Sources and Assessment of Flood Risk

5.2.1. Definition

Fluvial flooding, also known as main river flooding, occurs when heavy or prolonged periods of rain or snow melt causes a river to exceed its capacity. Floodplains and adjacent open spaces in the natural environment help manage and convey fluvial flooding, mitigating the potential widespread impact. The impact of fluvial flooding on urban environments can be severe, causing significant social, economic and environmental impacts.

5.2.2 Assessment

The risk of flooding from fluvial sources is shown in the Fluvial Flood Risk Web Map. This breaks down the probability of fluvial flooding based on the EA’s Flood Zone categories. These Flood Zones are defined as follows:

• Flood Zone 1 – Land having a less than 1 in 1,000 annual probability of river or sea flooding.
• Flood Zone 2 – Land having between a 1 in 100 and 1 in 1,000 annual probability of river flooding; or land having between a 1 in 200 and 1 in 1,000 annual probability of sea flooding.
• Flood Zone 3a High Probability – Land having a 1 in 100 or greater annual probability or river flooding; or Land having a 1 in 200 or greater annual probability or sea flooding.
• Flood Zone 3b The Functional Floodplain – This zone comprises land where water has to flow or be stored in times of flood. Local planning authorities should identify in their SFRA’s areas of functional floodplain and its boundaries accordingly, in agreement with the EA.

The EA’s Flood Zones are based on the undefended flood scenario and do not account for the flood risk in an area that benefits from flood defence assets. Section 5.10 of this document provides further information on the local Flood Zone 3 approach and the LLFA definition of each.

The River Thames is an EA designated main river that flows along the north-western borough boundary. It is a non-tidal river throughout Kingston, but poses fluvial flood risk to the properties in its hydrological catchment.

The River Hogsmill and the Surbiton Stream are EA designated main rivers that flow through the borough. The Surbiton Stream is a tributary of the River Hogsmill, and the River Hogsmill is a tributary of the River Thames. The River Hogsmill flows through the middle of the borough from the east to the north-west of the borough (in the middle of the borough). The Surbiton Stream is situated to the south-west of the River Hogsmill (in the south-west of the borough). Both the River Hogsmill and the Surbiton Stream pose fluvial flood risk to the properties nearby.

The Beverley Brook is an EA designated main river that flows along the north-eastern borough boundary. It poses fluvial flood risk to the properties in its hydrological catchment.

Compared to the River Thames, the River Hogsmill, the Surbiton Stream and the Beverley Brook are flashier systems that have a faster response to heavy or prolonged periods of rainfall. For further information on the flood risk posed by the River Hogsmill, the Surbiton Stream and the Beverley Brook, see the Fluvial Flood Risk Web Map.

The Fluvial Flood Risk Web Map highlights areas at risk of fluvial flooding that currently benefit from flood defences. Flood defences are present for the River Hogsmill, the Surbiton Stream and the Beverley Brook within the borough. Structural failure of defences presents a residual risk due to breaching or overtopping onto defended areas.

The map also highlights the areas benefitting from flood defences on the ‘EA Flood Map for Planning (River and Sea) – Areas Benefitting from Flood Defences’ layer. The ‘Areas Benefitting from Flood Defences’ layer shows that there are no areas within the borough that benefit from flood defences, despite there being flood defences along the rivers previously mentioned. The ‘Areas Benefitting from Flood Defences’ layer only includes areas that are protected up to and including the 1 in 100 year return period event (i.e. a Standard of Protection up to and including the 1 in 100 year return period event). Therefore, the flood defences located along the River Hogsmill, Surbiton Stream and Beverly Brook within Kingston likely protect the local area for return period events that are below the 1 in 100 year event.

FRAs for development proposals should consider both ‘actual’ and ‘residual’ flood risks if a proposed site is protected by flood defences. FRAs must define the Standard of Protection of the local defences and address the residual risk associated with the specific defence asset. Section 6.3 defines development proposal requirements.

5.2.3 Impacts of climate change

Based on the EA’s UK climate change projections for peak river flow and peak rainfall intensity, it is expected that climate change will place a greater number of people, properties and infrastructure at risk of fluvial flooding. The frequency and severity of fluvial flooding would increase, increasing the need for flood defence and mitigation measures for the River Thames, the River Hogsmill, the Surbiton Stream and the Beverley Brook. The Web Mapping shows a range of fluvial flood extents and depths for different climate change scenarios.

5.3 Tidal Flood Risk

5.3.1.Definition

Tidal flooding occurs during extreme high tide and / or storm surge events. Water flows from the sea towards land, leading to the inundation of low-lying areas. This includes tidal river flooding, rivers whose level and flow are influenced by tides.

5.3.2 Assessment

There is currently no tidal flood risk within Kingston.

5.4 Surface Water and Ordinary Watercourse Flood Risk

5.4.1 Definition

Surface water (i.e. pluvial) flooding occurs because of high intensity rainfall resulting in water ponding or flowing over ground before entering an 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, leaving soil, drainage channels and other drainage systems incapable of draining water 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 (RoFSW) mapping.

5.4.2 Assessment

The extent of surface water flooding varies throughout Kingston. The borough has several large areas of green space, especially in the south of the borough. These areas help mitigate against the impact of surface water runoff due to vegetation and soil percolation reducing peak runoff. However, the borough also has heavily urbanised and densely populated areas. Most of the ground coverage in these areas is impermeable, increasing overland flows as less water is able to drain away through infiltration. These flow paths will flow towards topographical low points and result in a higher peak runoff rate. This places people and buildings along these overland flow paths at risk of surface water flooding.

The Surface Water Flood Risk Web Map highlights areas identified as at risk of surface water flooding. The mapping consists of the flood extent and depth of rainfall scenarios with a 3.3% (1 in 30 year), 1% (1 in 100 year) and 0.1% (1 in 1000 year) chance of occurring in any given year. The RoFSW map layers are national scale datasets that are optimised for viewing at 1:10,000 scale. They are not appropriate for identifying surface water flood risk at a property level scale. Information on surface water flood risk requirements and guidance for FRAs can be found in Section 6.5.2. Kingston has also defined the surface water 1% (1 in 100) rainfall scenario as Flood Zone 3a within the borough. Further information can be found in Section 5.10.3.

5.4.3 Impacts of climate change

Based on the EA’s UK climate change projections for peak rainfall intensity, it is expected that climate change will place a greater number of people, properties and infrastructure at risk of surface water flooding. The EA’s Adapting to a Changing Climate report (2016) highlights that wetter winters and more intense rainfall will cause more surface water runoff, leading to more localised flooding.

There are no EA specific surface water models that include climate change scenarios. This SFRA uses the RoFSW 1 in 1000 year return period event as the climate change scenario for the 1 in 100 year return period event. The extent and depth mapping for both the 1 in 100 year and 1 in 1000 year return period events are found in the Surface Water Flood Risk Web Map. The increase in surface water runoff as a result of climate change will place sewers and the drainage network under greater pressure, increasing the likelihood of sewer-related flooding. See Section 5.6 for information on sewer flooding.

5.5 Groundwater Flood Risk

5.5.1 Definition

Groundwater flooding occurs because of the water table rising, which can result in water emerging through the ground and causing flooding in extreme circumstances.  Groundwater flooding often occurs after extensive periods of heavy rainfall, potentially occurring for weeks or months. During these periods, a greater volume of water infiltrates through the ground, causing an underlying aquifer to rise above its regular depth. Springs and low-lying areas, where the water table is likely to be closer to the surface, pose greater risk of groundwater flooding. Groundwater flooding can occur in areas where the underlying soil and bedrock are vulnerable to becoming saturated. Ground composition and aquifer vulnerability are significant influences on the potential rate of groundwater flooding.

5.5.2 Assessment

The bedrock geology for the entirety of Kingston is the Thames Group, a geology type comprised of clay, silt, sand and gravel. The geological units generally have discontinuous distributions. Although often thin, they have great significance for engineering projects, due to their often highly permeable lithology and the development of hard calcareous concretions at several levels. Water moves less easily through this geology as it has a low hydraulic conductivity with clay, silt and sand.

The superficial geology for Kingston is a range of different deposits, including the Alluvium, River Terrace Deposits (Undifferentiated), and Sand and Gravel of Uncertain Age and Origin. The River Terrace Deposits (Undifferentiated), and Sand and Gravel of Uncertain Age and Origin are comprised of sand and gravel, geology with a higher hydraulic conductivity than those comprised of clay and silt. The Alluvium is a geological unit comprised of clay, silt and sand. Water moves less easily through this geology as it has a low hydraulic conductivity. Area-specific information on groundwater flood risk and geology is shown in Groundwater, Sewer and Artificial Flood Risk Web.

5.5.3 Impacts of climate change

There have been no investigations completed to identify how groundwater flood risk may be influenced by climate change. However, there are several potential ways in which climate change could impact groundwater flood risk. Rainfall intensity and duration variability could lead to a long-term decline in groundwater storage and an increase in groundwater drought periods and severity. The EA’s UK climate change projections for peak rainfall intensity predicts rainfall intensity to increase during future rainfall events. This could result in an increased frequency and severity of groundwater-related floods.

5.6 Sewer Flood Risk

5.6.1 Definition

Sewer flooding can occur due to:

• Drainage system failure (such as blockage or collapse).
• An increased flow and volume of water entering a sewer system, resulting in the system exceeding its hydraulic capacity and surcharging.
• Sewer outfall points being either blocked or submerged due to high water levels, causing back up in the system and flooding.

These issues can result in water overflowing from gullies and manholes, causing flooding in the local area.

5.6.2 Assessment

The sewer system in Kingston is owned and operated by TWUL. The borough is predominantly served by separate surface water and foul sewer systems. Modern sewer systems are designed to be separate systems, typically accommodating up to 1 in 30 year rainfall events in surface water sewers. However, sewer system segments across the borough vary in capacity due to age. Older segments have a lower capacity and may not be designed to accommodate rainfall events as significant as 1 in 30 year events.

TWUL have responsibilities for all ‘public sewers’ (the drainage network which serves more than one property, including associated manholes) under the Water Industry Act 1991. Typically gullies or drains and the interconnecting pipework which drain into sewers are the responsibility of the private landowner or, for those draining the highway, the Highways Authority. Due to the interconnection between these different assets, any associated flooding may be caused by a combination of factors, therefore all relevant parties should be involved in subsequent investigations and, where necessary, work to resolve the root cause.

Consideration needs to be given to the existing sewer network as part of all new development proposals. Kingston Planning and Building Control requires that applicants for major developments provide a drainage assessment form, detailed drainage design plans and check to see if a flood risk assessment is needed as part of the planning application that capacity exists in the public sewerage and water supply network to serve their development. As new developments are added into the catchment area additional capacity stress is applied to sewers, increasing the chance of them becoming overloaded. Sewer flood risk is something that can therefore increase throughout the borough. The Groundwater, Sewer and Artificial Flood Risk Web Map contains information regarding recorded sewer flood incidents.

5.6.3 Impacts of climate change

The predicted impacts of climate change on sewer flood risk are closely linked to potential changing rainfall patterns and intensity, as well as changes in surface water flood risk. Based on the EA’s UK climate change projections for peak rainfall intensity, the UK can expect an increase in rainfall intensity during future rainfall events. This would increase the flow and volume of water attempting entry into the drainage system, increasing the chances of overload. Overload can lead to surface water surcharging, resulting in localised flooding above ground. It can also cause an increase in untreated wastewater overflows entering the environment through combined sewer overflows into rivers, causing widespread damage.

5.7 Artificial Sources of Flood Risk

5.7.1 Definition

Artificial flooding can occur because of infrastructure failure or human intervention. Sources of artificial flooding include reservoirs, canals, ponds and other artificial structures. The probability of a structural breach is low; however, the potential extent of damage can be significant. Artificial source failure could leave many properties at risk of flooding.

5.7.2 Assessment

The Groundwater, Sewer and Artificial Flood Risk Web Map shows the risk of flooding from reservoirs. It shows the largest area that could flood if a reservoir were to fail and release the water it holds. Information presented on the Web Map displays the worst-case scenario, providing data that could be used for emergency planning purposes. For further details on emergency planning and other FRA requirements, refer to Section 6.

5.7.3 Impacts of climate change

Due to the complex nature of reservoirs and other large artificial infrastructure, the potential impacts of climate change on these structures are complex and varied. Climate change could result in extreme fluctuations in water levels due to changes in frequency and intensity of rainfall. This could therefore have a knock-on effect on reservoir yields.

5.8 Residual Risk of Flooding

5.8.1 Fluvial Defence Breach / Failure

The flood defences for the River Hogsmill, the Surbiton Stream and the Beverley Brook within the borough offer some protection against flooding from fluvial sources, however, risk still remains. Overtopping or failure of the flood defence assets could occur. Defences can also be overtopped due to wind and wave actions. In addition, structural failure of the flood defence assets can lead to these features being breached. The Fluvial Flood Risk Web Map shows the location of the flood defences, that could cause the residual risk because of the fluvial flood defence breach / failure.

For proposed developments within Flood Zone 2 of the River Hogsmill, the Surbiton Stream and the Beverley Brook, an assessment analysing the residual risk should be considered as part of an FRA. The probability of residual risks linked to overtopping and flood defence asset failure is small; however, the potential damage extent and potential risk to life is significant. Section 6 of this document contains further information on development requirements.

5.8.2 Flood warnings and alerts

The EA issues Flood Warnings and Alerts to specific areas when flooding is possible (flood alerts) or when flooding is expected (flood warning or severe flood warning). This enables the EA, residents and businesses to prepare, as required, to mitigate against the potential impacts of fluvial flooding. Severe flood warnings are issued where there is potential for risk to life.

The River Hogsmill, the Surbiton Stream and the Beverley Brook have smaller hydrological catchments compared to the River Thames. This makes them ‘flashier systems’ that respond faster to hydrological changes. As a result, these catchments may have shorter lead times for flood warnings and alerts. The flood warning / alert areas within Kingston can be seen from the Fluvial Flood Risk Web Map.

5.9 Historic Flooding

Kingston has historic flooding information for several different flood sources. The EA’s ‘Historic Flood Map’ dataset shows the maximum extent of all individual recorded flood outlines in Kingston. TWUL also provides historic flooding information on flood incidents reported to them. The historical records of sewer flooding (DG5) from TWUL are provided on the four-digit postcode areas. The DG5 database highlights a number of areas within the borough to be at a high risk of sewer flooding, with the following areas being particularly vulnerable (numbers in brackets indicate number of records of sewer flooding incidents between 1988 - 2021):

• Surbiton KT5 9 (409)
• Chessington KT9 1 (134)
• Kingston KT2 6 (229)
• Kingston KT1 3 (126)
• Chessington KT9 2 (207)
• New Maldon KT3 5 (126)
• Surbiton KT5 8 (185)
• Surbiton KT6 4 (113)
• Kingston KT2 7 (160)
• New Maldon KT3 4 (112)
• Worcester Park KT4 7 (157)

Applicants are advised to review these as part of the planning application process. Applicants are also advised to review the PFRA, LFRMS and SWMP for more information. If there are any queries regarding the records, they are advised to contact Kingston LLFA. For further FRA guidance, see Section 6.The EA’s ‘Historic Flood Map’ dataset can be viewed as part of the Fluvial Flood Risk Web Map, whilst the TWUL flood records dataset can be viewed as part of the Groundwater, Sewer and Artificial Flood Risk Web Map.

5.10 Flood Zones 3a and 3b – Fluvial / Surface Water

The two components of Flood Zone 3 are Flood Zone 3a and Flood Zone 3b. The EA has defined fluvial Flood Zones 3a and 3b and these are presented in Section 5.2.1. The designations for fluvial Flood Zone 3b should be further defined by LPAs in conjunction with the EA as described in Section 5.10.1. In addition to this, Kingston has included a surface water designation for Flood Zone 3a. Further information on Flood Zone 3b (fluvial), Flood Zone 3a (fluvial) and Flood Zone 3a (surface water) is detailed below. The Policy Web Map highlights the extents of Flood Zone 3a (fluvial), Flood Zone 3a (surface water) and Flood Zone 3b (fluvial) within the borough.

5.10.1 Functional Floodplain / Flood Zone 3b (Fluvial)

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. This SFRA defines Flood Zone 3b (functional floodplain – fluvial) as the following:

“Land within EA modelled fluvial flood risk extents predicted for up to and including 1 in 20 year return period events, allowing for the impact of flood defences. It also includes land featured as part of the EA’s Flood Storage Areas dataset.”

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 sources.

The Flood Zone 3b definition within this SFRA does not include surface water flood risk. It should be noted that a policy recommendation is made in Section 7 for Kingston to potentially impose additional requirements for developments within the 1 in 30 year RoFSW extent.

5.10.2 Flood Zone 3a (Fluvial)

The PPG defines Flood Zone 3a as land that has a ‘High Probability’ of fluvial or tidal flooding. There is a greater than 1 in 100 (> 1%) annual probability of river flooding, or a greater than 1 in 200 (> 0.5%) annual probability of sea flooding.

In accordance with the PPG, this SFRA defines Flood Zone 3a (fluvial) as the following:

“Land within EA modelled fluvial flood risk extents predicted for up to and including 1 in 100 year return period events.”

The extent of Flood Zone 3a (fluvial) within Kingston is shown in the Policy Web Map.

5.10.3 Flood Zone 3a (Surface Water)

This SFRA defines Flood Zone 3a (surface water) as the following:

“Land within EA modelled surface water flood risk extents predicted for up to and including 1 in 100 year return period events.”

The extent of Flood Zone 3a (surface water) within Kingston can be seen from the Policy Web Map. The Flood Zone 3a (surface water) extent uses the RoFSW map published by the EA. This is the most consistent and representative surface water dataset currently available.

Flood Zone 3a has been split into ‘(fluvial)’ and ‘(surface water)’ within the Policy Web Map, and the definitions of Flood Zone 3a (fluvial) (Section 5.10.2) and Flood Zone 3a (surface water) (Section 5.10.3) explain which datasets have been used for each. 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. Flood Zone 3a (fluvial) and Flood Zone 3a (surface water) may exceed the 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 that Flood Zone 3 does not equal Flood Zones 3a plus 3b.

Where Flood Zone 3a (surface water) exceeds the EA’s Flood Maps for Planning’s Flood Zone 3, Flood Zone 3a (surface water) 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): Environment Agency and for minor developments Local Planning Authority applying relevant Standing Advice
• Flood Zone 3a (surface water): Local Planning Authority only

5.10.4 Permitted Development Rights

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 amended) 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.