Flood risk maps
The EA’s interactive ‘Flood Map for Planning (Rivers and Sea)’ can be accessed via the above link. The following flood risk zones should be used to inform site specific flood risk assessments (FRAs) prepared by developers in support of their planning applications and as the basis for undertaking the ‘sequential test’
- 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).
More detailed maps showing areas at risk of river flooding, the extent of the ‘functional flood plain’ (Flood Zone 3b), areas benefitting from flood defences, flood storage areas and the location of rivers and ordinary watercourses within the Borough are shown in Figure 4.1 (with insets 1 and 2) of the Strategic Flood Risk Assessment (SFRA).
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).
Figure 4.4 of Sutton’s SFRA Level 1 Report shows areas susceptible to groundwater flooding based on British Geological Survey data.
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.