4. Flood Risk Assessment Guidance

This sub-section provides developer specific guidance on a range of key requirements to ensure that development proposals are compliant. The guidance accompanies the information presented in the tables in Section 4.1.

Implementation of a sequential, risk-based approach is vital in determining the suitability of a site for development with regards to flood risk. For proposed development sites that require the application of the Sequential Test, and in some instances, the Exception Test, this document, and the Web Maps, provide the basis for applying these tests at a site-specific level.

Proposed development sites within multiple flood risk zones are classed under the highest Flood Zone present on site. For example, a site that partly falls under Flood Zone 1 and Flood Zone 2 is formally classified as a site in Flood Zone 2. The Flood Zone that each proposed site falls under helps inform the approach needed for the site and the information required for the planning application. The Sequential Test will need to be applied to steer the entire proposed site to the areas with the lowest risk of flooding. If the Exception Test is required, application is based on the highest flood risk zone the site is in and will need to be passed for the planning application.

Sustainable Drainage Systems (SuDS) incorporate a range of measures and management techniques designed to manage surface water runoff. All new developments should incorporate SuDS in line with the Non-statutory technical standards for sustainable drainage systems, unless there are practical and justifiable reasons for why they are not appropriate. The PPG highlights that the final decision on whether SuDS are truly inappropriate for a particular development proposal is a matter of judgement for the LPA.

The SuDS measures should aim to achieve greenfield runoff rates, providing management and attenuation features that ensure that surface water runoff is managed as close to the source as possible. Greenfield runoff conditions must be achieved for any greenfield sites. Development on current brownfield sites should also aim to achieve greenfield runoff rates where practical. To assist, several policy and guidance documents provide information to assist with the implementation of sustainable drainage. In addition to the London Plan, the London Sustainable Drainage Action Plan, and CIRIA guidance documents The SuDS Manual and Guidance on the Construction of SuDS provide important information.

The investigation needs to demonstrate the SuDS measures that the development will include and demonstrate how they connect with any piped drainage system required if infiltration is not possible. The submitted evidence needs to demonstrate that the London Plan drainage hierarchy has been followed, with surface water management features higher up the drainage hierarchy preferably incorporated:

1. Store rainwater for later use
2. Use infiltration techniques, such as porous surfaces in non-clay areas
3. Attenuate rainwater in ponds or open water features for gradual release
4. Attenuate rainwater by storing in tanks or sealed water features for gradual release
5. Discharge rainwater direct to a watercourse
6. Discharge rainwater to a surface water sewer/drain
7. Discharge rainwater to the combined sewer

To assist the application of the above hierarchy, reference should be made to the SuDS Suitability Mapping available in each borough’s SWMP to determine the potential of implementing infiltration-based SuDS. Where information is available, the SWMP indicates where infiltration based SuDS may be potentially suitable for use, where uncertainties exist and where they are unlikely to be suitable. Where infiltration SuDS are potentially suitable or uncertain, the applicant must provide site-specific infiltration testing or bore hole data to justify use of non-infiltration based surface water management techniques within their drainage strategy.

Not all developments that require a planning application have a bearing on a site’s existing drainage regime, or the potential impact of surface water flooding locally. This may include certain Minor developments that do not increase the built footprint of a site or do not introduce new building structures. Some of these cases may not present an opportunity to improve on-site water management. However, efforts should be made to improve the site’s drainage systems as the current regime may have wider flood risk implications for the area. For further information, contact the relevant LPA.

Further details on SuDS is provided in the tables in Section 4.1 and a West London-specific checklist for drainage strategies is accessible through the Checklist section.

Site-specific FRAs should be proportionate to the degree of flood risk, making the best use of available information. They should also be appropriate to the scale, nature and location of the development. For example, developments such as single house extensions would generally require a less detailed assessment as they tend not to significantly increase the number of people present in an area at risk of flooding. Conversely, new developments comprising of multiple houses in a similar location would generally require FRAs with greater detail. For further information, see the ‘Site-specific FRA’ key requirement section in Tables 4-1 (Major Developments), 4-2 (Minor Developments), and 4-3 (Change of Use), and the EA produced guidance on FRAs for Planning Applications.

The site-specific FRA requires potential flood depths to be addressed as part of flood risk management and emergency planning where there is a probability of flooding from any flood risk source. Depending on the circumstances, certain mitigation measures will need to be employed to demonstrate that the potential impacts of flood depth will be adequately addressed. The most appropriate measure depends on a range of different factors including flood risk source, the potential impact of the flood risk, and the vulnerability classification of the development amongst others. Where developments are proposed within Flood Zone 3a (surface water), flood plain compensation must account for predicted flood depths for the 1 in 30yr and 1 in 100yr RoFSW mapping or depths predicted by site specific modelling. Further details on the requirements of emergency planning can be found in Tables 4-1, 4-2, and 4-3.

For further guidance on the preparation and development of a site-specific FRA, the PPG has a checklist to provide guidance through the process. A West London-specific FRA checklist has also been developed as part of this SFRA and is accessible through the Checklist section.

As part of, or separate to, site-specific FRAs, information demonstrating how surface water runoff generated by the development site may need to be presented. As FRAs are not required for all developments, producing a separate drainage strategy may be advisable. A drainage strategy is a report that demonstrates how surface water could affect a site of interest and the surrounding areas. A strategy is required for all Major developments not categorised as ‘Change of Use’. This includes sites identified as being at risk of surface water flooding, and those that have a history of surface water flooding. All Minor developments and developments categorised as ‘Change of Use’ which modify existing surface water drainage will also require a Drainage Strategy. The report needs to demonstrate how water is expected to behave on a site, determine the site’s infiltration potential, runoff rates, and flow pathways, both before and after the proposed development is in place.

A Drainage Strategy Submission Checklist has been created alongside this SFRA (accessible through the Checklist section) and aligns with the standards and key policy requirements highlighted throughout Section 4. This template should be completed and included with any drainage strategy submissions to the relevant LPA. All six of the boroughs already have guidance forms that should also be completed and included with drainage strategy submissions where required:

• Barnet: SuDS Assessment Form
• Brent: SuDS Proforma (XLS / PDF)
• Ealing: SuDS Proforma (XLS / PDF)
• Harrow: SuDS Proforma (XLS / PDF) and Standard Drainage Requirements
• Hillingdon: SuDS Flows and Volumes Proforma
• Hounslow: Drainage Assessment Form

Further details on the SuDS requirements and SuDS implementation to address the impact of future growth are contained in Section 4.1 (Tables 4-1, 4-2, and 4-3) and Section 5.1 respectively.

As described in Section 3.3 and 3.4, a complex system of flood defences exists along the River Thames and its tributaries within the study area. The impact of these defences is shown on the Flood Management Infrastructure Web Map in the ‘Areas Benefitting from Defence’ layer and the location of the defences is shown in the ‘EA Flood Defences’ layer. To be classified as an Area Benefitting from Defence, the defence must provide a minimum standard of protection of 1 in 100yrs. This is why there can be areas with defences shown, but no associated Area Benefitting from Defence, as the defence in question likely provides a standard of protection less than 1 in 100yrs.

These defences are owned and maintained by a range of entities depending on their location and history. Most defences are owned and maintained by the EA, but others may be owned and maintained by riparian owners, Local Authorities or other entities. As defined in Table 4-1, any development within the stated buffer zones will need to contact the EA to determine the ownership and condition of any adjacent flood defences. Depending on the nature of the development, a Flood Risk Activity Permit may be required. An assessment of the condition of the defence and the associated impact on the Flood Evacuation Plan should also be considered.

Under section 21 of the FWMA, LLFAs are required to maintain a register of features and structures that are likely to have a significant impact on local flood risk. LLFAs have the power to ‘designate’ these structures and features on this register, which means that an application needs to be submitted to the relevant Borough to alter or remove one. For further information on asset registers and designated structures, contact the Borough directly. As defined in Tables 4-1, 4-2 and 4-3, any development within the stated buffer zones of an Ordinary Watercourse will need to contact the relevant LLFA to determine if any Ordinary Watercourse consents are required. The local of all mapped Ordinary Watercourses is shown on Flood Management Infrastructure Web Map.

In addition to the national, regional and local guidance available for flood risk management, LPAs provide supplementary guidance in the forms of Supplementary Planning Documents (SPD) and/or Supplementary Planning Guidance (SPG). These documents often supplement and operate in conjunction with Local Plans, providing guidance which goes over and above existing policies. Table 4-5 below highlights relevant SPD/SPG documents for each Borough which contains specific requirements on the information presented in Section 4.1. A complete list of SPG/SPD documents for each Borough can be found on their respective websites. It should also be noted that Boroughs may choose to adopt some or all of the Policy Recommendations made in Section 5 and developers should ensure they are applying the most recently published policies.

This sub-section provides development management specific guidance to ensure that the key requirements for individual planning applications can be effectively evaluated and assessed. Development should be considered at a strategic level, so it is important to identify how individual development proposals fit within a wider flood risk management strategy for a given area. The guidance accompanies the information presented in the tables in Section 4.1.

Implementation of a sequential, risk-based approach is vital in determining the suitability of a site for development with regards to flood risk. Developers need to apply the Sequential Test, and in some instances, the Exception Test, for any proposed development site that requires them. This document, and the Web Maps, provide the basis for applying these tests at a site-specific level.

Guidance on development in London, and the types of sites and locations to be considered, has seen a push towards certain considerations. The current London Plan identifies small site developments making an important contribution towards meeting housing objectives. In addition, the need to adopt a sequential approach to guide retail, commercial and leisure developments towards town centres is also of importance. These development objectives are consistent with the guidance and policies laid out in the draft of the new London Plan, making them important considerations for Boroughs when considering new development proposals.

The PPG contains information on development compatibility within different Flood Zones. This table works in conjunction with the PPG Flood Risk Vulnerability Classifications table to provide guidance on the types of development that may be considered as suitable within Flood Zones.

Sustainable Drainage Systems (SuDS) incorporate a range of measures and management techniques designed to manage surface water runoff. They are designed to mimic natural drainage as closely as possible, providing an alternative to ‘hard engineered’ traditional drainage. They provide opportunities to:

• Reduce the causes and impacts of flooding, providing opportunities to reduce the overall local flood risk.
• Minimise pollution from urban runoff at source.
• Enable groundwater recharge where infiltration is possible.
• Combine water management with green space, providing environmental, amenity and recreational benefits.

As highlighted in Section 2.2.2 of this document, Policy 5.13 of the London Plan is a key policy with regards to flood risk and water resource management. The policy provides the drainage hierarchy to ensure that reasonable measures are taken to sustainably manage and reduce the overall amount of rainfall being discharged from a development site. Developers should take measures to ensure that surface water management features higher up the drainage hierarchy are incorporated. The current London Plan drainage hierarchy is as follows:

1. Store rainwater for later use
2. Use infiltration techniques, such as porous surfaces in non-clay areas
3. Attenuate rainwater in ponds or open water features for gradual release
4. Attenuate rainwater by storing in tanks or sealed water features for gradual release
5. Discharge rainwater direct to a watercourse
6. Discharge rainwater to a surface water sewer/drain
7. Discharge rainwater to the combined sewer

Developers should aim to achieve greenfield runoff rates via their proposed SuDS measures and ensure that surface water runoff is managed as close to the source as possible. The proposed measures should be incorporated in line with the Non-statutory technical standards for sustainable drainage systems.

In December 2014, LLFAs became statutory consultees on major planning applications with surface water drainage implications. The associated Written Ministerial Statement, alongside the London Plan, demonstrate the importance of developers incorporating SuDS into their development proposals. This means that LPAs are required to consult LLFAs for expertise and technical advice on the management of surface water before reaching a decision on Major planning applications under the Town and Country Planning (Development Management Procedure) (England) Order 2015.

The issues that are analysed by LLFAs and LPAs for planning applications are referred to as ‘material planning considerations’, issues that are relevant to the decision making process. SuDS are a material planning consideration for major applications, and decisions on all planning applications require evidence that SuDS are implemented to ensure surface water is managed safely on site. Further information on material planning considerations, planning applications and the decision making process can be found on the Determining a Planning Application guidance page.

Submitted site-specific FRAs should demonstrate how flood risk will be managed now and in the future over the proposed development’s lifetime. The FRA needs to take climate change into account, and the vulnerability of land use classification of the development (Refer to Table 2 – Flood Risk Vulnerability of the PPG). An FRA should be provided with a planning application for developments in the following circumstances:

• New proposals in Flood Zone 2 or 3, including Minor Development and Change of Use. Minor developments include property sub-division (as this is ‘development’ defined by Section 55 of the Town and Country Planning Act 1990) and extensions that exceed the parameters of Permitted Development defined by Planning Portal Guidance.
• Proposals for development areas that are 1 hectare or greater in Flood Zone 1.

• New proposals, or a Change of Use in development type to a more vulnerable class, where the proposed development could be affected by sources of flooding other than rivers and the sea.
• Proposals within areas with critical drainage problems as designated by the EA (note that this does not include Critical Drainage Areas as defined by the Borough SWMPs – there are currently no such areas defined by the EA within the West London sub-region at the time of publication of this SFRA in March 2018).

As early as possible, development management should refer this SFRA and the Web Maps to developers, highlighting the key areas that developers should take note of as it could impact their proposals. For development proposals in areas at risk of fluvial or tidal flooding, there is a statutory requirement for LPAs to consult with the EA before planning permission is granted under the Town and Country Planning (Development Management Procedure) (England) Order 2015. For advice on when the EA should be consulted, and guidance for where fluvial / tidal flood risk is an issue, the EA has developed Standing Advice. In addition, the PPG has a checklist which can aid in the process of reviewing a site-specific FRA.

Developers may need to demonstrate how surface water runoff generated by the development site will be managed. This may be demonstrated through a drainage strategy, a report that should demonstrate how surface water could affect a site of interest and the surrounding areas. A strategy is required for all Major developments not categorised as ‘Change of Use’. All Minor developments and developments categorised as ‘Change of Use’ which modify existing surface water drainage will also require a Drainage Strategy.

DEFRA published the Non-statutory technical standards for sustainable drainage systems in March 2015. The document sets out non-statutory technical standards for SuDS relating to:

• Flood risk outside the development
• Peak flow control
• Volume control
• Flood risk within the development
• Structural integrity
• Designing for maintenance considerations
• Construction

These standards should be used for the assessment of surface water drainage strategies submitted with planning applications.

The impact of flood defences along the River Thames and its tributaries can be seen on the Flood Management Infrastructure Web Map in the ‘Areas Benefitting from Defence’ layer. In addition, the location of the flood defences can also be seen on the map in the ‘EA Flood Defences’ layer. Places classified as Areas Benefitting from Defence provide a minimum standard of protection of 1 in 100yrs.

Most defences are owned and maintained by the EA, but others may be owned and maintained by riparian owners, Local Authorities or other entities. As highlighted in Tables 4-1, 4-2 and 4-3, developments within the stated buffer zones will need to contact the EA to determine the ownership and condition of any adjacent flood defences. Developers may also need to submit a Flood Risk Activity Permit depending on the nature of the development. Developers should also consider providing an assessment of the condition of the defence and the associated impact on the Flood Evacuation Plan.

If a development is within the buffer zone of an Ordinary Watercourse (as defined in Tables 4-1, 4-2 and 4-3) developers are required to contact the relevant LLFA to determine if any Ordinary Watercourse consents are required. The local of all mapped Ordinary Watercourses is shown on Flood Management Infrastructure Web Map.

Developers are required to follow any borough-specific requirements when addressing flood risk as part of their planning application submission. LPAs may provide guidance through Supplementary Planning Documents (SPD) and/or Supplementary Planning Guidance (SPG) that accompany the Local Plan. These are listed in Table 4-5. Development management should refer to these when reviewing planning applications as they support the borough-wide objectives for flood risk management.

This sub-section provides guidance on managing flood risk at a strategic level. The NPPF and PPG highlight that developments should be directed away from the highest areas of risk and that developments should be made safe without increasing flood risk elsewhere. Due to development needs and demands, the NPPF identifies that it may not always be possible to completely avoid flood risk areas. The guidance provided in this sub-section supports borough level strategic planning and supplements information presented in the tables in Section 4.1.

The NPPF highlights the need for a sequential, risk-based approach to be considered for development. This approach aims to keep development out of Flood Zones 2 and 3, and areas at risk from other sources of flooding, where possible. Implementation of the sequential, risk-based approach requires proposed development sites to be reviewed through the application of the Sequential Test, and in some instances, the Exception Test. This document, and the Web Maps, provide the basis for applying these tests, at the site-specific level.

Strategic application of the Tests for Allocated Sites, if required, are generally completed as part of the Local Plan development process by LPA officers. This process should be informed by the initial screening assessment completed for current Allocated Sites provided in Appendix A. Recommendations for Level 2 SFRAs are made in Section 6 where further flood risk information and assessment may be required to inform the Tests. Guidance is provided in the following sections for application of the Test at the Local Plan / strategic scale.

The Sequential Test ensures that a sequential approach is followed to steer new development to areas with the lowest probability of flooding. This document provides the evidence base for the Sequential Test to be applied at a borough-wide level in preparation for a Borough’s Local Plan and associated Allocated Sites. If the application of the Sequential Test demonstrates that development can be allocated in Flood Zone 1, then the Sequential Test has been passed. However, some lower flood risk areas may not be suitable for development due to various other reasons. In these instances, the Sequential Test should be applied to guide the development to the lowest risk area appropriate for the development type. This increases the possibility of facilitating development which is at the lowest risk of flooding in line with the relevant vulnerability of land use classification. The PPG flowchart demonstrating the ‘Application of the Sequential Test for Local Plan Preparation’ provides guidance.

The following process is recommended to complete the Sequential Test for site allocations during Local Plan development based on the PPG development vulnerability classification:

1. Complete a screening assessment of all sites to identify flood risk sources and how they might be impacted by Climate Change. The Web Maps should be used to identify flooding from all sources as detailed in Section 3. Climate change guidance is provided in Section 3.10. Note this screening assessment has already been completed for sites allocated at the time writing this SFRA – Refer to Appendix A and the Policy Web Map.
2. Assess how long it is anticipated each development will be present for (the ‘design life’). A design life of 100yrs for residential development and 60yrs for non-residential development is recommended if no other information is available.
3. Any ‘Highly Vulnerable’ developments should be located within Flood Zone 1. If this is not possible due to a lack of suitable sites, then locations in Flood Zone 2 can be considered where the Exception Test can be passed. If no suitable sites exist in Flood Zones 1 or 2, then further opportunities for development locations should be sought (this could be within or outside the Borough)
4. A similar process can then be applied to ‘More Vulnerable’ developments with priority given to locations within Flood Zones 1 and 2. If there are no suitable sites, then Flood Zone 3a can be considered – noting that the Exception Test will need to be passed.
5. ‘Less Vulnerable’ developments can then be located within remaining sites in Flood Zones 1, 2 and 3a (in that order of preference). This development classification is not appropriate for Flood Zone 3b.
6. ‘Essential infrastructure’ should also be preferentially located in the lowest risk Flood Zone available for the type of infrastructure. This development can be located in Flood Zone 3a or 3b after passing the Exception Test.
7. ‘Water compatible’ development should be allocated last as they generally have the fewest constraints with regard to flood risk.

Where proposed site allocations are at a risk of flooding from one or more sources, Level 2 SFRA recommendations are made in Section 6 for specific Allocated Sites within each Borough. The Level 2 SFRA can provide site-specific flood risk management recommendations and an assessment of whether the site could pass the exception test on this basis.

The Exception Test should be applied after the Sequential Test if it has been determined that a proposed development cannot be located in an area with a lower flood risk. To pass the Exception Test and ensure that flood risk to both people and property is effectively managed across the proposed developments lifetime, the PPG sets out two considerations that need to be achieved. These considerations are:

• The development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared; and
• A site-specific FRA must demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall.

The PPG ‘Application of the Exception Test for Local Plan Preparation’ flowchart provides guidance on applying the Exception Test for Local Plans. The flowchart highlights that following the borough-wide level Sequential Test, the Exception Test will need to be applied if certain development sites are not in an appropriate location. Guidance for what is deemed an appropriate location is based on NPPF flood risk policy as highlighted in Section 2.2.1. A Level 2 SFRA may also be used to assess Allocated Sites in more detail to determine if the Exception Test can be passed. Recommendations for Level 2 SFRA assessments are made in Section 6.

Under Section 21 of the FWMA, LLFAs are required to maintain a register of features and structures that are likely to have a significant impact on local flood risk. LLFAs have the power to ‘designate’ these structures and features on this register, which means that owners and developers need to submit an application to the relevant Borough to alter or remove one. These registers are available to the public and access can be arranged through the relevant LLFA if the information is not already published on the Local Authority website.

To ensure that future developments incorporate flood risk management measures in line with the findings of this SFRA, a set of strategic and site-specific policy recommendations are provided in Section 5.3. These recommendations could be adopted as part of the flood risk planning policies within the developing Local Plans. Recommendations supporting the implementation of SuDS, the requirement of site-specific FRAs and drainage strategies, and strategic implementation of the Sequential Test are provided amongst other policies designed to guide flood risk management.