3. Sources and Assessment of Flood Risk

Surface water flooding occurs as a result of high intensity rainfall when water is ponding or flowing over the ground surface before it enters the underground drainage network or a watercourse. Ordinary Watercourse flooding occurs under similar circumstances but is associated with non-main river watercourses or ditches. Surface water flooding is often exacerbated by the intensity or duration of the rainfall event overwhelming drainage points, leaving soil, drainage channels and other drainage systems incapable of draining water away at a sufficient rate. Extreme weather conditions can also lead to ordinary watercourses exceeding their capacity, overwhelming systems and causing water to flow onto land.

For the purposes of this SFRA, the risk of flooding from ordinary watercourses is covered within the ‘surface water’ terminology. This aligns with the inclusion of ordinary watercourse flood risks within the EA’s Risk of Flooding from Surface Water mapping.

The majority of the ground coverage in the sub-area is impermeable as it is heavily urbanised. This can compound surface water flooding as the runoff rate is greater on impermeable grounds compared to permeable areas. In addition, less water is able to drain away through infiltration, which increases the surface water flood risk in these areas.

The Surface Water Flood Risk Web Map highlights areas identified at risk of surface water flooding from all sources. The map also highlights Critical Drainage Areas (CDAs). These areas are defined locally by a Borough’s SWMP and do not include areas with critical drainage problems as designated by the EA. Heavy rainfall and severe weather leave CDAs at risk from multiple flood risk sources, mainly surface water flooding but typically heavily interrelated with sewers and/or watercourses. For further information on how surface water flood risks have been incorporated into Flood Zones 3a refer to Section 3.11.

Groundwater flooding occurs because of the underground water table rising, which can result in water emerging through the ground and causing flooding in extreme circumstances. This source of flooding tends to occur after extensive periods of heavy rainfall. During these periods, a greater volume of water infiltrates through the ground, causing underlying aquifers to rise above its regular depth below the ground’s surface. Springs and low-lying areas, where the water table is likely to be closer to the surface, pose greater risks of groundwater flooding. Groundwater flooding can occur in areas where the underlying soil and bedrock can become saturated with water. Therefore, ground composition and aquifer vulnerability are significant influences on the potential rate of groundwater flooding.

A majority of the sub-region is underlain by Thames Group (also referred to as London Clay) bedrock, a composition of silty clay/mudstone, sandy silts and sandy clayey silts of marine origin. This geological unit generally has a low hydraulic conductivity which means water does not easily move through it. However, because of this characteristic and poor drainage, ponding can occur if London Clay is downhill of aquifer outcrops. Other predominant bedrock geology types are Lambeth Group compositions and White Chalk, both of which are predominantly found in the northwest of the sub-region. White Chalk in particular can be prone to groundwater flooding due to its high hydraulic conductivity and low effective porosity, meaning it can become saturated quite quickly due to intense rainfall and recharge the water table. In areas with a high water table, water can move through chalk and out onto the surface. Superficial deposits in the region are predominantly River Terrace Deposits which are comprised of sand and gravel, with lenses of silt, clay or peat. Area-specific information on groundwater flood risk are shown in Sewer, Groundwater & Artificial Flood Risk Web Map.

Sewer flooding can occur due to sewer infrastructure failure or due to an increased flow and volume of water entering a sewer system which exceeds its hydraulic capacity, causing the system to surcharge. If sewer outfall points are either blocked or submerged due to high water levels, water can back up in a sewer system and cause flooding. These issues can result in water overflowing from gullies and manholes, causing flooding in the local area. Blockages caused by sediment or debris can further exacerbate the probability of sewer flooding.

Drainage in the sub-region is serviced by Thames Water Utilities Ltd (Thames Water), who provide surface water, foul and combined sewer systems. Modern sewer systems are designed to be separate surface water and foul water systems, typically accommodating up to 1 in 30 year rainfall events. However, sewer system segments across London vary in capacity due to age. Older segments have a smaller capacity and may not be designed to accommodate rainfall events as significant as 1 in 30 year events. Combined sewer systems are also prevalent within older areas of London, including eastern parts of Ealing and Hounslow, leading to increased environmental risks were flooding to occur.

The Thames Water historical sewer flooding dataset provides details on the number of reported sewer flood incidents within a four-digit postcode area. Further information on historical sewer flooding is shown in the Sewer, Groundwater & Artificial Flood Risk Web Map.

Artificial Flooding occurs when the failure of infrastructure or human intervention results in flooding. Artificial flood sources include reservoirs, canals, water retention ponds, docks and other artificial structures. Though the probability of a structural breach is low, the potential extent of damage is significant. Flooding from an artificial source could leave many properties at risk.

The Sewer, Groundwater & Artificial Flood Risk Web Map shows potential reservoir breach inundation mapping, which displays the largest area that could potentially flood if a reservoir were to fail and release the water it holds. The information displayed by the Web Map is a worst-case scenario, providing data that could be used for emergency planning purposes. Further details on emergency planning and other FRA requirements, refer to Section 4.4.

Each Borough has differing levels of historic information. The majority of this comes from the EA’s Recorded Flood Outline dataset which shows all EA records of historic flooding and the EA’s Historic Flood Map which shows the maximum extent of all individual recorded flood outlines. This can be viewed on the Fluvial & Tidal Flood Risk Web Map. Further information can be obtained from the Borough Flood Investigation Reports produced under Section 19 of the FMWA (see Table 3-3). The Boroughs can investigate any flood event deemed necessary. Where they, as LLFAs, carry out these investigations, they must notify the relevant RMAs and publish the results of the investigations.

Applicants are advised to contact the respective Borough as part of planning application submissions to check against any other records that may exist and review published PFRAs, LFRMSs and SWMPs.

The NPPF set out how the planning system should help minimise vulnerability and provide resilience to the impacts of climate change. The Web Map shows the impacts of climate change on the various sources of flood risk as follows:

• Fluvial Flooding – A proportion of the main rivers within the sub-region have been assessed for impacts of climate change using the allowances defined in the EA Guidance for Flood Risk Assessment (2016). The Fluvial & Tidal Flood Risk Web Map includes flood mapping for the following climate change scenarios for main rivers as detailed in Table 3-4.

• Tidal Flooding – The Fluvial & Tidal Flood Risk Web Map shows the potential tidal defence breach inundation area for the year 2100 epoch. This is considered an appropriate representation of climate change impacts on tidal flooding for the purposes of this SFRA.
• Surface Water and Ordinary Watercourse Flooding – The Surface Water Flood Risk Web Map shows a range of surface water flood event annual probabilities (3.33%, 1% and 0.1%). There are no climate change scenarios available for the Risk of Surface Water Flood Map. Applicants should apply the correct climate change allowance for peak rainfall intensity to any assessment of surface water flood risk using the latest guidance. In line with this guidance, local exceptions may also apply where local evidence supports using other data or allowance. Where local modelling has been undertaken which uses the latest climate change allowances, this should be used. Otherwise, the six West London boroughs consider that without any other higher confidence data:
  • the 1% annual probability extent is considered to represent the current likely risk; and
  • the 0.1% annual probability extent represents the potential climate change adjusted impact of current risk.
• Groundwater, Sewer and Artificial Flooding – No specific climate change impact assessments have been completed for these flood risk sources. Existing flood risk mapping should be used until updated information is made available.

The Flood Risk and Coastal Change PPG defines functional floodplain as “land where water has to flow or be stored in times of flood” (PPG Table 1 in Paragraph 065). The PPG states that the extent of the functional floodplain, also known as Flood Zone 3b, should be defined by LPAs within their SFRAs. This allows for the incorporation of local circumstances and must be agreed with the EA and the boroughs’ LLFAs.

PPG Paragraph 078 states that the functional floodplain is usually defined, as a minimum, as land that would naturally flood up to and including a 1 in 30 year* return period event or designed to flood in 1 in 1000 year events. *Note: at the time of original publication of this SFRA, this was the 1 in 20 year return period event. Flood storage areas designed to protect downstream communities from flooding should also be included in the functional floodplain. This ensures that land that is required for current or future flood management features can be safeguarded from development, directly aligning to Paragraph 100 of the NPPF. The area identified as functional floodplain should take into account the effects of defences and other flood risk management infrastructure. Areas which would naturally flood, but which are prevented from doing so by existing defences and infrastructure or solid buildings, will not normally be identified as functional floodplain – further clarification of this is provided below in Section 3.11.2.

This SFRA adopts a definition for Flood Zone 3a that includes fluvial, tidal and surface water flood extents as described in the bold text boxes below. The extents are shown in the Web Map. The mapping extents have been split to aid applicants and the LPA through highlighting the flood risk source(s) which a site may fall within. The policy requirements are identical regardless of the flood source.

The Flood Zone 3b (functional floodplain) definition is adopted to ensure that future development is steered away from the most ‘at risk’ flooding extents from fluvial and tidal sources. The Flood Zone 3b definition within this SFRA does not include surface water flood risk, but it should be noted that a policy recommendation is made in Section 5 that may be adopted by some boroughs that could impose additional requirements for developments proposed within Flood Zone 3a (surface water) and the 1 in 30yr RoFSW extent.

The surface water mapping used, the Risk of Flooding from Surface Water (RoFSW) map published by the EA, is the most consistent and representative data currently available. Further information about this dataset can be found in its associated November 2013 guidance note (note that the published name for the Risk of Flooding from Surface Water map was previously the ‘updated Flood Map for Surface Water’ – the underlying dataset is the same).

Flood Zone 3a is defined as:
• Land within EA modelled fluvial flood risk extents predicted for up to and including 1 in 100 year return period events – Flood Zone 3a (fluvial / tidal)
• Land within EA modelled tidal flood risk extents predicted for up to and including 1 in 200 year return period events – Flood Zone 3a (fluvial / tidal)
• Land within EA modelled surface water flood risk extents predicted for up to and including 1 in 100 year return period events – Flood Zone 3a (surface water)

 

 

 

 

 

The River Thames model was updated in 2023 and incorporates the defended fluvial extents for Flood Zone 3b, and the undefended fluvial extents for Flood Zone 3a within Hounslow in accordance with EA guidance. Any tidal breach scenarios should be assessed separately to identify the risks associated against policy.

Flood Zone 3a has been split into ‘(fluvial / tidal)’ and ‘(surface water)’ within the Web Map, and the above definition explains which datasets have been used for each. Flood Zone 3a may exceed EA’s Flood Map for Planning’s Flood Zone 3 in some locations. This is due to differences in the modelling methodologies used to define fluvial and surface water flood risk mapped extents. It is for this reason why Flood Zone 3 does not equal Flood Zones 3a plus 3b.

Where Flood Zone 3a exceeds Flood Zone 3, Flood Zone 3a as defined by this document takes precedence and should be applied accordingly. It should also be noted that, Flood Zone 3a always includes land also defined as Flood Zone 3b. This is no different from how Flood Zone 2 incorporates land within Flood Zone 3. Sites within Flood Zone 3a and / or 3b as defined by this SFRA will be treated as if it were in Flood Zone 3 with regards to the needs of the NPPF, PPG and site-specific FRA submission requirements. Where a location is mapped to be within two or more Flood Zones, the requirements for the highest risk zone must be applied.

In line with nationally defined responsibilities for management of flood risk, applications will be assessed by the organisations defined as follows:

• Flood Zone 3a / 3b (fluvial / tidal): Environment Agency and for minor developments Local Planning Authority applying relevant Standing Advice
• Flood Zone 3a (surface water): Local Planning Authority only

 

This approach is consistent with existing statutory requirements and means that there is no change to the permitted development rights and policy requirements listed within the Town and Country Planning (General Permitted Development) (England) Order 2015 as these only relate to the EA’s Flood Zones 1, 2 and 3. For this reason, site-specific FRAs are still required for developments requiring prior approval (in relation to change of use permitted development rights) if located within EA Flood Zones 2 or 3.

The definition of Flood Zones 3a and 3b within this SFRA cannot amend the General Permitted Development policies without the adoption of an Article 4 Direction. Article 4 Directions are detailed on each Borough’s respective website and should be reviewed by applicants to ensure all requirements are met.

As noted in the previous section, areas which would naturally flood, but which are prevented from doing so by existing defences and infrastructure or solid buildings, will not normally be identified as functional floodplain (Flood Zone 3b). The impact of flood defences has been accounted for in the definition of the Flood Zones in the previous section. This section provides clarification of interpretation of Flood Zones 3a and 3b where the land being considered for development is already occupied by infrastructure or solid buildings that influence the extent of the functional flood plain.

As required by NPPF Paragraph 103, a site-specific FRA is required for all development proposals in Flood Zone 3 (the combination of Flood Zones 3a and 3b as defined in Section 3.11.1). The site-specific FRA must provide appropriate evidence to demonstrate which parts of the site are classified within Flood Zones 3a or 3b based on flood mapping and existing use. Any parts of the site that are mapped as being within Flood Zone 3b that are already occupied by infrastructure or solid buildings that provide a physical obstruction to existing flooding can be interpreted as being Flood Zone 3a. Any areas that do not meet this criterion should be interpreted as Flood Zone 3b. The use of this approach as a method for defining land exempt from Flood Zone 3b means that front and rear gardens are included within the functional floodplain when within Flood Zone 3b. Similarly, open spaces (defined in Section 336 of the Town and Country Planning Act 1990) are included within the functional floodplain where they have a level of risk probability of Flood Zone 3b.

The assessment process that should be used within the site-specific risk assessment to determine Flood Zones 3a / 3b within a development is recommended as follows:

1. Identify Flood Zones: Identify the extents of Flood Zone 3a and 3b within the site.
2. Assess Current Risk: Assess the interactions of existing infrastructure and / or solid building footprints with the Flood Zones (this can be done through comparison of existing flood mapping or undertaking appropriate flood risk modelling).
3. Determine Functional Floodplain: Determine areas acting as functional floodplain (this could include standing water and / or overland flow paths and assessment of flood depth where this information is available).
4. Assess Development Impact: Assess the impact of the proposed development on the functional floodplain (including the storage/flow route function of the land surrounding the proposed buildings) and associated potential change in flood risk on and off site.
5. Assess Potential Mitigation: Assess and select appropriate flood mitigation and resilience / risk reduction measures (using appropriate flood risk modelling techniques where required and recognising the predicted depth of flooding where this information is available). Within Flood Zone 3b development should be directed to making use of the existing built footprint. No additional building footprint would be acceptable in within Flood Zone 3b.
6. Determine Development Approach: Based on evidence (including site specific flood risk modelling where required) and guidance below, determine if development could be possible on all or part of the site through resilience, mitigation and / or compensation.

Development may be possible within land classified as Flood Zone 3b (fluvial / tidal) when it is directed to the areas occupied by existing infrastructure or solid building footprint (which are not currently floodable). The proposals must provide mitigation and resilience against flood risks, must not increase flood risk elsewhere, and aim to provide an improvement to the current situation by reducing the levels of risk. Proposals will not be acceptable where they introduce additional development footprint outside the existing solid footprint areas. Where beneficial to flood risk and / or other planning requirements it may 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 (taking advice from the Environment Agency as appropriate). To enable development, further guidance is provided in Section 4 (including detailed requirements for Major, Minor and Change of Use / change to Prior Approvals development proposals) and Standing Advice is available online.

Potential development must still align to the PPG’s Flood Risk Vulnerability and Flood Zone Compatibility table through not increasing the development’s vulnerability. The applicant must submit evidence to demonstrate the application of the Sequential Test and the passing of the Exception Test as appropriate in those parts of the site classified as Flood Zone 3a.