6.1. Review & Update - Technical Content

A SFRA is a live document which is to be used to assist in allocating sites for future development and general decision making. It is essential that the data contained within the SFRA is as up to date as possible to ensure that decisions are made on the best information available. Events that may trigger review and update are summarised below:

  • Changes to the NPPF and associated Flood Risk and Coastal Change PPG which form the basis of the SFRA. *Note: the PPG was updated in August 2022.
  • Updates to any of the overarching legislation which may alter the responsibilities of the Boroughs – including the new London Plan and RFRA documents, which are both currently undergoing consultation and are expected to be finalised in late 2019.
  • Changes to the flood risk information. There is a need to ensure developers and the LPA are provided with the best available information.
  • Developments in flood risk knowledge. There is a need to ensure that site-specific FRAs are informed by the most up-to-date information and planning decisions are made on the best available data.
  • Significant updates of baseline flood risk information (such as a major update to the Risk of Flooding form Surface Water map or Flood Map for Planning) or following a major flooding event within the sub-region.

6.2. Review & Update - Mapping

The knowledge of flood risk is constantly changing and improving and the SFRA should reflect this. Not only could this enhanced knowledge highlight risk areas which were not previously at risk, it could also free up areas which may have been at risk but are no longer considered to be so. This could free up land for potential future development.

The Web Maps developed to support this SFRA provides a flexible platform for ensuring the most up-to-date information is available. Several Web Map layers are maintained externally by the EA and will be updated automatically when the EA publishes revised data – these layers include:

  • EA Flood Map for Planning (River and Sea) – Flood Zone 2
  • EA Flood Map for Planning (River and Sea) – Flood Zone 3
  • EA Flood Map for Planning (River and Sea) – Areas Benefiting from Flood Defences
  • EA Flood Map for Planning (River and Sea) – Flood Defences
  • EA Flood Warning Areas
  • EA Recorded Flood Outlines
  • EA Historic Flood Map
  • Risk of Flooding from Surface Water Extent: 3.3 percent annual chance
  • Risk of Flooding from Surface Water Extent: 1 percent annual chance
  • Risk of Flooding from Surface Water Extent: 0.1 percent annual chance
  • Risk of Flooding from Reservoirs – Maximum Flood Extent

It should be noted that all Flood Zone 3a and 3b layers are static and may require update under the following circumstances:

  • Updated main river flood extents are made available by the EA (the EA undertake periodic review and update of main river flood models and associated predicted flood extents)
  • Updates to the Risk of Flooding from Surface Water Map – these occur when LLFAs undertake local surface water flood risk studies and provide revised surface water flood extents to the EA to update the national mapping

As described in Section 3.11.1, PPG changes in August 2022 have led to alterations being necessary for Flood Zone 3b (fluvial / tidal). Not all main river models have yet been updated by the EA since the original publication of this SFRA (March 2018), and it is upon individual boroughs to update their Flood Zone 3b mapping accordingly. This means that not all of the online mapping within this SFRA has currently been consistently updated to reflect the PPG changes. The main river models that have been updated since March 2018 are:

  • Ruislip – Park Wood and Pinn Meadows FAS (2021)
  • Silk Stream (2019)
  • Thames (Datchet to Teddington) – Thames and Tributaries domains (2023)

Boroughs where changes have been made following the PPG updates, and where affected by the above new mapping, are Harrow (published TBC 2024) and Hounslow (published September 2024). As new modelling is released and boroughs’ Flood Zone 3b (fluvial / tidal) are updated in this SFRA’s online mapping, this section will be updated.

The remaining Web Map layers are current as at the date of publication of the SFRA (March 2018) and may require update in the future. The above list will be amended as and when future updates are made.

6.3. Level 2 SFRA Recommendations

As detailed in Section 4, a high level screening assessment of currently Allocates Sites within each Borough was undertaken as part of this SFRA. This assessment includes a spatial analysis of the percent of site area within each of the defined Flood Zones, the potential impact of climate change, potential interactions with other sources of flood risk, an initial appraisal on whether the Sequential Test and Exception Test are required and a recommendation on if assessment through a Level 2 SFRA would be appropriate. Allocated Site specific recommendations are included in Appendix A in a spreadsheet format that can be filtered on a borough-by-borough basis and other assessment parameters as required.

The assumptions applied for the assessment are summarised below:

  • If proposed use of site is unknown, then vulnerability classification is assumed to be ‘more vulnerable’ (residential)
  • Waste management sites with an unspecified use have been given an unknown vulnerability classification as these sites could be either ‘More vulnerable’ or ‘Less Vulnerable’ dependent on the use
  • If proposed use is ‘Other’ these have been given an unknown vulnerability classification
  • If proposed use is ‘Commercial’ these have been given an unknown vulnerability classification as these sites could be either ‘More vulnerable’ or ‘Less Vulnerable’ dependent on the use
  • Gypsy & Traveller sites have been given a ‘Highly Vulnerable’ classification as these sites could be either ‘Highly vulnerable’ or ‘More Vulnerable’ dependent on the property type
  • Sites with 0% of areas in Flood Zones 2 and 3a/b do not require the Sequential Test (on the basis that other forms of flood risk are generally manageable on a site by site basis)
  • Less vulnerable sites within Flood Zone 2 and 3a/b require the Sequential Test
  • More vulnerable sites within Flood Zone 2 and 3a/b require the Sequential Test
  • More vulnerable sites in Flood Zone 3a require the Sequential and Exception Tests
  • Highly vulnerable sites in Flood Zone 2 require the Sequential and Exception Tests
  • Highly vulnerable sites are not suitable in Flood Zones 3a/b
  • Water Compatible use in Flood Zones 2 and 3a/b require the Sequential Test
  • Level 2 SFRA recommended where Sequential & Exception Tests are needed and Flood Zone 3 extent is greater than 20% of site (and will be a significant constraint on development) or if the site is currently less than 20% in Flood Zone 3, but will be more than 20% under the selected climate change scenario (1 in 100yr event +35%)
  • Allocated Sites in Barnet were not supplied with a spatial extent – only centre point coordinates. The development size is assumed to be a 100m diameter circle around the centre point for the purpose of this analysis
  • Flood Zone 3a for surface water is defined using the full 1 in 100 extent from the EA Risk of Flooding from Surface Water Map
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