Flood risk

CC1: Flood risk 1. All development should be directed to areas at lowest risk of flooding from all sources, taking account of climate change, and should reduce the causes and impacts of flooding both on and off-site. 2. Inappropriate development in areas with a high or medium risk from all sources of flooding will not be supported. 3. The functional flood plain (Flood Zone 3b) is safeguarded for the purpose of storing and allowing water to flow in times of flood. 4. Development in any area at risk of flooding will be required to demonstrate compliance with the sequential test and, where necessary, the exception test as set out in national policy and guidance and latest Strategic Flood Risk Assessment (SFRA). Exceptions to this requirement for sequential testing include: a) Development in accordance with a Development Plan allocation that has been sequentially tested. Developments for more vulnerable uses than the allocated use will still require sequential testing. b) Minor development or changes of use (excluding change of use to a camping, caravan or mobile home site). c) Development where a site-specific FRA demonstrates that all development is located outside areas of flood risk from all sources, now and in the future. 5. A site-specific Flood Risk Assessment is required for: a) All development in areas at risk of flooding (now or in the future). b) All sites greater than 1ha. To demonstrate that there is no increase in flood risk onsite or harm to third parties offsite. 6. All development must adhere to the advice in the Strategic Flood Risk Assessment, including demonstrating in a site-specific flood risk Assessment all the following: a) The risks from all sources of flooding now and taking account of climate change allowances for the lifetime of the development, including – but not limited to – fluvial flooding, groundwater flooding and surface water flooding. b) where appropriate, detailed modelling of any ordinary watercourse or surface water risk within or adjacent to the site, to define in detail the area at risk of flooding and model the effect of climate change. c) A sequential approach to site layout, directing the most vulnerable uses to the areas of lowest flood risk. d) The feasibility of safe access and escape routes to the site and emergency planning procedures for the lifetime of the development. e) Resilient and resistant construction methods for managing residual risk. f) Opportunities to reduce the causes and impacts of flooding both on and offsite. g) The drainage characteristics of the site and the feasibility of Sustainable Drainage Systems (SuDS), taking account of climate change. h) cumulative impacts on flood risk and, where appropriate, how the proposal will contribute to catchment-wide modelling or mitigation measures. Developments should take opportunities for betterment of existing flood risks both within the site and in surrounding areas. i) the impacts on vulnerable groups have been considered. 7. Sites including 50 dwellings or more, or 1 ha or more for non-residential sites, will be required to produce a strategic drainage strategy which will inform the masterplanning and comprehensive delivery of SuDS across the site, maximising drainage and other co-benefits (such as biodiversity, landscape and amenity). This must consider climate change allowances and residual risk. 8. Development must not be located above existing culverts. Culverting of open watercourses is not permitted except where essential for infrastructure crossings and where no practicable alternatives exist. New culverts must comply with the Council’s Culvert Policy and CIRIA C786 (as replaced). 9. Development proposals must use the most up-to-date flood risk data and consult the Environment Agency and Lead Local Flood Authority (LLFA) as appropriate. 10. A Surface Water Drainage Strategy should be required for all developments in Chesham and High Wycombe. This will seek to reduce existing flood risk in Chesham and High Wycombe, which may include making a developer contribution towards wider flood alleviation works, as appropriate. 11. A Surface Water Drainage Strategy is required for all developments in higher sensitivity catchments set out in the SFRA Level 2 Cumulative Impact Assessment. 12. New settlement ‘NESS’ site policy areas shall be accompanied by an overall surface water drainage strategy. This shall set out how the cumulative impacts of potential peak rates and volumes of water from development sites would impact on peak flows, duration of flooding and timing of flood peaks on receiving watercourses. Definition of Flood Risk 7.486 Flood is formally defined within the Flood and Water Management Act 2010, as occurring “where land not normally covered by water becomes covered by water”. Flood risk is defined as the combination of the probability of a flood occurring and the consequences or harm should that flooding occur. 7.487 Development proposals must meet the Flood Risk Vulnerability and Flood Zone Compatibility Table set out in the NPPF. Inappropriate development in areas with a high or medium risk from all sources of flooding will not be supported. 7.488 For the purposes of this policy, ‘Residual risk’ comes in two main forms: Residual risk from flood risk management infrastructure; and Residual risk to a development once any sitespecific flood mitigation measures are considered. Residual risk is the risk remaining after avoidance, control and mitigation have been utilised. 7.489 The term ‘Inappropriate Development’ in (2) this refers to the Flood Risk Compatibility in Planning Practice Guidance for Flood Risk and Coastal Change Table 2 and the NPPF Annexe 3 Flood Risk Vulnerability Classification. Split of responsibility 7.490 The responsible bodies for flood risk management on Buckinghamshire’s watercourses are as follows. The Flood and Water Management Act place a duty on all flood risk management authorities to co-operate with each other: 7.491 Buckinghamshire Council is designated a Lead Local Flood Authority (LLFA). The core responsibility of the LLFA is co-ordinating the management of flooding that arises from local sources in Buckinghamshire under the auspices of a Local Flood Risk Management Strategy. Local sources are defined as surface water, ordinary watercourses and groundwater flooding. 7.492 The LLFA role is a co-ordination role bringing other key stakeholders such as the EA, Highways and Water Companies together to ensure joined-up management of flood risk, rather than the LLFA undertaking the works itself. The LLFA ‘s other main responsibilities are the Local Flood Risk Management Strategy investigations, land drainage consenting and enforcement. The LLFA is also a statutory consultee to the Local Planning Authority on surface water drainage arrangements on major planning applications. 7.493 The Environment Agency are responsible for taking a strategic overview of the management of all sources of flooding and coastal erosion. Also has operational responsibility for managing the risk of flooding from main rivers, reservoirs, estuaries and the sea (as well as being a coastal erosion risk management authority). 7.494 Canal & Rivers Trust are responsible for canals. 7.495 Riparian owners are responsible for maintaining the watercourse or ditch running through, underneath, or adjacent to the boundary of their land. If the watercourse forms the boundary of the land, the riparian ownership responsibilities are shared equally between the landowners on either side. 7.496 Buckinghamshire Council Highways are responsible for fixing drainage and flooding issues on adopted highways and roads in Buckinghamshire. 7.497 Water and sewerage companies are responsible for flooding and flood risk from public sewers and water mains. 7.498 Bedford Group of Internal Drainage Boards (IDBs) responsible for managing water levels and watercourses in their area (https://www.idbs.org.uk/about-us/boards-drainage-district/ ). A small area of North/Northwest bucks is covered by the Buckingham and River Ouzel IDB. Early development proposals 7.499 It is recommended that as part of the early discussions relating to development proposals, developers discuss requirements relating to site-specific flood risk assessment and drainage strategies with both the Local Planning Authority and the Lead Local Flood Authority (LLFA), and the Environment Agency /Internal Drainage Board to identify any potential issues that may arise from the development proposals. 7.500 There may be the need to undertake detailed modelling of any ordinary watercourse within or adjacent to the site to define in detail the area at risk of flooding and model the effect of climate change. The published SFRA should be used as a starting point to identify where modelling of watercourse would be required. 7.501 Flooding can occur away from the flood plain because of development where off-site sewerage infrastructure and capacity is not in place ahead of development. Therefore, early engagement with the relevant water company is also important in preparing planning applications. 7.502 Groundwater guidance including Groundwater monitoring will need to be undertaken from the start of October to the end of April to provide suitable information as part of the Flood Risk Assessment. Planning application stage 7.503 At the planning application stage, developers will need to undertake more detailed hydrological and hydraulic assessments of the watercourses where required, using channel, structure, and topographic survey. This should happen particularly where there are no detailed hydraulic models. The modelling should verify flood extents with the latest climate change allowances. Developers should engage at the earliest opportunity with the EA and LLFA on detailed modelling requirements. Sites in Local Plan versus sites not in Local Plan 7.504 For clarity, the council uses the term ‘Development Plan’ as this policy applies to both this Local Plan, the Minerals and Waste Local Plan, and Neighbourhood Development Plans. For sites allocated within the Local Plan, the Local Planning Authority should use the information in the SFRA produced for the Local Plan evidence base to inform the Exception Test. 7.505 For developments that have not been allocated in the Local Plan, developers must undertake the Sequential Test followed by the Exception Test (if required) and present this information to the Local Planning Authority for approval. In areas at risk of flood this will need to demonstrate that there are no sites available in a lower flood risk zone. This assessment will need to include allocated sites. The scope of the sequential test should relate to the scale and location of a development and should be agreed with the Council beforehand. The Council needs to be satisfied that the sequential test assessment justifies the site being developed before planning permission can be granted. Only new water compatible and essential infrastructure, if the Exception Test is passed, is appropriate in areas defined as the functional floodplain. Applicants should refer to tables 1 to 3 in the National Planning Practice Guidance. Depending on vulnerability of users, developments may need to demonstrate that they can meet the Exceptions test before being deemed appropriate. 7.506 The policy makes provision for exceptions, including in situations where a site-specific flood risk assessment demonstrates that no built development within the site boundary, including access or escape routes, land raising or other potentially vulnerable elements, would be located on an area that would be at risk of flooding from any source, now and in the future (having regard to potential changes in flood risk). 7.507 Developers should consider flood resilience measures for new developments. New development and re-development of land should wherever possible seek opportunities to reduce overall level of flood risk at the site, for example by: a) Reducing volume and rate of runoff using SuDS. b) Relocating development to areas with lower flood risk. c) Creating space for flooding. 7.508 For the purposes of this policy, ‘Minor development’ means: a) Minor non-residential extensions: industrial/commercial/leisure etc. extensions with a footprint less than 250 square metres. b) Alterations: development that does not increase the size of buildings e.g. alterations to external appearance. c) Householder development: For example, sheds, garages, games rooms etc. within the curtilage of the existing dwelling, in addition to physical extensions to the existing dwelling itself. This definition excludes any proposed development that would create a separate dwelling within the curtilage of the existing dwelling e.g. subdivision of houses into flats. 7.509 Green and blue Infrastructure should be considered within the mitigation measures for surface water runoff from potential development and consider using areas at risk of flooding as public open space. Consideration must be given to the potential cumulative impact of development on flood risk. This requires catchment-based modelling or developer contributions toward the outputs of such modelling where this has been done on another site. When designing a building, the finished floor levels should be a minimum of either 600mm or 300mm above the 1 % Annual Exceeded Probability (AEP), plus climate change peak flood level, depending on the development vulnerability classification. 7.510 Developments should look for opportunities for betterment where surface water flooding issues are present, which could be implemented through masterplans for individual strategic sites or new settlements. Proposals should integrate water conservation through rainwater harvesting and water butts and promote land management practices to attenuate runoff and alleviate potential issues downstream. Finally, development proposals should identify opportunities to help fund future flood risk management through developer contributions to reduce risk for surrounding areas and identify opportunities to make space for water to accommodate climate change. 7.511 The management of residual risk can be aided by safeguarding land from development that is required, or likely to be required, for current or future flood management. 7.512 Culverting open watercourses will not be acceptable, except where essential to allow highways and/or other infrastructure to cross, and where there are no reasonably practicable alternatives to culverting. In any case, the LLFA will not support development above existing culverts. Where new culverts are necessary, their design must comply with Buckinghamshire Council’s Culvert Policy, secure Land Drainage Consent from the Lead Local Flood Authority and be designed in line with CIRIA’s Culvert, Screen and Outfall Manual (C786). Planning permission does not negate the requirement for land drainage consent, nor does it guarantee that consent will be given. Access and Escape 7.513 Planning applications must ensure development is ‘safe’, dry pedestrian escape from the floodplain and emergency vehicular access should be possible for all residential development. If at risk, then an assessment should be made to detail the flood duration, depth, velocity and flood hazard rating in the 1% AEP plus climate change flood event, in line with Flood Risk Guidance for New Development FD2320 (as replaced). 7.514 A route can only be completely safe in flood risk terms if it is always dry. By placing the future occupants of the development, including potential vulnerable people into this area of existing flood risk and safe access and escape issues, this will create an increased burden on key service providers. These are the local authority, emergency services and other associates' parties during a time of flood. 7.515 Safe refuge areas should be provided wherever there are significant residual risks to developments associated with extreme flood events and/or rapid inundation. 7.516 Access and escape should be considered at the site, but also in the vicinity of the site, for example, a site may have low surface water risk, but in the immediate locality, access/ egress to and from the site could be restricted for vehicles and/ or people. Floodplains 7.517 The extent of the functional floodplain (Flood Zone 3b) is defined through the local plan’s latest Strategic Flood Risk Assessment. 7.518 Development proposals should identify long-term opportunities to remove development from the floodplain and to make space for water. Development concerning brownfield sites in the functional floodplain should seek to reduce risk to the wider floodplain and provide flood risk betterment. Other Important Considerations 7.519 Consult the Lead Local Flood Authority if an ordinary watercourse is within or immediately adjacent to the site area. Consult the Environment Agency if the site contains or is immediately adjacent to a main river. If alterations or discharges are proposed to the watercourse, a land drainage consent will be required. Where necessary, blockages of nearby culverts may need to be simulated in a hydraulic model to confirm residual risk to the site. 7.520 Surface water risk should be considered in terms of the proportion of the site at risk in the 3.3% AEP (30-year), 1% AEP (100-year) or 0.1% AEP (1,000-year) events, whether the risk is due to a wider overland flow route. Surface water risk and mitigation should be considered as part of a detailed site-specific Flood Risk Assessment and Surface Water Drainage Strategy. 7.521 Sites where there is a canal within or immediately adjacent to the site area, developers should consult the Canals and Rivers Trust. Any proposed alterations to the canal or discharges must be consented by the Canals and Rivers Trust prior to decision on planning applications. If a site is located within 250m of a landfill site, there could be amenity, dirt, and contamination issues. Sites could be sensitive from the perspective of controlled waters and therefore any redevelopment must ensure there is no pollution risk to the water environment. 7.522 Please see the SFRA and any other technical guide from the council for advice on development affected by Chalk streams including the sensitive management of run off and drainage. The SFRA and Water cycle studies have been produced to inform this local plan and utilise the best and latest available data. However, developers preparing planning applications will need to use the most up to date flood risk and water resources information available, in consultation with the Environment Agency and Lead Local Flood Authority. For instance, developers should also refer to Section 19 flood investigation reports and flood incident data held by the LLFA. 7.523 Users of the Local Plan must check for the latest information on flood risk from the Environment Agency and other responsible bodies. It is important to note that the information relating to flood risk in this Local Plan and the latest SFRA is based upon the best data available at the time of writing. Mapping of flood risk, however, always involves a level of uncertainty and can never be an exact science. It should be recognised that areas that are highlighted as having high risk of flooding now or in the future in this document may not flood and areas with low risk are not guaranteed to be safe. 7.524 To address cumulative and catchment-wide impacts, the policy requires development to look at flood risks beyond the site boundary, developers should be encouraged to implement sustainable solutions which manage flood risk. Development proposals within catchments draining into the River Ouzel and Upper Great Ouse should also consider the requirements of the Marston Vale Surface Waters Plan, which sets further policies for surface water runoff draining towards the area of the Forest of Marston Vale, to the south and west of Bedford. 7.525 The surface water drainage strategy for NESS sites should be used to develop and implement appropriate drainage sub catchments and specific runoff rate and volume requirements for each phase of the development. Depending on the number of land parcels and developers involved in a new settlement area, this may lead to be an opportunity for Buckinghamshire Council to lead in the preparation of an overarching drainage strategy, as previously developed for the Princes Risborough Expansion Area. 7.526 The surface water drainage strategy required for sites in High Wycombe and Chesham is to address locations that are nationally significant ‘Flood Risk Areas’ for surface water flood risk within the 2018 Environment Agency Preliminary Flood Risk Assessment. The strategies that inform development should help to ensure all developments in Chesham and High Wycombe have considered the rapid response nature of the catchments to surface water flooding when designing safe access and escape routes. Strategic drainage strategies 7.527 Given the scale of strategic sites, developers will be required to submit a site-wide drainage strategy ahead of individual plot applications, to ensure the potential amenity, biodiversity and water quantity and quality improvements are maximised. If such measures are implemented at plot scale, there is the risk that while they would be implemented their benefits would not be maximised as they would be developed in isolation to other plots. For the purposes of this policy, strategic is defined as 50 dwellings and above or 1 hectare or more site area of non-residential development. 7.528 A strategic site-wide Drainage Strategy should consider local sources of flooding including ordinary watercourses, surface water and groundwater flooding across the expansion area. This would identify drainage issues, make allowance for extreme weather events caused by climate change and inform a strategic Sustainable Drainage scheme, making it clear to developers what is required from individual development parcels as these come forward. 7.529 Further guidance is available in the latest version of the Strategic Flood Risk Assessment.