Agenda Reports Pack (Public) 15/10/2012, 19.00 - Meetings ...

1. Mapping Flooding RiskSURFACE WATER FLOODING: Overview1.1. In contract to river and coastal flooding, capabilities to map and model (andhence provide warnings for) surface water flooding are very limited. Surfacewater flooding can be defined as flooding that occurs due to extreme rainfall andthe inability of the water to drain away quickly enough, hence forming pools ofwater. Pools may also form due to water coming out of drains at other locations.However, the reasons behind the inferior drainage capability of an area can bequite varied and are often interlinked. For example, an urban sewerage system(designed to convey surface water runoff into a nearby watercourse) might beunable to discharge water if the watercourse levels are too high, which was thecase in certain areas of Sheffield during the summer floods of 2007.1.2. Many factors affect the likelihood of surface water flooding:► Intensity of rainfall: Rainwater drains away naturally over long periods of time,but if rain falls in intense bursts, the drainage system may be unable to cope. Theprobability of this type of intense rainfall occurring in the future is likely toincrease due to climate change.► The location of the rainfall: The direction of travel of surface water is directlyinfluenced by the topography of an area. Small changes in the location of rainfallcan have a significant impact on where the water ends up.► The capacity and condition of the sewerage and drainage system: This willobviously affect the rate at which rainwater can drain away. Most of the UKsewerage system was built before the Second World War, and so deterioration isanother key issue.► The type of surface material: The permeability of surface material affects theamount of runoff. Urban areas, such as Hammersmith and Fulham, are moresusceptible to surface water flooding than rural areas because they arecharacterised by a significant quantity of built-up (and hence impermeable) areas.► The saturation (or the soil moisture deficit) of the ground: If the ground issaturated, or in fact too dry, large amount of rain that falls will be converted intorunoff.► River levels: High river levels will hinder the sewerage systems ability todischarge water.► Planning and development: Permitting additional homebuilding and otherdevelopment will reduce the amount ofpermeable space available and is also likely toreduce the number of open watercourses.Sustainable drainage systems can beincorporated into new property developments tohelp to reduce the surface water runoff.SURFACE WATER FLOODING: Mapping1.3. As noted above the capabilities to map andmodel surface water flooding are currently verylimited. At present the Council is reliant uponinformation provided by Thames Water, as well- 13 -Page 172