Local Plan Regulation 18 - Managing flood risk from all sources

Map - Flood risk zones

5.48 Most of the borough and the majority of its population are in the Environment Agency's Flood Zones 2 and 3 which indicates a medium to high risk of flooding from the River Thames. However, the actual extent of flood risk from the river is mitigated by flood defences such as the Thames Barrier and river walls which provide a high level of flood protection. If these defences were to ever fail or be breached though, large parts of the borough could be impacted by flood water with some areas experiencing deep and fast flowing flood water creating a risk to life. Flood defences are proposed to be raised in the future, but climate change impacts such as sea level changes mean that the extent of the area potentially impacted by such a breach or overtopping event is much larger than for the current risk.

5.49 Flood risks from the River Thames are focussed mainly in the central and southern parts of the borough. However, surface water flood risks are spread across the borough, and many streets are at risk during a major storm event. The brough's sewer system is a combined sewer system which means that stormwater run-off from roofs, roads and other impermeable surfaces and foul water from properties all flow into the same network of sewers and these can rapidly reach capacity during storms, causing flooding.

5.50 As well as flooding at external ground level, sewers can also surcharge and cause flooding internally inside properties, particularly at basement floor levels as foul water can back up in the drainage system and then flow out of fittings such as toilets and showers etc contaminating the property.

5.51 Basements are also potentially vulnerable to ingress of groundwater by seeping through floors, walls service ducts etc if they are not properly waterproofed. There is an increased potential for elevated groundwater in some parts of the borough although it should be noted that the Environment Agency has not designated any Groundwater Source Protection Zones in the borough that require specific protection. For further information on additional issues related to Basements and Lightwells, including a definition of a basement, see Policy DH10.

When is an FRA Required

5.52 To ensure that development proposals consider flood risks as part of the planning process, a Flood Risk Assessment (FRA) will be required as set out in the policy wherever there is a medium/high risk of flooding (either for the present day or in the future) from the River Thames or from major storm events. Major developments and any development proposals that include a basement floor element will also need to be supported by an FRA due to the potential for groundwater and sewer flood risks. Where relevant, FRAs must also consider potential impacts from major storms or flood defences failing or being breached which would mean developments are impacted by flood water in areas normally protected from such impacts. As well as implementing property level protection measures, flood evacuation plans should also be developed in case of such a flood emergency.

5.53 The requirement for an FRA to be submitted can be triggered by 1 or more flood risks being present now or in the future on a development site. As so much of the borough is located in the Environment Agency's Flood Zones 2 and/or 3 this is often the reason for an application to include an FRA, but if there are other potential sources of flooding for a site which are not covered in the FRA, then it will be deemed to be unacceptable.

5.54 FRAs should outline the mitigation measures to be implemented in the development. Examples of property level measures that may be suitable include raised floor levels, use of flood resistant materials and fittings such as doors and windows, structural waterproofing measures, raising of fittings such as electrical sockets etc, protecting drains from back-flow from the sewer and development where necessary of flood evacuation plans.

5.55 Further guidance on FRA requirements and mitigation measures is included in the Hammersmith and Fulham Strategic Flood Risk Assessment and the council's Planning Guidance and Climate Change SPDs.

Sequential and exception tests

5.56 Development proposals will need to be directed towards areas of lower risks from flooding where possible by applying the Government's Sequential Test procedure. This will have already been carried out for a limited number of allocated sites in the borough, information on which can be found in the council's SFRA which is currently being prepared. For all other sites, the developer will need to carry out their own Test. If the Sequential Test shows that the development cannot be located in an area with lower flood risks, then the Exception Test should be applied (unless it is exempt as set out in the NPPF). Allocated sites may have already been assessed, in which case it is not required to be repeated unless the proposals or local flood risks have changed.

5.57 The Sequential Test applies to development proposals in Flood Zones 2 or 3 which relates to flood risks from the River Thames. It is not required for Flood Zone 1, unless other flood risks are present, e.g. such as surface water flood risks.

5.58 In order for a site to pass the Exception Test, information will need to be provided in the FRA to show that the development would provide wider sustainability benefits that outweigh the flood risks and also that it can be made safe for its lifetime without increasing flood risks elsewhere. Full details of the Sequential and Exception Tests are contained in the NPPF and the national guidance on Flood risk and Coastal Change

5.59 As most of the borough is at current and/or future risk of flooding from at least one source, the council recognises that development proposals are unlikely in many cases to be able to identify alternative, lower flood risk sites for development. However, FRAs should still carry out this assessment before moving onto satisfying the requirements of the Exception Test and providing details on suitable flood mitigation measures.

Basements

5.60 Policy DH10 on Basements and Lightwells sets out full requirements in relation to basement proposals, but as basements are more susceptible to flood risk, they also need detailed assessment in terms of flooding, particularly as they can be impacted by groundwater and internal sewer flooding in addition to flooding from the River Thames or from surface water.

5.61 As noted in the Policy, certain development proposals such as self-contained basements or those that include sleeping accommodation at basement level will not be permitted in locations with the highest flood risks associated with the River Thames or a major storm event (either currently or in the future). This is to avoid inappropriate development in those areas as there could be a potential risk to life from fast flowing and deep flood water.

5.62 Basement developments can be considered in other locations, subject to the provision of an acceptable FRA showing that all relevant flood risks can be managed and occupants remain safe.

5.63 The FRA will need to show how groundwater risks have been assessed and taken into account for new basements and/or extensions to ensure that flood risks are not increased. FRAs must confirm the inclusion and provide details of appropriate structural waterproofing measures, which must be suitable for the proposed use. This is especially important for developments classified as "more" or "highly" vulnerable to flooding and includes change of use proposals that create more/highly vulnerable uses on sites previously used for less vulnerable activities.

5.64 The use of British Standard BS 8102:22 Code of Practice for Protection of Below Ground Structures against Water Ingress is recommended to guide the structural design measures for basements to ensure flood risks are managed both on-site and off-site.

5.65 Similarly, "more" or "highly"' vulnerable developments that include proposals at basement level, must confirm the installation of a pumped device (or equivalent) to mitigate against the potential risks of the sewer surcharging and flooding the property internally through fittings such as toilets.

5.66 The majority of the sewer network in the borough is a combined system which means that it is used to manage both foul water and surface water flows. Sewer flooding can occur when high volumes of surface water are directed into the sewer during heavy rainfall events and the system surcharges back into properties due to lack of capacity. The number of properties at risk of sewer flooding is likely to increase with climate change with interventions like sewer surcharge protection being installed.

5.67 Sewer flood risks are therefore often linked to surface water flood risks present on site. Policy CR3 sets out detailed requirements on managing surface water risks by implementing sustainable drainage measures. Policy CR9 on water efficiency is also relevant to flood risk management as minimising foul water flows into the sewer can help the dry weather flows in sewers to a minimum saving as much capacity as possible for stormwater flows when it is required.

Riverside sites and flood defences

5.68 H&F has existing flood defences along much of its riverside boundary which protect the borough from potential flood risks from the Thames. We are also protected by the Thames Barrier. However, sea levels are rising and there is a requirement, as set out in the Environment Agency's Thames Estuary 2100 Plan for provisions to be made to ensures that these defences, including the river wall defences, are maintained and also raised in the future.

5.69 Where developments are proposed along the riverside, these need to take account of future requirements for maintaining/raising the defences and either demonstrate in the FRA how those defences will be raised as part of the development proposals or show that they could be raised in the future.

5.70 For Hammersmith and Fulham, defences will need to be raised to the following heights in the future:

• By 2050 to a level of 5.90-6.00m AOD
• By 2090 to a level of 6.30-6.40m AOD

5.71 Riverside developments will also need to be set back from flood defences. For sites that are proposed within 16m of the flood defences, will need to demonstrate that they have the Environment Agency's agreement for this and that sufficient space will be provided in order to facilitate future flood defence maintenance and upgrade works.

5.72 Developers are encouraged to consult the Environment Agency early on in their design process for river side sites, particularly where there could be implications for the flood defences. It should also be noted that there is a separate environmental permitting system for works in proximity to the flood defences administered by the Environment Agency which also needs to be complied with separate to the planning policy requirements set out here.

5.73 The Local Plan also has standalone policies covering other issues relating to the riverside environment.