Flood risk

Flood Risk Management Plans

Strategic flood risk assessment

The National Planning Policy Framework and associated National Planning Practice Guidance set out national planning guidance for development in relation to flood risk. A risk based approach is taken and categorises land uses into different vulnerabilities, which are appropriate to different flood zones.

The purpose of the Level 1 SFRA was to collate and analyse the most up to date readily available flood risk information for all sources of flooding, and provide an overview of flood risk issues across the study area. The borough wide mapping deliverables for Wandsworth Borough are presented in the Level 1 SFRA Appendix A Figures 5.1 – 5.9.

Level 1 SFRA

This provides guidance on:

  • The application of the Sequential Test when allocating future development sites to inform their Local Plans, as well as by developers promoting development on windfall sites.
  • Managing and mitigating flood risk, the application of sustainable drainage systems (SuDS), and the preparation of site specific Flood Risk Assessments (FRAs).
  • Potential flood risk management objectives and policy considerations which may be developed and adopted as formal policies within developing Local Plans.

Level 2 SFRA

Using the strategic flood risk information presented within the Level 1 SFRA, Wandsworth Council undertook the Sequential Test to document the process whereby future development is steered towards areas of lowest flood risk. Where it was not possible to accommodate potential development sites outside those areas identified to be at risk of flooding, the Exception Test may be required, as set out in Table1-1. This Level 2 SFRA Report provides information to support the application of the Exception Test for future development sites. All sites assessed in this Level 2 SFRA have been allocated in the Council’s Local Plan - Site Specific Allocations Document (adopted March 2016).

Updates

Updated national surface water flood risk mapping released by the Environment Agency (the updated Flood Map for Surface Water (uFMfSW)) coupled with a review of the Surface Water Management Plans (SWMPs) for the Boroughs has been used to identify areas at risk from surface water flooding. Drainage catchments (DCs) have been identified for the administrative area and mapped with the uFMfSW. However, it should be noted that this national mapping has the following limitations: Use of a single drainage rate for all urban areas; It does not show the susceptibility of individual properties to surface water flooding; The mapping has significant limitations for use in flat catchments; No explicit modelling of the interaction between the surface water network, the sewer systems and watercourses;  In a number of areas, modelling has not been validated due to a lack of surface water flood records, and as with all models, the uFMfSW is affected by a lack of, or inaccuracies, in available data.

In addition, British Geological Survey (BGS) Susceptibility to Groundwater Flooding mapping has been used to provide an understanding of areas where the underlying geology may enable the presence of groundwater below or at the surface level. Based on geological and hydrogeological information, the digital data can be used to identify areas where geological conditions could enable groundwater flooding to occur and where groundwater may come close to the ground surface. Note, it is a susceptibility set, it does not indicate hazard or risk, i.e. it does not provide any information on the depth to which groundwater flooding occurs or the likelihood of the occurrence of an event of a particular magnitude.

The BGS state that the dataset is suitable for use for regional or national planning purposes where the groundwater flooding information will be used along with a range of other relevant information to inform land-use planning decisions. It might also be used in conjunction with a large number of other factors, e.g. records of previous incidence of groundwater flooding, rainfall, property type, and land drainage information, to establish relative, but not absolute, risk of groundwater flooding at a resolution of greater than a few hundred metres. The susceptibility data should not be used on its own to make planning decisions at any scale, and, in particular, should not be used to inform planning decisions at the site scale. The susceptibility data cannot be used on its own to indicate risk of groundwater flooding.

In addition to the flood risk analysis contained within the SFRA, further details of the Sequential and Exception test process can be found in the NPPF and associated National Planning Practice Guidance. In addition to this the Council has applied the Sequential Test to the sites contained within the preferred options Site Specific Allocations Document. This is due to be updated.

Read the results in the Flood Risk and Development Sequential Test Report.

Preliminary Flood Risk Assessment (PFRA)

As part of the new duties the Council commissioned URS (formerly Scott Wilson) in 2011 to undertake a Preliminary Flood Risk Assessment (PFRA) and a Surface Water Management Plan (SWMP).

The PFRA provides a high level summary of significant flood risk, based on available and readily derivable information, describing both the probability and harmful consequences of past and future flooding. The scope of the PFRA is to consider flooding from the following sources; surface runoff, groundwater, sewers and ordinary watercourses and any interaction these have with main rivers and the sea. 

Surface Water Management Plan (SWMP)

The SWMP outlines the preferred surface water management strategy for Wandsworth and includes consideration of flooding from sewers, drains, groundwater and runoff from land, small watercourses and ditches that occur as a  result of heavy rainfall.

Note: Since the production of the Strategic Flood Risk Assessment, Surface Water Management Plan and the Preliminary Flood Risk Assessment, the Environment Agency has published new mapping including surface water and flood zone mapping on the 13 December 2013 which the Council is now using. The mapping and background data are available to download via the Environment Agency.