Managing exceedance

For rainfall events with a return-period in excess of 30 years, surface flooding of open spaces such as landscaped areas or car parks is acceptable for short periods, but the layout and landscaping of the site should aim to route water away from any vulnerable property, and avoid creating hazards to access and egress routes. No flooding of property should occur as a result of a one in 100 year storm event (including an appropriate allowance for climate change). In principle, a well-designed surface water drainage system should ensure that there is little or no residual risk of property flooding occurring during events well in excess of the return period for which the sewer system itself is designed.

Deciding on the approach

Understanding the problem and recognising the opportunities enables designing for exceedance to be successful. A wide range of measures can convey and store exceedance flow, making best use of the existing urban area through minor topographical changes, for example those made to the profile or a highway, footpath or kerb. In places this may mean increasing the resistance or resilience of an area or property to withstand or recover from exceedance respectively.

The range of measures to select will depend upon the problem, opportunity and the available funding. These include conveyance, storage, property level protection and engagement. Minor modifications to topography and the layout of spaces are often critical to manage exceedance flows from urban drainage systems. The water is typically shallow when it moves over the surface until it reaches a low spot to pond. For example, the removal of a 100 mm kerb blocking a flow pathway can make the difference between flows being conveyed away from or towards properties.