Strategic Flood Risk Assessment

The full Strategic Flood Risk Assessment report and Strategic Flood Risk Assessment Addendum created by Metis Consultants Ltd is available to download from our website.

8.1 Review and Updates

8.1.1 Technical Content

The SFRA has been developed using the policy, legislation and information available at the time of writing (May 2021). The SFRA is intended to be used to assist various parties consider flood risk when making planning decisions about the location and design of proposed future developments and flood risk management. 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 a 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.
• Updates to any of the overarching legislation which may alter the responsibilities of Kingston.
• Significant updates to the available flood risk information used to develop the SFRA. There is a need to ensure applicants and the LPA are provided with the best available information.
• Improved understanding of local 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.
• Updates following any significant flood risk investigation work conducted by Kingston
• Following a major flooding event within Kingston.

8.1.2 Mapping

The knowledge of flood risk is constantly changing and improving, and the SFRA should reflect this. This could enhance knowledge, highlighting risk areas which were not previously known, and could free up areas which were, but are no longer considered to be at risk. This approach could free up land for potential future development.

The Web Maps developed to support this SFRA provide 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 Storage Areas
• EA Flood Alert Areas
• EA Flood Warning Areas
• 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

The remaining Web Map layers are current at the date of writing this SFRA (May 2021) and will require updating in the future. It should be noted that the Flood Zone 3a and 3b layers are static and will require updating under the following circumstances:

• Updated main river flood extents are made available by the EA. The EA undertake periodic review and updates of main river flood models and associated predicted flood extents.
• Updates to the RoFSW map – If RoFSW data are factored into future Flood Zone 3b extents (as per recommendation 6 in Section 7.6.1), updates would occur when Kingston undertakes local surface water flood risk studies that provide surface water flood extents to the EA to update national mapping.

8.2 Level 2 SFRA

A high-level screening assessment of currently Allocates Sites within Kingston 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 B in a spreadsheet format that can be filtered on assessment parameters as required.

The assumptions applied for the assessment are summarised below:

• Flood Zone 3a (surface water) is defined using the full 1 in 100 year extent from the EA Risk of Flooding from Surface Water Map as per this Level 1 SFRA.
• Small Site Programme with an unspecified use has been given an unknown vulnerability classification as these sites could be either ‘More vulnerable’ or ‘Less Vulnerable’ dependent on the use.
• If the proposed use is ‘Estate Renewal’ the vulnerability classification is assumed to be ‘More Vulnerable’ (residential).
• 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.
• A Level 2 SFRA recommended where:
  
  • Sequential & Exception Tests are needed
  • Flood Zone 3a (fluvial and / or surface water) extent is greater than 20% of site (and will be a significant constraint on development)
  • The site is currently less than 20% in Flood Zone 3a (fluvial and / or surface water), but will be more than 20% under the selected climate change scenario (1 in 100 year event +35% for fluvial, 1 in 1000 year event for surface water).

A Level 2 SFRA is a detailed assessment of all potential sources of flood risk for identified sites that require site-specific assessment. These allocation sites and / or windfall sites are identified as either part of the Local Plan or through the Level 1 SFRA.

The Level 2 SFRA builds on the strategic flood risk information presented in a Level 1 SFRA. If a Level 1 SFRA identifies that it is not possible to allocate all development outside of flood risk areas, then a Level 2 SFRA may be required. A Level 2 SFRA may also be required if it is believed that applicants will submit a high number of applications on sites that are not identified in the Local Plan.

This Level 1 SFRA has identified that not all developments can be located outside of flood risk areas. It is recommended that a Level 2 SFRA is produced to achieve the following:

• Identify the potential development sites that require a site-specific assessment.
• Complete a detailed site-specific assessment considering all sources of potential flood risk.
• Provide the information needed to apply the Exception Test where appropriate.
• Identify any site-specific requirements, including policy, mitigation measures, and FRA requirements.
• Provide a set of recommendations for each assessed site.

The Level 2 SFRA will consider flood risk from all sources now and in the future with climate change considerations as per NPPF requirements. The NPPF sets out how the planning system should help minimise vulnerability and provide resilience to the impacts of climate change. NPPF and NPPG describe how FRAs should demonstrate how flood risk will be managed over the lifetime of the development, taking climate change into account. Site-specific FRAs should confirm the impact of climate change using latest guidance. The assessment will provide details on aspects such as extent, velocity, depth, and hazard ratings. The information presented will support proposals in submitting the necessary information to meet the requirements.