Draft Flood Risk Supplementary Planning Document

2.3 Part 3 of FRA: Assessing flood risk

2.3.1 This Section will assist applicants in competing Part 3: Assessing flood risk of the FRA Proforma (Appendix 2).  It will also inform the identification of appropriate mitigation measures and thus help to complete Part 5: Managing and mitigating flood risk of the FRA Proforma (Section 2.5).

2.3.2 Section 2.1 provides a starting point to understanding the sources of flood risk that affect a site with this section adding further detail regarding the exact nature and degree of risk. Further detail on each element of the Proforma is provided below along with references to key sources of information. In particular, Appendix E of the SFRA provides a useful overview of risk for each settlement area.

2.3.3 The level of assessment will depend on the degree and type of flood risk, scale and nature of development and its vulnerability classification. For example, a development within a high risk area may require additional modelling to confirm the exact nature of risk specific to that site. Applicants are advised to take advantage of the pre-application services set out in Section 2.1 in order to scope the likely level of assessment required.

Topography, Geology and Landscape features

2.3.4 When considering flood risk at the site level it is important to understand how the topography, geology and landscape features of the site influence the nature and degree of risk. For example, lower ground levels could cause floodwater to pond, steep slopes may increase run-off rates and vegetation will affect the speed that water flows away. Similarly, the underlying ground conditions of a site will influence saturation and run-off rates and thus affect the time period over which a site may be affected by flooding.

2.3.5 Whilst information within the SFRA will provide a useful overview across the Borough, site surveys and ground investigation reports may be necessary to confirm the exact nature of local topography and ground conditions affecting a site and its influence on flood risk within the site and surrounding area. 

Sources of information: SFRA Appendix B, Figure B1 & Appendix E; Site Survey

Watercourses

2.3.6 There are five Main Rivers in Elmbridge. These are shown below along with the catchment area they are located within.

  • River Wey (Lower Wey)
  • River Mole (Middle and Lower Mole)
  • Dead River
  • River Rythe
  • River Thames (Lower Thames)

2.3.7 There are also a number of smaller Ordinary Watercourses and Drainage Ditches in the Borough. These are small streams, ditches and drainage channels that form tributaries to the Main Rivers. 

Sources of information: SFRA Appendix B, Figure B4 & Appendix E

Flooding from Rivers

2.3.8 The Environment Agency undertakes regular modelling to determine the risk of flooding from rivers and categorises areas into low, medium, and high risk (Flood Zones 1, 2 and 3 - Table 10) in relation to a range of annual probability events. This is published on their Flood Map for Planning (Rivers and Sea). The Functional Floodplain (Flood Zone 3b) is not separately defined from Flood Zone 3 on the Flood Map for Planning (Rivers and Sea) but identified by the Council within the SFRA, in discussion with the Environment Agency. The SFRA mapping therefore reflects the zones as depicted in the Flood Map for Planning (Rivers and Sea) and identifies Flood Zone 3b. More information on how this has been defined is provided below. It is therefore essential when checking Flood Zone extents to always refer to the Council's SFRA mapping[1]

Sources of information: SFRA Appendix C and Appendix E

Table 10: Flood Zones

Flood Zone            

Flood Zone definition

Risk

Flood Zone 1  

Less than 1 in 1,000 chance of river flooding each year (0.1% annual probability)

Low

Flood Zone 2 

Between 1 in 100 and 1 in 1,000 chance of river flooding each year (between 1% and 0.1% annual probability)

Medium

Flood Zone 3a 

Land having a 1 in 100 or greater chance of river flooding each year (greater than 1% annual probability)

High

Flood Zone 3b 

Undeveloped land within the 1 in 20 year outline where water has to flow or be stored in times of flood, or land purposely designed to be flooded in an extreme flood event

Functional Floodplain

2.3.9 The modelling of the annual probability events does not take into account the presence of defences.

Defining the Functional Floodplain in Elmbridge

2.3.10 In accordance with national policy and guidance the Council has defined the Functional Floodplain within the SFRA. Those areas within the 1 in 20 year (5%) or greater flood extent have been mapped. Within this outline, undeveloped areas, where water has to flow or be stored in times of flood, are defined as Functional Floodplain. However, in Elmbridge, there are some areas within the 1 in 20 year (5%) or greater flood extent that are already developed and are prevented from flooding by the presence of existing infrastructure or solid buildings. Whilst these areas will be subject to frequent flooding, it may not be practical to refuse all future development. As such, in accordance with national policy, existing building footprints where they can be demonstrated to exclude floodwater will not be defined as Functional Floodplain. Consideration of what is developed or undeveloped will be determined on a case by case basis, having regard to the presence of existing (non-floodable) buildings on the site and the routing of floodwater through the site in times of flood. Appendix 3 contains further details of the Council's approach to development in these areas.

Depth, velocity and hazard

2.3.11 A range of additional data is available for each of the Main Rivers including information on the flood extent, depth and velocity (speed of flow) (Table 11).

Table 11: Modelling information for Main Rivers

River catchment

Information available

Lower Wey

1D-2D model. Flood extent, flood depth and velocity for each annual probability event

Lower Mole (Esher Railway Bridge to confluence with Thames at Molesey)

1D model. Flood extent for each annual probability event

Middle Mole (From Sidlow in Reigate to Esher Railway Bridge)

Flood extent, flood depth, and velocity for each annual probability event.

NB: The Environment Agency is currently remodelling this section of the River Mole.

Dead River

1D-2D model. Flood extent, flood depth, velocity and hazard rating for each annual probability event

Lower Thames (Hurley to Teddington)

1D-2D model. Flood extent, flood depth, velocity for each annual probability event.

NB: The Environment Agency is currently remodelling this section of the River Thames. It is anticipated that the Flood Map for Planning (Rivers and Sea) will be updated with the results of this modelling in April 2015

River Rythe

The Environment Agency is currently undertaking a modelling study for the River Rythe.

2.3.12 The information on flood extents is depicted within the SFRA mapping. More detailed information on depth, velocity and hazard as well as the model itself are available as a range of 'products' that can be obtained from the Environment Agency. Product 4 relates to depth, velocity and hazard. This information is essential to inform the design of the scheme and mitigation measures including safe access/egress routes, finished floor levels etc or as a basis for undertaking additional modelling. This information is provided free of charge to householders. 

To obtain a flood risk 'product' contact the Environment Agency via: enquiries@environment-agency.gov.uk or telephone 03708 506506 
Sources of information: Additional modelling may be required depending on the information available within the flood risk 'products' for the relevant watercourse, the nature of the proposals and mitigation measures required

 Taking account of climate change

2.3.13 As part of the modelling studies for rivers in Elmbridge simulations have been run for the 1% annual probability (1 in 100 year/Flood Zone 3) including the implications of climate change. It should be noted however, that this does take account of the presence of defences.

2.3.14 This means that sites indicated to be in lower risk areas (Flood Zone 2) could in future be in a higher risk zone (Flood Zone 3a). In areas where the impact of climate change indicates a greater flood extent the Council will reclassify and treat the area as 3a. 

Sources of information: SFRA Appendix C, Appendix E
 
Dry Islands

2.3.15 The floodplain within Elmbridge is relatively flat but there are areas of slightly higher ground which are less prone to flooding than the land around them or may not flood at all. These areas would however, be surrounded by water in times of flood and are often referred to as 'dry islands'. During times of flood it may be difficult to find a dry route of escape and those located in these areas may be unable to leave or require rescuing by the emergency services. Therefore, applicants should not only look at risk on the proposed site but also in relation to the wider area. 

Sources of information: SFRA Appendix C, Appendix E

 Historic river flooding

2.3.16 There is a long history of flooding from rivers in Elmbridge particularly in the Lower Thames, Lower Wey and Middle Mole catchments. There are no recorded flood events on the Dead River or the Lower Mole since the completion of the Lower Mole Alleviation Scheme in 1991.  Historical records of flooding are shown in the SFRA mapping.

Sources of information: SFRA Appendix C, Appendix E

 Existing flood risk management schemes

2.3.17 The Environment Agency's Asset Information Management System (AIMS) contains details of flood defence assets associated with Main Rivers. The majority of watercourses are not formally defended but may be informally defended by the presence of higher ground.

2.3.18 Formal flood defences are present on both sides of the River Mole (Lower Mole Catchment) from West End, Esher to its confluence with the River Thames. These defences form part of the Lower Mole Flood Alleviation Scheme and are shown as an 'Area Benefitting from Defences for Flood Zone 3' on the SFRA maps. 

Sources of information: SFRA Appendix C, Appendix E, Current Thames Catchment Flood Management Plan[2] and Draft Thames River Basin District Flood Risk Management Plan[3]
1. [The Environment Agency issue updates to their mapping on a quarterly basis. Where updates affect Elmbridge, this will be reflected within the SFRA mapping [back]
2. Current Thames Catchment Flood Management Plan - https://www.gov.uk/government/publications/thames-catchment-flood-management-plan [back]
3. Draft Thames Basin River Basin District Flood Risk Management Plan - https://consult.environment-agency.gov.uk/portal/ho/flood/draft_frmp/consult?pointId=s1407245469487 [back]

Flooding from Land (Surface Water)

2.3.19 Overland flow and surface water flooding typically arise following periods of intense rainfall, often short duration, that is unable to soak into the ground or enter the drainage system. It can run quickly off land and result in localised flooding. 

Risk of flooding from surface water

2.3.20 The Environment Agency has undertaken modelling of surface water flood risk at a national scale and produced mapping identifying those areas at risk of surface water flooding during three annual probability events: 3.33% annual probability (1 in 30 year) (High); 1% annual probability (1 in 100 year) (Medium/Low) and 0.1% annual probability (1 in 1,000 year) (Low) . Remaining areas would be classified as Very Low (<0.1% annual probability). The latest version of the mapping is referred to as the 'updated Flood Map for Surface Water' (uFMfSW) and is reflected within the SFRA mapping.

2.3.21 If a site is at High or Medium/Low Risk of flooding and there is a history of flooding in the area (see below) a topographic survey and/or additional modelling may be required to assess the degree and nature of risk.

2.3.22 Estimated depths of surface water flooding can be obtained from Surrey County Council and will be important in informing the design of mitigation measures.

Historic surface water flooding

2.3.23 Historical records of flooding are recorded by Surrey County Council, Elmbridge Borough Council, Highways England and the Environment Agency and are shown in the SFRA mapping. 

Sources of information: SFRA Appendix C, Appendix E, Topographic Survey and or Site Walkover may be required; Surrey County Council

Groundwater flooding

2.3.24 Groundwater flooding usually occurs in low lying areas underlain by permeable rock and aquifers that allow groundwater to rise to the surface through permeable subsoil following long periods of wet weather.

Risk of flooding from groundwater

2.3.25 If a site is indicated to either have potential for groundwater flooding to occur at the surface or for properties situated below ground level the Council will require a detailed ground investigation report and hydrology report to be prepared as part of an FRA. Development, particularly subsurface development e.g. basements, in these areas may affect groundwater flows. Even though the displaced water may find another course this may have implications for the surrounding area. FRAs must include proposals for mitigation measures e.g. perimeter drainage to ensure that existing sub-surface flows are not disrupted.

Historic groundwater flooding

2.3.26 Historical records of flooding are shown in the SFRA mapping. If the site has been affected by flooding in the past and the source is unknown this is likely to warrant further investigation. 

Sources of information: SFRA Appendix B, Figures B2, B3 and B5, Appendix C Appendix E, Ground Survey Investigation/Hydrology Report may be required

Flooding from Sewers

2.3.27 Flooding from the sewer system can occur:

  • During periods of heavy rainfall;
  • If the system becomes blocked by debris;
  • If the system surcharges due to high water levels in receiving watercourses

Historic sewer flooding

2.3.28 Thames Water holds records of internal and external property flooding by postcode area which is re-produced in the maps within the SFRA. An asset location plan can also be provided by Thames Water should this be necessary. 

Sources of information: SFRA Appendix B, Figures B7 and B8; Thames Water asset location plan
Applicants are reminded to liaise with Thames Water about connections to the sewer system and to submit information with any planning application to demonstrate sewer capacity either exists or will be provided prior to occupation of the development.

Flooding from reservoirs and other artificial sources

2.3.29 There are four large water supply reservoirs present in Elmbridge,

  • Queen Elizabeth II Storage, Walton
  • Bessborough Reservoir, Walton
  • Knight Reservoir, Walton
  • Island Barn Reservoir, East and West Molesey

2.3.30 Thames Water Utilities Ltd is responsible for the management of these reservoirs and ensuring all the required safety standards are met. The Environment Agency is the enforcement authority and all reservoirs are regularly inspected. Reservoirs in the UK have an extremely good safety record and therefore present minimal risk. The Environment Agency publishes mapping that identifies areas that could be flooded if a large reservoir were to fail and release the water it holds. This is known as the 'Risk of Flooding from Reservoirs' and is available on the Environment Agency website. There are also number of other artificial waterbodies in Elmbridge shown in the SFRA mapping. 

Sources of information: SFRA Appendix B, Figure B4; Risk of Flooding from Reservoirs[4]
4. Risk of flooding from reservoirs - http://watermaps.environment-agency.gov.uk/wiyby/wiyby.aspx?topic=reservoir#x=357683&y=355134&scale=2 [back]