PPS 15: Planning and Flood Risk
Annex C: Benefits of and constraints on Sustainable Drainage Systems
C5 Experience of SuDs in England and Scotland indicates that they can help reduce flood risk. While the benefits of such systems are secured principally at the river-catchment scale, their use can make a significant contribution towards the sustainability of individual developments in:
- managing environmental impacts at source, rather than downstream; and
- managing water run-off rates thereby reducing the impact of development on flooding.
C6 Such benefits depend on the identification and application of clear design and maintenance objectives tailored to local circumstances. It requires developers to work in partnership with a number of disciplines and agencies (planners, drainage engineers, architects, landscape architects, ecologists and hydrologists) from the earliest stages of the development process.
C7 Surface water management using sustainable drainage systems can be implemented at all scales. It may start with good housekeeping measures and soakaways for individual premises and progress through the use of infiltration devices, tank storage, basins and wetlands for development at a more significant scale. At any level, it can help to reduce the need for investment in flood management and protection works by mitigating the intrinsic additional flood risk that new development might otherwise generate.
C8 While there are tangible benefits to the use of sustainable drainage systems, there are also constraints on the choice of system. The surface structures that may be needed can take more space than conventional systems although it is often possible for them to be integrated into the surrounding land use, e.g. in public open space or road verges. Other limitations to infiltration devices can occur where:
- the soil is not very permeable;
- the water table is shallow;
- the groundwater under the site may be put at risk; or
- infiltration of water into the ground, particularly if concentrated in a limited area, could adversely affect ground stability.
C9 For example, infiltration from particular types of development may be prohibited in groundwater protection zones18 or be subject to the need for investigation and appropriate additional treatment prior to discharge.
C10 Particular care is needed in designing sustainable drainage systems with appropriate capacity to handle run-off at their location. Contingency measures may be required to ensure that problems are not made worse when the intensity and/or duration of rainfall creates a situation where the quantity of run-off exceeds that for which the system was designed. In extreme events, sustainable drainage systems may, like other drainage systems be overwhelmed because they will only deal with the rainfall event for which they are designed. They may assist, however, in reducing the initial impact of extreme events.
18 See Policy and Practice for Groundwater Protection in Northern Ireland (DOE Environment and Heritage Service , 2001) available at www.ehsni.gov.uk/