drainage Systems (SUDS) are increasingly being specified as solutions for managing surface runoff and relieving pressure on existing drainage networks. This programme looks at the reasons for adopting SUDS, investigates different types of SUDS systems, and explores techniques and regulations for their correct installation and maintenance. Programme Notes These notes are based on NBS Shortcut 11: SUDS: Sustainable Urban Drainage Systems and its associated bibliography. 1 BLD

2 About the contributors Bob Sargent, UK Head of Hydrology for Hyder Consulting, and immediate past-president of the Chartered Institution of Water and Environmental Management. Discussion points What is the prime suspect in the recent spate of floods in the UK? Discuss and prioritise the implications of - climate change, inadequate piped drainage, insufficient sustainable drainage, too much hard standing. What is an Environmental Impact Assessment and what factors should it consider in relation to SUDS? What does CIRIA stand for? What should be consulted before, during and after a SUDS scheme is implemented? 2 BLD

3 BUILDING SURVEYING SUDS Sustainable Urban Drainage Systems With concerns about flooding and soil erosion becoming widespread, what are the options for managing rainwater run-off with minimal reliance on, and investment in, mains infrastructure? Here we examine the technicalities, approvals and consents needed for Sustainable Urban Drainage. Many of the historic drainage systems that lie beneath our feet are a combination of surface water and foul water. The early Victorian sewers tended simply to discharge the combined effluent into a river. After London s Big Stink in 1868 (so vile that Parliament s tactic of perfuming the curtains in order to mask the stench, proved ineffective), pipes were routed further afield extended downstream (towards the estuary in London s case) or eventually, by the end of the 19 th century, into sewage treatment works. Modern developments are generally obliged to separate out sewage and surface water to avoid heavy rainfall placing an intolerable burden on the treatment facilities into which they would otherwise both discharge. The resultant flooding from combined water is known as hydraulic flooding, where the existing sewer can no longer cope with the excessive flows. Currently, in east London, when the combined sewer overloads, the excess discharges intentionally into the River Lee and Thames to reduce the risk of overloading London's sewage treatment works and flooding properties. A commission has been set up to rectify this 3 BLD

4 before the 2012 Olympics. Nowadays, where possible, designers are urged to route surface water to discharge into natural watercourses or into the soil via controlled infiltration methods. An additional consideration is the need to reduce pollution and damage to the water-table arising from these natural flows of water. Pollution may derive from the surfaces that have been drained (the contaminated run-off from lead roofs and oily driveways, for example); while damage can be caused by an unmediated rate of flow into the watercourses into which the drains discharge, leading to scouring and flooding during severe rainstorms. The Environment Agency normally only require pollution hazard assessments for lorry parks, garages, industrial sites and major commercial sites although some less hazardous areas may fall into its remit. If the rainwater goods from domestic roofs are not sealed but discharge to an open grating, for example, authorisation is required. Building Regulations (E&W) Approved Document H: 2002, 'Drainage and Waste Disposal', sets out the benchmarks for dealing with rainwater disposal. It states that adequate provision shall be made for rainwater run-off from roofs and paved areas to discharge, in order of priority, into a soakaway, a watercourse or, where these are 'not reasonably practicable', into a sewer. Proving that something is 'not reasonably practicable' is not easy and nowadays it is rare to discharge rainwater drainage into a non-natural means of disposal. In essence, when designing drainage systems, soakaways (see Shortcut 45 forthcoming) and watercourse connections must be explored and proven to be inappropriate before plans to take rainwater into a run-off sewer will be considered for approval. After all, AD H states that methods of drainage 'other than connection to a public surface water sewer are encouraged where they are technically feasible'. Scottish Planning Policy (SPP) 7 Planning and Flooding effectively says the same. Planning Policy Guidance (PPG25) and Planning Policy Wales Technical Advice Note (TAN 15) (both entitled Development and Flood Risk ) encourage local planning authorities to adopt sustainable drainage wherever practicable. The objective of SUDS is to manage water run-off rates, but equally to provide habitat for wildlife, protect water quality and, where appropriate, to encourage natural groundwater recharge. To this end, local controls on run-off quantities and qualities have to be managed close to source. Therefore, CIRIA advise that, where possible, run-off be limited by recycling at source, using water butts, incorporating green roof design, etc. It also suggests that pollution can be kept to a minimum 'by keeping paved areas clean' (though not by washing, but by 'sweeping hard surfaces regularly'). Other helpful tips include the memorable: 'Litter and animal faeces can be kept out of drainage systems by education and the provision of bins'. A number of Housing Associations now insist on low volume baths to limit water use and drainage strain at source. Given the legislative and practical constraints, Sustainable Urban Drainage (SUDS) systems use a variety of techniques to control surface water run-off locally, avoiding or considerably reducing these problems: 4 BLD

5 Filter strips and swales Filter strips are areas of ground, covered in vegetation, that absorb run-off from adjacent hard surfaces. Swales resemble long shallow ditches, and can have the additional function of carrying the collected water elsewhere. These features slow down the flow of surface water, and remove pollutants through the filtering effect of the planting, which may be slow-growing grasses or wild flower mixtures. Gradients must be gentle to avoid erosion, and deep hollows where planting may become waterlogged should be avoided. Permeable surfaces or paving Paving is traditionally laid to falls to gullies or slot drainage, with below-ground pipework to sewers. In a SUDS scheme, the run-off may have local treatment and disposal, e.g. via 5 BLD

6 soakaways. A more radical solution is to use permeable paving, where the use of belowground pipework can be eliminated or much reduced. Soakaways, infiltration trenches and filter drains These features all include some degree of below-ground water storage which is designed to delay the release of water to the sewerage system, or into the surrounding ground. Details on soakaways are contained in (Shortcut 45 forthcoming) but, in general, the trench and drain filters usually rely on an excavated pit or trench filled with coarse aggregate or rubble often with a geotextile lining and sometimes a precast concrete or brick structure to retain the fill. Vertical pipes capped by inspection covers may be used to monitor water levels and assist maintenance. A filter drain is essentially an infiltration trench with a horizontal pipe to assist water flow. Alternatively, plastics honeycomb units, again wrapped in geotextile, can be used to create underground voids where water can be held before percolating out harmlessly. Basins and ponds These store water above ground, temporarily or permanently. Flood plains and detention basins are normally dry but flood harmlessly during heavy rainfall. Balancing and attenuation ponds are permanent water features that moderate the flow by storing the run-off into the basin (or pond) during flood conditions and releasing it after the peak has passed. Retention ponds retain the water for longer periods to improve quality, by allowing pollutants to settle or by active biological treatment of the water. With the addition of specific planting to form constructed wetlands, the quality improvement is even greater, allowing secondary or tertiary treatment of effluent from septic tanks or sewage treatment units. Basin and pond lining can be of compacted clay or bentonite, or sheet material (including geosynthetic clay). Regulatory framework Because SUDS is a new and developing phenomenon and departs radically from traditional pipe away and treat remotely systems, the permissions and approvals necessary as the project evolves may be unfamiliar and conflicting. For example, planning permission may be needed for some features although regional planning guidance (or regional spatial strategies) will tend only to have generic requirements. The more specific requirements will be spelled out in structure plans, local plans and supplementary planning guidance (SPGs). These tend to require the applicant/ agent to demonstrate that where SUDS is not practicable, a full 6 BLD

7 environmental impact report is compiled, including such details as the aims of the SUDS approach to drainage and local soil and hydrology characteristics. In Scotland, the Drainage Impact Assessment (DIA) requires developers to document the impact of the proposed development on the catchment, including soil porosity and classification data and proof of attenuation for critical rainfall periods. Building Regulation approval is needed where a SUDS system is used for run-off from building roofs and paving associated with buildings. The following agency approvals may be required: Sewerage Undertaker for connections to adopted sewers, and sewerage systems that are to be adopted. Highway Authority for highway construction and drainage. Local Authority where public land is to be maintained. The environmental regulator (e.g. Environment Agency, Scottish Environmental Protection Agency) for consent to discharge, e.g. to a watercourse. The final thing to note is that unlike most correctly installed piped drainage systems, sustainable urban drainage systems require close monitoring and maintenance from regular de-silting and the disposal of sediment to grass cutting. This needs to be factored in at an early stage. Some councils will adopt publicly accessible systems under Section 106 Agreements on the provision of a maintenance plan and requisite payment arrangements. In some cases, adoption by the statutory sewerage undertaker may be possible, although it has no duly to accept the land drainage flow. References: Construction Industry Research and Information Association (2000) Publication C521 Sustainable urban drainage systems Design manual for Scotland and Northern Ireland, CIRIA. Construction Industry Research and Information Association (2000) Publication C522 Sustainable urban drainage systems Design manual for England and Wales, CIRIA. Construction Industry Research and Information Association (2001) Publication C523 Sustainable urban drainage systems Best practice manual for England, Scotland, Wales and Northern Ireland, CIRIA. Construction Industry Research and Information Association (2004) Publication C625 'Model agreements for sustainable water management systems. Model agreements for SUDS', CIRIA. Construction Industry Research and Information Association (1995) Report R 134 'Sediment management in urban drainage catchments', CIRIA. Department for Transport, Local Government and the Regions (2001) Planning Policy Guidance Note (PPG25) 'Development and floor risk', DTLR Environment Agency (2003) Sustainable drainage systems (SUDS) - A guide for developers, EA. Planning Policy Wales (2004) Technical Advice Note (TAN) 15 Development and Flood Risk, Welsh Assembly Government. Scottish Executive Development Department (2001) Planning Advice Note 61 Planning and sustainable urban drainage systems, SEDD. Scottish Executive Design Development (2004) Scottish Planning Policy (SPP) 7 Planning and Flooding, SEDD. 7 BLD

8 Recommended Reading: National SUDS Working Group (2004) Interim Code of Practice for Sustainable Drainage Systems, CIRIA. Construction Industry Research and Information Association (2006) Publication C635 'Designing for exceedance in urban drainage - good practice', CIRIA. Office of the Deputy Prime Minister (2002) 'Approved Document Part H: Drainage and waste disposal', ODPM. Written and illustrated by Austin Williams. 8 BLD