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Management of Surface Water Run Off From Developments

Sur 1: Management of surface water run-off from developments

Credits Available: 2 (mandatory elements)

It is a mandatory requirement to limit surface water run-off from a developed site and reduce discharge to sewers and nearby watercourses. Post construction peak run-off rates from the site into watercourses must be the same as pre-construction run-off rates, i.e. the development of the site should not alter the run-off rates from the site into watercourses.

Where surface run-off rates for the developed site (based on a 1 in 100 year event of 6 hours duration) exceed pre-development rates the excess volume of water run-off must be reduced by infiltration and or used in non-potable water consuming applications, such as toilet cisterns and washing machines.

Further credits are available for using SUDS (Sustainable Urban Drainage Systems) to improve water quality prior to its run-off or discharge from the site or by putting in place agreements which detail ownership, operation and maintenance of the SUDS systems.

SUDS systems generally reduce run-off from sites by encouraging infiltration of the water into underlying natural soils and often utilise water storage features, such as ponds and lagoons, as part of a complete system. SUDS systems limit the potential of the surface run-off water becoming contaminated by encouraging run-off water to enter the subsurface and undergo infiltration into natural soils or discharge into a watercourse as soon as possible. Any specified SUDS system must not discharge into watercourses following rainfall depths below 5mm, in order to preserve water quality.

Detailed requirements for compliance and credit weightings are outlined in the table below.

Management of surface water run-off criteria table

Taken from Code for Sustainable Homes, Sur1, Page 124-125

Evidence Required

Design Stage:

Mandatory Elements require a statement from the appropriately qualified professional that they are qualified in line with the Code and that they provide a report containing the following information:

Demonstration of compliance with peak rate of run-off and volume rate of run-off

Areas of permeable and impermeable surfaces (pre and post development)

A flood risk assessment

Drawings showing pre development drainage

Drawings showing proposed drainage

Confirmation that a drainage system failure would not increase flood risk to site or surrounding area

Design specifications, calculations and drawings to support achievement of credits

Post Construction

Mandatory elements require confirmation from the developer or appropriately qualified professional that the solutions have been implemented which were presented at the design stage. Alternatively if the solutions are different, evidence must be provided to design stage requirements.

Evidence of maintenance responsibilities for SUDS (Sustainable Urban Drainage Systems)

Information provided in the Home User Guide

Confirmation of Flood Risk Assessment

Key Points for Compliance

To establish the risk of post development flooding at the site, a Flood Risk Assessment (FRA) should be undertaken. The level of detail required within the FRA report depends on site size and development density. Small sites with an area less than or equal to 2000m2 will require a less detailed FRA, which may comprise a short report containing information from the local water or sewage utility company, the site investigation, local knowledge and the environment agency, produced by the client's engineer. Sites with an area less than 10000m2 will require a more detailed FRA report, with further detail required for a densely packed site over a sparsely packed site.

Allowances now have to be made for climate change over the lifetime of the development in all post construction calculations.

Credits cannot be awarded if the development of the site has been undertaken against the recommendations of the Environment Agency regarding flood risks at the site.

Credits can be awarded independently

1 = ensuring there is no discharge into the watercourses for rainfall depths up to 5mm

1 = ensuring that run off from all hard surfaces will receive an appropriate level of treatment as defined in the SUDS manual.

November 2010 Amendments to Definitions

Appropriately Qualified Professional

This now refers to a person with experience and skills to deal with the use of SUDS within the design and construction of a development. They must understand the situation of each site in particular and have experience of SUDS based solutions and calculations at design stage.

Level of Treatment

If there is more than one level of treatment for surface water the level should be regarded as the number of SUDS components in series through which run off passes from the originating surface to the discharge point. Where a SUDS component has more than one treatment process it might be considered to provide more than one level of treatment.

Tidal Estuary

Defined as a semi-enclosed body of water which has free connection with the open sea and within which seawater is measurably diluted with fresh water derived from land drainage. Tidal rivers cannot be included as part of the estuary.

Note: Additions to Special Cases have also been made to include instances listed in Technical Guidance notes 001:

  • Maximum flow rate set by sewerage undertaker
  • Highways and impermeable areas
  • Derelict sites
  • No change or reduction in impermeable area
  • Sites with existing infrastructure/planning approval

Additional Information

Methods for the calculation of peak run-off rates can be found as follows:

  • Climate Change: PPS25 Development and Flood Risk: Practice Guide
  • Greenfield Sites of less than 50 ha: IH Report 124, Flood Estimation for Small Catchments (Marshall and Bayliss 1994)
  • Greenfield Sites of 50 ha to 200 ha: IH Report 124, Flood Estimation for Small Catchments (Marshall and Bayliss 1994)
  • Greenfield Sites of more than 200 ha: Flood Estimation Handbook (Centre for Ecology and Hydrology 1999)
  • Brownfield Sites: if existing drainage is known, use best practice simulation modelling; if not known the run-off should be calculated using the Greenfield models with Soil Type 5
  • Limiting Discharge
  • 100 year peak rate event: Excess volume run off

Volume Run Off Calculations can be made by:

  • Calculation Methodology: refer to The SUDS Manual
  • Allowance for Climate Change
  • Criterion A or B
  • 5 l/s Flow Rate

Further information on how to produce a site specific flood risk assessment plan is detailed within Planning Policy Statement 25: Development and Flood Risk - Practice Guide (2009)

Information regarding SUDS systems can be found within the The SUDS Manual (CIRIA C697,2007) and Preliminary rainfall run off management for developments (EA/DEFRA, 2007)

Useful document templates detailing SUDS system ownership, legislation and maintenance agreements:

Product Focus

Rain water harvesting systems (providing water for toilet flushing/washing machines)

Professional Services

If a Flood Risk Assessment (FRA) or advice regarding SUDS systems is required for your development the following professional services could be considered:

  • Flood Risk Consultants
  • Flood Risk Assessors
  • Hydrologists
  • Hydrological consultants