River flood Hazard level: High ?

In the area you have selected (Washington) river flood hazard is classified as high based on modeled flood information currently available to this tool. This means that potentially damaging and life-threatening river floods are expected to occur at least once in the next 10 years. Project planning decisions, project design, and construction methods must take into account the level of river flood hazard. Surface flood hazard in urban and rural areas is not included in this hazard classification, and may also be possible in this location. Please see 'Urban Flood' for consideration of urban surface and river flooding.The following is a list of recommendations that could be followed in different phases of the project to help reduce the risk to your project. Please note that these recommendations are generic and not project-specific.

Climate change impacts: In western, central and eastern continental US, medium confidence in an increase in the number of extreme rainfall events. In northeast US, medium confidence in an increase in days of intense precipitation. In southeastern US, model projections are inconsistent in changes in rainfall. In Alaska, high confidence in an increase in days of intense precipitation and the number of extreme rainfall events. The present hazard level may increase in the future due to the effects of climate change. It would be prudent to design projects in this area to be robust to river flood hazard in the long-term.

Recommendations

• LOCATION ASSESSMENT: The high-level information available in this tool may indicate the presence of river flood hazard in your project area. Before committing significant resources to this issue, a study of the surrounding landscape can help you assess whether more detailed assessment and/or intervention should be considered. More information
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  Since flooding is a very local phenomenon, the hazard information provided by this tool should be considered the preliminary action in defining river flood hazard level. Flood hazard can vary dramatically over short distances, depending on factors such as local topography and distance to waterways. To further define potential flood risk, a local assessment should be undertaken to identify whether any sources of flooding exist (rivers/streams). If they do, it would then be necessary to identify whether the location of interest is situated in an area that may become flooded, usually by identifying whether the area is located within a natural floodplain (see Guidelines for Environment Risk Assessment and Management). The objective of the location assessment is to gain a greater understanding of the likely flood hazard at your location, through the interpretation of the local landscape.

  • Be aware that the hazard level provided by the tool should be considered a first estimate of flood hazard. Further clarification of the local river flood hazard will be required.
  • Consider the position of your project in the landscape by assessing local topography (i.e. how flat or steep your area is). Flooding is most likely to occur in relatively flat areas beside rivers (‘floodplains’), in local low-points in the land surface (‘depressions’), or along valley bottoms. If your project is situated on the top or side of a hill then you are unlikely to be directly at risk from river flooding; if your project is situated at the base of a hill or on flatter terrain then you are more likely to be at risk. Note that flood risk may not always originate from the closest point on the river; you may also be at risk from flood waters that overflow from the channel upstream before flowing downhill over the floodplain.
  • Check whether there are any streams or rivers in your project area. You can use tools such as published maps, Google Earth, or open source GIS software such as QGIS, to identify nearby waterways. Do not dismiss small streams as these can swell rapidly and dramatically in flood conditions and cause serious local flooding. In some areas, rivers may not run all year around; such ‘ephemeral’ rivers are still capable of causing significant flooding at certain times (this type of river is most common in arid and semi-arid regions).
  • Flash-flooding is unlikely to be represented in this tool, be aware that any small streams may pose a risk that is unaccounted for.
  • If any such streams or rivers are present, and your project is not obviously protected by its position in the landscape, assume that the hazard level for your location is higher than ‘Very Low’.
  • Identify debris that may have been deposited during previous flooding. The presence of large objects, such as boulders, in the vicinity of even seemingly small waterways may indicate the presence of powerful flood waves. Such behaviour is particularly prevalent in small, steep tropical catchments.
  • Attempt to identify the origin of the hazard identified by the tool. This will likely be a large river system located with the selected region.
  • Identify whether this hazard poses a risk to your asset. Is this river connected to your site via a floodplain? The position of your project in the landscapewill be key in defining whether there is a risk.Flooding is most likely to occur in relatively flat areas beside the identified waterways ('floodplains'). If your project is situated on the top or side of a hill then you are unlikely to be directly at risk from river flooding; if your project is situated at the base of a hill or on flatter terrain then you are more likely to be at risk. Note that flood risk may not always originate from the closest point on the river; you may also be at risk from flood waters that overflow from the channel upstream before flowing downhill over the floodplain.
  • Identify whether there are any additional sources of flood hazard present, that may not be captured in the Think Hazard! tool. In particular, flash-flooding and flooding from small waterways is unlikely to be represented in tool. Do not dismiss small streams as these can swell rapidly and dramatically in flood conditions and cause serious local flooding. In some areas, rivers may not run all year around; such 'ephemeral' rivers are still capable of causing significant flooding at certain times (this type of river is most common in arid and semi-arid regions).

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• EXISTING INFORMATION: The high-level information available in this tool may indicate the presence of flood hazard in your project area. However, because flood hazard can change dramatically over short distances, the exact geographical location of your project should be checked against pre-existing flood hazard information. More information
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  Acquisition of the available flood hazard data should be undertaken for all sites and all hazard levels. Information about flood hazard can take multiple forms, this includes spatial maps of flood hazard, flood zoning information, reporting of previous events and local knowledge.

  Local flood zoning information, typically from government planning departments, can provide a good indication of the likely flood hazard for specific locations. Flood hazard maps provide an estimate of the likelihood of flooding at a given location. Flood hazard maps may be available from government agencies or through local consultants, and may be available at a variety of scales. Depending on the quality of the data, local scale flood hazard maps can provide a reliable understanding of the flood risk at a given location.

  In addition to local/country level flood hazard maps, large-scale flood hazard maps have emerged in recent years (see GAR15, Aqueduct [used in this tool], SSBN). These data are available for most locations and may provide further insight into the spatial extent of flood hazard over large scales. The limitations of these data, particularly those produced over large scales, cannot be overstated. Flood hazard data are typically derived from the output of computer models. These flood models themselves are subject to significant uncertainty and therefore their output should be used tentatively. Flood hazard maps produced over large scales may highlight where there is a predominant flood hazard and will indicate the need to do more localised flood modelling with better topography and river channel data in these areas for detailed planning and engineering design. However, owing to their coarse resolution, national or regional food hazard maps should not be used to provide information at local (building) scales and certainly not to inform engineering design.

  Observations of previous flood events can also be used to define flood hazard. Monitoring of flood events is becoming more routine, with a number of centres building archives of previous events (see Dartmouth Flood Observatory[http://floodobservatory.colorado.edu/], Disaster Charter [https://www.disasterscharter.org/web/guest/home], and Copernicus [http://www.copernicus.eu/]). The identification of previous events occurring at a given location will clearly indicate the presence of a significant hazard. This can be used to refine the hazard levels provided by the ‘Think Hazard!’ tool. However, the non-existence of an event in these archives should not be used to infer the non-existence of a hazard. It simply means that an event has not yet been recorded officially.

  Documentation of previous events, through local news reports, can also provide a useful insight into previous events, and help to define flood risk at a more local scale. Organisations such as Reliefweb (reliefweb.int) also represent a useful resource, providing information about previous events and their consequences. In addition to flood hazard modelling, mapping and documentation, local knowledge can provide a useful source of additional information that can be used to identify flood hazard. Local government officials may provide written records, observations or insight into previous flood events that have occurred. Likewise, local residents may have a good understanding of the local flood behaviour, particularly if they have resided near the site location over a significant period of time. These sources of local knowledge should be used to further inform the likely flood hazard level at a given location.

  • Consult local flood zoning information (if readily available).
  • Consult local open access flood hazard maps (if readily available).
  • Obtain local ‘soft’ knowledge (interview council, local residents, etc.).
  • Obtain evidence of previous events (Darthmouth Flood Observatory etc.), if available.
  • Obtain detailed flood hazard maps for your project from government agency, or local/international consultancy (if available).
  • Purchase and compare multiple flood hazard datasets for your project from government agency and/or local/international consultancies (if available).
  • Identify existing local flood defence levels and/or structures (such as river embankments, storm drains), and who is responsible for maintaining them.

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• TECHNICAL EXPERTISE: Consultation with flood risk experts will provide a more detailed understanding of the risk posed to your asset by flooding. The level of guidance required will depend upon the level of hazard present, the vulnerability of the asset and local legislation that might apply. More information
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  Professional guidance can take a number of forms, ranging from informal advice to a full site specific Flood Risk Assessment (FRA).The required level of consultation will depend largely on the vulnerability of the project or development, the anticipated flood hazard level and the level of FRA required (if any) by local legislation.

  Firstly, informal advice can represent a useful and effective means to gain a greater understanding of flood hazard. Professional forums and even professional social-media platforms can provide a means to access professionals with expertise in the field of flood risk assessment. Academics, with expertise in your location of interest, may also prove a useful resource. They may have an intricate knowledge of flood hazard, as well as being able to identify key datasets in the region.

  A more detailed understanding of flood risk can be obtained via a flood risk appraisal. A flood risk appraisal differs from a site specific FRA, in that it provides a more localised view of flood risk (than provided by the Think Hazard tool), but still a broad view of flood risk. These studies are typically desk studies that will attempt to provide a generic assessment of flood risk by pooling together available information, and perhaps undertaking some coarse scale modelling. A flood risk appraisal will highlight key areas where a more detailed study may be required. Flood risk appraisals should be undertaken by consultants who have specific expertise in undertaking and delivering flood risk assessments. Preferable, the consultant will also have local expertise, and consequently be familiar with available data and information as well as relevant local legislation.

  A site specific FRA represents the most detailed appraisal of flood risk at a given location. FRA’s will typically include engineering level site assessments, perhaps including detailed flood modelling. FRA’s can provide detailed information about flood risk, and inform the design process as to the appropriate level of flood defences or site adaptation required. FRA’s can be expensive, and should be undertaken by consultants with expertise in site specific flood risk assessments. The consultant should also have extensive experience undertaking FRA’s under the given climatology, topography, as well as being familiar with local legislation. If the project involves construction, there may be relevant building standards that apply to the project regarding flood risk, e.g. drainage for highways and embankment stability. They will need to have appropriate skills in hydrology, hydraulics and computational river modelling. Always ask for detailed examples of relevant experience from any consultant you wish to hire for these purposes.

  The level of professional guidance will depend largely on the vulnerability of the project or development and the identified hazard level. High vulnerability assets are assets that would be acutely impacted should they become inundated. For example, the Planning and Policy Statement 25 (PPS 25) in the UK, provides a classification of flood risk vulnerabilities that may be useful in determining the vulnerability of a project. PPS 25 defines partitions vulnerability into 5 categories:
  Essential Infrastructure - all infrastructure that cannot be relocated, regardless of hazard level.
  Highly vulnerable - assets that would acutely exacerbate the impacts of a flood, should they become inundated. Examples include emergency response units, hospitals, power stations, installations of hazardous materials.
  More vulnerable - assets that would result in significant damage should they become inundated. Examples include residential buildings, educational and landfill facilities.
  Less vulnerable - assets that would not significantly exacerbate the impacts of a flood, but would result in damages. Examples include shops, cafes, non-residential establishments, waste treatment (non-hazardous), agricultural buildings and land.
  Water compatible - infrastructure that does not result in damages, during a flood. Examples include flood defences, amenity open space, sports and recreational ground, docks and marinas.

  A Highly Vulnerable asset, such as a hospital, will require a detailed understanding of flood risk. Therefore, for all hazard intensities greater than Very Low, a site specific FRA would be highly recommended.

  • For all assets potentially vulnerable to flooding, consider commissioning a site specific FRA, with the aim being to provide a detailed understanding of local flood risk.

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• PROJECT LOCATION: Consider relocation of the project. If local flood risk information confirms your project is exposed to a high level of flood hazard, relocation to areas not prone to flooding is recommended where viable. This decision will need to be undertaken with the consideration of other hazards. More information
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  Depending on the local flood hazard information available, relocation of the project may become the most suitable and cost effective course of action to take. When considering the relocation of a project, it is important to consider both the likelihood and the consequence of flooding. The likelihood of flooding is alluded to by the hazard levels provided in this tool.

  The consequence of flooding, that being the damage or loss of life that results from a flood, depends upon a number of factors including the vulnerability of the exposed asset. High vulnerability assets are assets that would be acutely impacted should they become inundated. For example, the Planning and Policy Statement 25 (PPS 25) in the UK, provides a classification of flood risk vulnerabilities that may be useful in determining the vulnerability of a project. PPS 25 defines partitions vulnerability into 5 categories:
  Essential Infrastructure - all infrastructure that cannot be relocated, regardless of hazard level. Examples include water treatment works and essential transport infrastructure (including evacuation routes).
  Highly vulnerable - assets that would acutely exacerbate the impacts of a flood, should they become inundated. Examples include emergency response units, hospitals, power stations, installations of hazardous materials.
  More vulnerable - assets that would result in significant damage should they become inundated. Examples include residential buildings, educational and landfill facilities.
  Less vulnerable - assets that would not significantly exacerbate the impacts of a flood, but would result in damages. Examples include shops, cafes, non-residential establishments, waste treatment (non-hazardous), agricultural buildings and land.
  Water compatible - infrastructure that does not result in damages, during a flood. Examples include flood defences, amenity open space, sports and recreational ground, docks and marinas.

  These classifications provide a useful blueprint to determine the vulnerability of a given project or asset. Understanding vulnerability, along with hazard level, is a crucial component when determining whether a location is suitable for development.

  Before relocation is considered, it is necessary to obtain a more detailed understanding of the flood risk present at the specific site location. If this more detailed information indicates the present site to be at risk, it may be possible to relocate your project to a different site within the same administrative area that is known to be less susceptible to flooding. For high flood hazard areas, relocation to areas with a lower hazard level is recommended where possible. Only water compatible projects, and essential infrastructure that cannot be relocated, should be permitted in these areas. For medium flood hazard areas, only water compatible, essential infrastructure and less vulnerable assets should be permitted. The relocation of more vulnerable assets is highly advised. For low flood hazard areas, highly vulnerable assets should be considered for relocation. For both medium and low hazard levels, a cost benefit analysis would be a recommended course of action to take before a project is considered for relocation. This analysis aims to identify whether relocating the asset, or making the asset resilient to flood is the most cost effective measure. It may be that building flood resilient structures or implementing flood defences is more cost effective than moving the project to a different location.

  • Confirm exact hazard level using local data before considering relocation
  • Only water compatible and essential infrastructure should be permitted in flood prone areas. All other projects should be relocated to non-flood prone areas. Relocation may be to a site within the same administrative area that has been identified as having a lower flood hazard level following a local flood risk assessment.

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• EARLY WARNING SYSTEMS: Identify early warning systems (EWS) that may exist in your project area. Receive warning messages and be aware of what happens when a warning is triggered. Ensure that the warnings will be received in the facilities built and the people working or living there. Flood early warning systems are designed to provide communities with advanced warning of an imminent flood event based on weather forecasts, rainfall and upstream water levels, and can be used to trigger deployment of portable flood defences or evacuation to mitigate impacts of a flood. More information
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  Early Warning Systems (EWS) are a key tool that can be utilised to reduce the damage that results from a flood event. These should consist of collecting real-time or forecast information on floods, warning protocols and procedures, and protocols and procedures that explain how to act upon flood warnings. If sufficient warning can be given, then mitigation procedures can be implemented; these may include evacuation, movement of vulnerable assets/material or the implementation of temporary/moveable flood defences. The availability of EWS may therefore significantly reduce flood risk. EWS may vary from local scale procedures that already exist for your location, through to large scale EWS. Local EWS are likely to be run by government agencies, emergency responders or even local communities. Large scale EWS also exist; GLOFAS is a global EWS for large scale flood events. If the river system that poses a hazard is represented, then GLOFAS can be used to provide a provisional EWS.

  • If your project provides a critical service, consider implementing basic measures to ensure the project can continue to function in the event of a flood such as having procedures in places to move vital equipment to safe areas.
  • Enquire whether an EWS exists for the project area. This is likely to be run by the government agency responsible for rivers but other stakeholders, such as emergency responders, reservoir or hydroelectric plant operators, or even community run systems may exist.
  • Global EWS may be applicable, check whether the hazardous river systems are included in GLOFAS (http://www.globalfloods.eu/). GLOFAS will likely only be applicable to large scale river systems.
  • Ascertain the feasibility of a real-time connection to the EWS. This can be as simple as receiving a text message or email from the EWS operator when a flood is expected.
  • Ensure that any received early warning can be rapidly and clearly disseminated to all staff at the project location
  • Develop a set of protocols that define the actions to be taken when an early warning is received. Depending upon the sophistication of the EWS, a range of protocols may be required to define actions for different warning levels. Ensure that staff are aware of the EWS, understand how long they will have to respond to a warning, and are familiar with the response protocols.
  • For critical or networked assets, protocols should warn dependents of possible service interruption.
  • For critical or networked assets, protocols should warn backup assets that their service(s) may be required imminently.

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• INTERDEPENDENCY: Consider vulnerability of other assets within the project's dependency network. If your project is interdependent with other projects, it is important to assess the vulnerability of the entire network if the service provided is critical. More information
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  In addition to the specific location of a project or development it is also necessary to consider other locations that, should they be impacted during a flood, would adversely affect your project or development. The consideration of other locations applies particularly to crucial infrastructure that is required to remain operational during a flood event. It is also important to ensure that escape routes remain available at all times during flood events. Therefore transport links, not located in the vicinity of the project or development may need to be considered.

  • If your project provides a critical service and is reliant on external services (e.g. power supply, computer networking and communications) then the vulnerability of these external services to flood should also be considered. Externally provided services may be based in a different physical location and therefore be subject to a different level of flood hazard.
  • Access to the project or development may become restricted during a flood event. Ensure that escape routes are available at all times.
  • If the service provided by your project is critical, the effect of flooding on your project’s ability to provide the service should be assessed. All dependent assets must be prepared to continue operating without your project for a sustained period. The backup services should themselves be assessed for flood hazard to ensure they cannot be exposed to the same flood event.

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• FLOOD HAZARD MANAGEMENT: Your project or development should consider flood management measures, such as restoration of natural wetlands, removal of impermeable surfaces, or implementation of flood defences at the project site and in the upstream catchment. More information
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  Flood management includes all measures that may alleviate flood risk. This can range from management of upstream catchment areas, through to the implementation of flood defences. Management of upstream catchment areas are sometimes defined as being ‘Green’ protective measures, as they often involve the restoration of natural wetlands or the removal of impermeable surfaces. Some examples of Green protective measures include:

  • Measures to reduce runoff: replacing impermeable surfaces with natural landscapes and afforestation are measures that will reduce the rate at which rainfall moves across the surface of a catchment and into a river.
  • Storage of runoff: wetlands and reservoirs can store excess water during extreme flows, reducing the magnitude of the flood event. Often the restoration of natural wetland areas will alleviate flood risk, whilst also providing additional benefits to local eco-systems.

  The implementation of flood defences are sometimes referred to as ‘Grey’ protective measures, as they involve the implementation of built structures to protect areas or transport water away as quickly as possible. Examples of Grey measures include:

  • Capacity enhancement of rivers: bypass channels and channel deepening/widening are measures that can be used to increase the amount of water that can pass through a river channel.
  • Flood defences: Flood defences can be implemented to separate the hazard source (river) from the receptor (e.g. a building).

  The flood management options available will depend upon the size of the development, the resources available and the willingness/ability of other land users in the catchment area to participate in a flood management scheme. In the case of large developments, encompassing significant proportions of a river catchment, a number of flood management procedures will be available, including measures to reduce and store rainfall-runoff. For small-scale developments, the implementation of upstream, flood alleviation measures may not be possible. Instead the implementation of flood defences, aiming to protect the structure or development from a floodplain, or the development of flood resilient structures, may represent the available options. Regardless of the measures implemented, it is important when implementing flood management measures that they are not treated in isolation; their impact on other areas may be significant.

  The World Meteorological Organization (WMO) identified that attempts to control flooding, via traditional Grey methods, in single locations or developments, have had little success in reducing flood losses and damages. Instead the WMO has outlined what it calls Integrated Flood Management (See WMO- Integrated Flood Management). Integrated Flood Management moves away from measures of flood control, such as engineered flood defences in single locations, and instead promotes the management of the water cycle as a whole. This approach not only aims to alleviate flood risk in highly vulnerable areas, but also aims to create opportunities from flooding by maximising the net benefits from the use of floodplains. This may include resorting natural floodplain areas or replenishing agricultural land. Where possible, this integrated approach to flood management represents the most ideal approach to alleviate flood risk. However, such approaches will likely require a framework for collaboration across many different sectors and organisations. The Associated Programme on Flood Management (APFM) provides documentation, guidance and test cases outlining how to implement Integrated Flood Management strategies.

  • Identify whether flood management procedures are in place in the local region, and how your development will integrate within the existing flood management framework.
  • For large developments, consider implementing flood management procedures, such as the provision of storage areas for excess river flow, or the implementation of measures to reduce rainfall run-off.
  • Consider implementing an Integrated Flood Management approach where possible (see APFM).
  • If your project provides a critical service, consider implementing basic measures to ensure the project can continue to function in the event of a flood such as locating critical electronic equipment above ground level.
  • Undertake a cost-benefit analysis to assess including moderate flood defence features.
  • Consider ensuring accessibility to the project in the event of flood
  • Consider flood-resilient design if the asset provides a critical service. Flood resilient design enables a building to continue to function in the event of a flood; typically it involves raising all critical services to above the maximum plausible flood height.
  • Consider either flood defences and/or flood resilient design.
  • Flood defences aim to prevent water from reaching the project when a flood is occurring. The defences should include provision for site access by ensuring access routes cannot be flooded.
  • Flood resilient designs should allow a building to continue to function even if it becomes flooded. They typically involve elevating all essential building services to above the maximum plausible flood height (details to include examples of stilts, floating structures, etc.)

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• DO NOT INCREASE HAZARD: Built infrastructure may alter flood hazard. Constructing a significant piece of infrastructure can significantly alter the landscape and potentially influence how an area responds during a flood. Any alteration of the landscape should be undertaken with consideration as to how this will influence the local hydrology. More information
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  It is important when building a new development to consider how it will impact upon local hydrology. Built infrastructure can significantly change how an area responds to rainfall and how any area transports water, potentially increasing flood risk. For example, replacing large permeable areas with impermeable surfaces or structures will increase rainfall-runoff. Sufficient drainage systems will need to be in place to transport the excess water that previously would have been absorbed by the landscape, away from the site. However, this will increase the speed at which rainfall enters local river networks, potentially magnifying flood risk in downstream areas.

  Obstruction of water ways, for example by the development of a poorly located bridge, can also increase adjacent flood risk, as well us upstream. This will create the potential for flooding to occur more regularly and to a greater magnitude. Any development that encroaches upon the drainage capacity of a channel can also magnify flood risk by allowing water borne debris to gather, further restricting flow.

  Buildings and developments also have the potential to exacerbate downstream flood risk by reducing floodplain storage capacity. If a development removes space that previously stored excess water, then the scale and/or speed of run-off will increase. These considerations also apply to flood defences and channel conveyance measures. The implementation of flood defences will, by its nature, remove some amount of floodplain storage and exacerbate flood risk elsewhere. Similarly, measures to increase river channel capacity, with a view to making an area more drainage efficient, will likely increase the magnitude of flooding downstream.

  Aside from direct human impacts, the impacts of a development on local flood behaviour should also be considered from an environmental viewpoint. Floodplains are valuable ecological resources that will provide a habitat for a wide variety of plants and animals. All developments should be considered in terms of their potentially negative impact on local ecosystems, either through the removal of natural floodplain areas, or through the amplification of flooding.

  • Ensure that any implemented protection measures do not exacerbate flood risk in other areas.
  • For larger projects, a detailed Flood Risk Assessment (FRA) will be required.

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