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.

7.1 Overview

The impact of climate change on various flood risk sources is widely identified as the biggest factor which may increase the risk of flooding across the UK. Several important drivers, including development, planning, and infrastructure maintenance, are also key contributors to an increased risk of flooding. For example, a decrease in permeable ground cover due to urban development may increase the risk of surface water flooding.

The demand for more housing may mean a larger number of developments being proposed for sites within higher risk Flood Zones, placing them at greater risk of flooding. The combined impact of climate change, development requirements and projected future population growth may have an impact on the flood risk presented by different flood sources. In addition, it may present a greater overall flood risk to people and properties due to the accumulative risk from different flood sources. To meet flood risk mitigation requirements whilst facilitating housing development needs, local policy targeting the impact of future growth on flood risk is required.

The NPPF and accompanying PPG state that a sequential, risk-based approach to the location of development should be applied. This would enable possible flood risk to people and property to be avoided whilst taking impacts of climate change into account. In addition, the Kingston Core Strategy (2012) identified that the policy approach of the Local Plan should ensure that climate impact considerations are a priority. This, supported by the evidence base presented in this SFRA, underpins the strategic and site-specific policy recommendations for the borough (see Section 7.6 for further information). It is proposed that these policy recommendations are incorporated into Kingston’s new Local Plan.

7.2 The Impact of Future Growth on Flood Risk

The current London Plan (2021) sets out ten-year housing targets for each LPA to help meet the demands presented by future growth. These ten-year targets are for the period 2019/20 to 2028/29. These housing targets are set in line with Policy H1 ‘Increasing housing supply’ which provides actions and requirements to ensure that LPAs meet their ten-year target. These targets are based on the 2017 London Strategic Housing Land Availability Assessment, which is part of the London Plan evidence base.

The ten-year target set for Kingston in the current London Plan is to deliver 9,640 new homes. In line with Policy H2 ‘Small sites’, a number of these new houses should be delivered through small sites (sites below 0.25 hectares in size) as a strategic priority. The policy provides guidance for LPAs on what they should do to support small site housing developments. The ten-year target for Kingston is to ensure that 2,250 of the 9,640 new homes delivered in this period are on small sites. At the time of writing (May 2021) Kingston are preparing a new Local Plan to set out a vision of the future and provide guidance on development. The new Local Plan is being prepared with increased housing targets in mind.

The London Plan recognises that London is at particular risk from surface water flooding, largely due to the extent of impermeable surface coverage in the city. The projected housing targets for the ten-year period could further exacerbate surface water flood risk by introducing even more impermeable surfaces. Policy SI5 ‘Water infrastructure’, Policy SI12 ‘Flood risk management’, and Policy SI13 ‘Sustainable drainage’ of the London Plan set out requirements to mitigate and manage flood risk in recognition of the pressing need for more housing. This SFRA adopts a definition for Flood Zone 3a that includes predicted surface water flood extents. The policy requirements are identical to the Flood Zone 3a (fluvial) requirements, helping the borough to manage surface water flood risk whilst addressing the need for housing. Further information on Flood Zone 3a (surface water) and relevant guidance is found in Section 5.10 and Section 6.3 respectively.

The NPPF and PPG recognise the impact of increasing development on flood risk throughout the country. They require that all developments need to demonstrate that they will remain safe for their lifetime without increasing flood risk elsewhere. The PPG defines the lifetime of residential developments as a minimum of 100 years unless reasons are stated otherwise. The lifetime of a non-residential development is locally defined as 60 years minimum. Information must be presented if applicants believe the individual characteristics of a proposed non-residential development means the lifetime should be something else. With the impact that an increasing number of properties could have on flood risk, it is vital that developments demonstrate that flood risk is not increased, but that flood risk is reduced overall wherever possible. To achieve these objectives, and those listed as part of the policies and guidance in Section 2, it is vital to ensure that the impact of future growth on flood risk is mitigated as much as possible.

Opportunities for the development and progression of strategic flood risk infrastructure schemes to address the cumulative impact of future growth on flood risk should be taken through the use of funding. Examples of such funding contributions include planning obligations under Section 106 (S106) of the Town and Country Planning Act 1990 and the Community Infrastructure Levy (CIL) under Part 11 of the Planning Act 2008. S106 funding allows for developers to enter into agreements with an LPA to make proposed development sites acceptable in planning terms. Similarly, CIL funding agreements allow potential for LPAs to provide contributions towards the costs of implementing infrastructure improvements required for the development of the area.

7.3 Property Level Resilience Measures

To ensure effective planning for climate change, the NPPF requires that policies support appropriate measures to ensure the future resilience of communities and infrastructure against climate change impacts. This includes ensuring that developments are appropriately flood resistant and resilient. The PPG defines flood resilience developments as buildings “designed and constructed to reduce the impact of flood water entering the building so that no permanent damage is caused”. They are also designed in a way to ensure that their structural integrity is maintained, and to ensure drying and cleaning is easier.

To assist applicants, MHCLG published Improving the Flood Performance of New Buildings: flood resilient construction (2007). It provides guidance on how to improve the resilience of new properties against different flood risk sources. Details of flood resistance and resilience plans need to be provided as part of the FRA and / or Drainage Strategy submitted as part of the planning application. The SFRA provides EA approved guidance on flood resistant and resilient measures for Kingston, including information on finished floor levels. For further information, see Section 6.3.

Policy D11 ‘Safety, security and resilience to emergency’ of the current London Plan (2021) outlines property level resilience measure requirements. The policy requires that “Development proposals should maximise building resilience and minimise potential physical risks, including those arising as a result of extreme weather, fire, flood and related hazards”. In addition, Policy GG6 ‘Increasing efficiency and resilience’ states that those involved in planning and development must “ensure buildings and infrastructure are designed to adapt to a changing climate, making efficient use of water, reducing impacts from natural hazards like flooding and heatwaves, while mitigating and avoiding contributing to the urban heat island effect.”

The evidence base which supported Kingston’s Core Strategy (2012) highlights the importance of flood resilient design of buildings. The accompanying requirements of Policy DM 4 ‘Water Management and Flood Risk’ highlights that “in areas which are susceptible to flooding, development proposals should ensure that the buildings are designed to be flood compatible or incorporate flood resilient measures to mitigate flood risk.”

If proposed development is categorised as a ‘minor extension’ or a ‘vulnerable development’, please see the EA Flood Risk Standing Advice information for minor extensions and vulnerable developments respectively. These sections provide additional guidance on appropriate property resistance and resilience measures.

7.4 Emergency Plans

Emergency planning is vital to ensure the potential impact of flooding is minimised. As climate change and urban development increase the risk of flooding, there is a greater need for cohesive emergency planning at strategic and site-specific levels.

Development needs to ensure that it does not impede on the emergency services or Kingston Emergency Planning Unit’s response to any flood events. A borough-wide emergency plan can provide policy context on how emergencies, including flood risk, are managed within the borough. This can help define the response structure to emergencies within the borough and provide guidance on deployment and co-ordination. It can also provide further policy context for local Flood Warning and Evacuation Plans. Applicants need to ensure that appropriate evacuation and flood response procedures are in place and aligned to the wider strategic plan. This will help Kingston to better manage the ‘actual’ and ‘residual’ risks associated with an extreme flood event on a strategic and site-specific level.

Kingston’s Emergency Plan highlights all emergency situations which the Council is prepared to deal with and what their role during emergencies include. Amongst the listed emergencies is flooding, highlighting Kingston’s scalable ability to respond to flood risk emergencies which could impact large numbers of people. The roles taken up by Kingston during a flood risk emergency include (amongst others):

• Leading the recovery and restoration to normality (working with businesses and community groups).
• Organising road closures, diversions and other highways duties.
• Clean up and waste management work.

7.5 Managing Residual Risk

Residual risks are the risks that remain after the effects of the mitigating actions have been considered. Under current climate conditions, these risks need to be quantified to ensure the remaining risks can and will continue to be safely managed. However, as climate change alters the rainfall occurrence, duration, and intensity, the residual risks from a mitigation measure implemented today could significantly change over time.

The London Plan (2021) identifies the importance of managing residual risk via Policy SI12 ‘Flood risk management’. It highlights the importance of strategies mitigating residual risk through resistance and then resilience, ensuring safe evacuation and quick recovery to address such risks are in place. Projections indicate that climate change could increase the severity and impact of flooding, making it challenging for the emergency services to gain access as required. Developments should be designed with the impacts of climate change in mind to ensure that the emergency services continue to have access in extreme events.

Considerations also need to be made to, as a residual risk measure, ensure that people can remain within them and be safe and comfortable in the unlikely event of such an extreme flood. As the collective understanding of climate change increases, risks and residual risks may need to be re-evaluated. This will enable the LLFA, management companies and users to implement further control measures in the future as necessary.

7.6 Recommended Policies

Using the findings presented throughout the SFRA as an evidence base, a set of policy recommendations for planning development and flood risk management are presented below. These recommended policies are intended for adoption as part of Kingston’s Local Plan. The policy recommendations set out strategic and site-specific principles to guide flood risk management for prospective development within the borough. The policies seek to address the cumulative impacts of increased urbanisation on strategic flood risk management issues, whilst acknowledging climate change and the necessity of development to help Kingston meet housing requirements.

7.6.1 Strategic Policies

• Kingston should implement measures through their Local Plan to deal with the Sequential Test acceptability of windfall site development (sites which become available for development unexpectedly) proposals at the strategic level. The measure could set out locations and quantities of windfall sites that would or would not be acceptable in Sequential Test terms (to provide input to the process defined in Section 6.5.1). This would help create efficiencies.
• Kingston should incorporate the draft London RFRA 2018 recommendations into future Local Plan policies and documents once finalised. This includes Recommendation 2 (Fluvial Flood Risk) and Recommendation 3 (Surface Water Flood Risk) which provide recommendations in line with Policy SI 12 and Policy SI 13 respectively of the current London Plan. The recommendations are summarised as follows:
  • Recommendation 2 – Planning policies should focus on making the most of the opportunities presented by regeneration and redevelopment on river corridors to reduce fluvial flood risk through location, layout and design of development. Opportunities should also look at flood compatibility, flood resilience and maximising open space for flood water.
  • Recommendation 3 – Developments should reduce surface water discharge in line with the Sustainable Drainage Hierarchy set out in Policy SI 13 of the London Plan, and the actions in the London Sustainable Drainage Action Plan (LSDAP) should also be taken.
• Kingston should make space for water storage by identifying strategic locations that are required for current and future flood risk management. These identified areas of land should be safeguarded via Local Plans to facilitate links between flood risk management and other environmental priorities. Kingston should work with the LLFA and EA to identify such potential locations through flood alleviation schemes.
• Kingston should use their Local Plan to ensure developments within CDAs and defined sub-catchments (as listed in Section 2.4.5) provide increased surface water drainage requirements. Examples could include increased storage through the use of SuDS to restrict off-site runoff rates to greenfield (or lower) conditions.
• Kingston should consider implementation of further surface water flood risk mitigation requirements for proposed developments within Flood Zone 3a (surface water) where the development is also within the 1 in 30 year RoFSW mapped extents. These requirements could be similar to those adopted for Flood Zone 3b (fluvial) Functional Floodplain with modifications as follows:
  • Development within the 1 in 30 year RoFSW mapped extent will be treated as if it were Flood Zone 3b (Functional Floodplain) as defined in PPG Table 1 (Paragraph 065).
  • Development may be possible within the 1 in 30 year RoFSW mapped extents outside of existing infrastructure or solid building footprints.
  • The development must not increase flood risk elsewhere and where possible reduce flood risk overall.
  • Where beneficial to flood risk and/or other planning requirements, it may also be possible for development to occur within the functional floodplain through the relocation (but not increase of footprint size) of an existing building’s footprint within a site.
• Kingston should ensure that all permissible basement developments within an area of fluvial, surface water and groundwater flood risk should be fitted with resilience measures. This should be in line with the flood risk thresholds as detailed in Table 6‑1 and Table 6‑2. Measures, for example, may include waterproofing of walls and floors.
• Kingston should set up mechanisms to enable the use of CIL charges to be used for flood alleviation schemes across the borough to address the cumulative impact of development on flood risk.
• Kingston should adopt the policies within this SFRA into their Local Plan.

7.6.2 Site-specific Policies

1. Kingston should ensure where possible that land within development sites are safeguarded for potential flood mitigation use through the active consideration of predicted flood mapping from all sources. This can be done as part of the planning process or as part of wider flood risk assessments such as a Level 2 SFRA.
2. Development proposed in ‘dry islands’ (areas within Flood Zone 1 that are surrounded by areas at higher risk of flooding, i.e., areas falling within Flood Zone 2 and 3) should be designed for safe access and egress in a flood event. ‘Dry islands’ are considered as flood risk areas due to the potential loss of important local services during flood events and lack of safe access routes. They require safe access and egress routes to be developed for the lifetime of the property, factoring in the impacts of climate change.

Kingston should ensure that developments maximise the use of existing green and open spaces for water to flow during times of flood. This includes green spaces around main rivers and ordinary watercourses.