Flood risk maps
- Flood Zone 1 ‘Low Risk’ (less than 1 in a 1000 annual probability of flooding);
- Flood Zone 2 ‘Medium Risk’ (between 1 in a 100 and 1 in a 1000 annual probability); and
- Flood Zone 3 ‘High Risk’ (over 1 in a 100 annual probability).
Surface Water Flooding (EA Updated Flood Map for Surface Water)The EA’s Updated Flood Map for Surface Water (uFMfSW) identifies the following categories of risk:
- ‘very low’ risk (less than 1 in 1000 annual probability);
- ‘low risk’ (between 1 in 1000 and 1 in 100 annual probability);
- ‘medium risk’ (between 1 in 100 and 1 in 30 annual probability);
- ‘high risk’ (more than 1 in 30 annual probability).
More detailed maps showing areas at risk of surface water flooding and drainage catchments within the Borough are available in Figures 4.2 and 4.3 of the Strategic Flood Risk Assessment (SFRA). The location of ‘critical drainage areas’ (CDAs) is shown in Figure 1 of the Borough Surface Water Management Plan (LBS, 2011).
With the exception of combined sewers in the Hackbridge/Beddington area, most of the Borough is served by separate foul and surface water sewers which are owned and maintained by Thames Water Utilities Ltd.
Surface water sewers are typically designed to deal with a rainfall event with up to a 1 in 30 annual probability of flooding. Figure 4.5 of the SFRA Level 1 Report, the Wards of Worcester Park, North Cheam, Rosehill and the northern part of Carshalton have been more affected by sewer flooding than elsewhere.