Ministerial guidelines
The Planning System and Flood Risk Management: Guidelines for Planning Authorities
Flood Zones A, B and C, the sequential approach and the Justification Test for development in areas at risk of flooding.
Department of the Environment, Heritage and Local Government and the Office of Public Works · November 2009 · 81 pages
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- Source-Pathway- Receptor Model For there to be flood risk, the three components of flood risk - the source of the hazard, the receptors affects by the hazard and the mechanism of transfer between the two - must all exist. Spatial resolution Defines the density of information produced from the flood risk assessment process across the area of interest. A mosaic of flood risk data produced by different tools and base data, with a range of certainty in the output. Strategic Environmental Assessment (SEA) Strategic Environmental Assessment (SEA) is the process by which environmental considerations are required to be fully integrated into the preparation and adoption of plans and programmes. The objective of the SEA process is to provide for a high level of protection of the environment and to contribute to the integration of environmental considerations into the preparation and adoption of specified plans and programmes with a view to promoting sustainable development. Strategic Flood Risk Assessment (SFRA) The assessment of flood risk on a wide geographical area against which to assess development proposed in an area (Region, County, Town). Surface water management This activity focuses on the assessment and management of flood risk within the urban environment from sources primarily resulting from intense rainfall. Surface water management should understand the performance of the urban drainage network, where exceedance flow routes would form and what impact this would have. Solutions to surface water flood risk can involve green infrastructure provision to capture and direct these exceedance flows to lower vulnerable areas or open space. New development can provide solutions to reducing runoff not only from the proposed development but also from existing areas. This should be considered in the SFRA in critical areas where development is planned upstream of flooding hotspots. Terms Definition 63 Guidelines for Planning Authorities The Planning System and Flood Risk Management
- Sustainable Drainage Systems (SuDS) A form of drainage that aims to control run-off as close to its source as possible using a sequence of management practices and control structures designed to drain surface water in a more sustainable fashion than some conventional techniques. Vulnerability The resilience of a particular group of people or types of property or habitats, ecosystems or species to flood risk, and their ability to respond to a hazardous condition and the damage or degree of impact they are likely to suffer in the event of a flood. For example, elderly people may be more likely to suffer injury, and be less able to evacuate, in the event of a rapid flood than younger people. Water Framework Directive (WFD) A European Community Directive (2000/ 60/ EC) designed to integrate the way we manage water bodies across Europe. It requires all inland and coastal waters to reach “good status” or “good ecological potential” in the case of heavily modified water bodies by 2015 through a catchment-based system of River Basin Management Plans (RBMP), incorporating a programme of measures to improve the status of all natural water bodies. Terms Definition 64
- What is flooding? Flooding is an event that occurs when land that is normally dry is covered by water. This can have both beneficial and adverse consequences. Deposition of sediments from river floods form the level floodplains alongside rivers and contribute to its agricultural fertility and its biodiversity. When it affects human activities it can threaten life, cause social and health impacts, damage buildings and their contents, interrupt communications and lead to pollution that can cause environmental damage. What causes flooding? Flooding at the coast is caused by higher sea levels than normal. Inland flooding is due to intense and or prolonged rainfall which exceeds the capacity for it to sink into the ground. As a result, it runs off over land and into rivers and artificial drainage systems. When the capacity of these is exceeded, the result is river and sewer flooding. The roofs of buildings, roads and hard standings prevent water soaking into the ground thus increasing run-off and exacerbating the problems. Flooding can also occur when water that has soaked into the ground fills the underground storage and overflows onto the surface. How will climate change influence flooding? Climate change could potentially have a very significant effect on flooding in the longer term. Higher sea levels and wetter winters, with more intense rain storms, together with possible increases in storminess could significantly increase both the frequency and intensity of flooding. For example, floods which currently have a 1 in 100 chance (1% probability) of occurring in any one year could occur much more frequently. How is flood risk measured? Flood risk is a combination of the likelihood of occurrence and the consequences of a flood occurring. This is normally expressed as: Flood risk = probability x consequences. Probability is difficult to estimate because it has to take account of the uncertainty of hydrological predictions based on the analysis of many years of flow records. Consequences are also complex to measure in terms of the potential loss of life, damage to property etc, which depend on the vulnerability of the land-use and property affected by the flood. 65 Frequently Asked Questions Guidelines for Planning Authorities The Planning System and Flood Risk Management
- What is a source-pathway-receptor model? A source-pathway-receptor model is a representation of the components of flood risk. The three components are a source of floodwater (prolonged or intense rainfall, high sea levels), the receptors (land, buildings and structures, people) that are affected by flooding and the pathway (rivers, floodplain, artificial drainage systems, overland flow etc.) through which the flood reaches those receptors. For a risk to arise, all three components must be present. Flood risk management is essentially concerned with protecting the receptors and/or modifying the pathways. What is a flood risk assessment? A flood risk assessment is the identification, quantification and communication of flood risk using the source-pathway-receptor model. It examines the sources of flooding and the pathways by which floodwaters might reach receptors, such as people, property and the environment to determine the likelihood of them being affected by flooding. It also examines the flood hazards that are likely to arise and the vulnerability of receptors to such hazards. Flood risk can be assessed at different scales, from the national, through regional and local to site-specific. For further information on individual assessment types see Table A2 in Appendix A. What is a flood hazard assessment? A flood hazard assessment is the identification, quantification and communication of the hazards due to flooding. It seeks to identify areas subject to particular hazards, such as deep or fast-flowing water, and to assess the likelihood of them occurring both now and in the future. What is a Strategic Flood Risk Assessment? A Strategic Flood Risk Assessment (SFRA) is an area-wide examination (up to county scale) of the risks of flooding to support spatial planning decisions such as the zoning of particular areas for development. It is an essential element in the adoption of the sequential approach to the consideration of flood risk in spatial planning. A generic example of SFRA mapping is provided in Fig. 4.1. 66
- Will it be very costly to model flooding on a county-wide basis? To ensure a proportionate response, flood risk assessments need only be as detailed as is necessary to obtain the information required to support the decision that is to be made. A staged approach should be adopted, therefore, comprising a flood risk identification stage to identify whether there are flood risks to an area or site, an initial assessment stage to confirm the sources of flooding and qualitatively appraise the risks and potential mitigation measures, and a detailed assessment stage to quantify the risks, appraise the effects of changes and the effectiveness of mitigation measures. At each stage the adequacy of the assessment to support the decision being made should be determined and the next stage should be initiated only if there is insufficient information to support the decision. What does the SFRA provide? The Strategic Flood Risk Assessment provides tools and methods to assist users in identifying the level of flood risk associated with an area to inform planning decisions. It supports the application of the sequential approach and provides data and maps to help in assessing sites against flood risk criteria. Where development is or would be at risk of flooding, it provides information on the mitigation measures considered deliverable to reduce the actual risk to that development and on the residual risks that would remain and how they might be managed. What does the SFRA not provide? The SFRA is an area-wide study and the level of detail is commensurate with its strategic nature. It does not provide suitably detailed site-specific information, such as design flood levels. A site-specific flood risk assessment is still required to cover in more detail all sources of flood risk for individual developments. The level of detail required for a site-specific flood risk assessment depends on the scale and nature of the development and the risks involved. 67 Guidelines for Planning Authorities The Planning System and Flood Risk Management
- Where does the SFRA fit in the planning process? The SFRA is not a statutory planning document. It is a consultation document that should be used to inform a development plan or local area plan, enabling the implementation of the sequential approach and the testing of development zoning against flood risk criteria. It can also be used to assist other planning decisions, such as development management, and emergency planning. In any instance, a site-specific flood risk assessment may be required when deciding on the grant of planning permission. What is the sequential approach? The sequential approach uses mapped flood zones alongside considerations of the vulnerability of different types of development to give priority to development in zones of low flood probability. Only if there are no reasonable sites available in zones of low flood probability should consideration be given to development in higher flood probability zones. The sequential approach should also be used within sites to help ensure that the most vulnerable uses are sited in the area of lowest probability. What is the Justification Test? Where there are insufficient sites available to locate development outside flood risk areas, it may be necessary, to meet the objectives of proper planning and sustainable development, for development to be sited within flood risk areas. The Justification Test is an examination of such proposals against proper planning and sustainable development criteria and, if these are satisfied, against flood risk criteria to ensure that risks are reduced to an acceptable level and that flood risk is not increased elsewhere. What is appropriate development? Appropriate development is development whose vulnerability to flooding is such that it is generally acceptable within a particular flood zone. In exceptional circumstances, development that is generally considered inappropriate because of its vulnerability to flooding may be justified as appropriate on the grounds of proper planning and sustainable development provided that the risks are reduced and/or managed to an acceptable level. 68
- How is vulnerability defined? Different types of development are classified as being highly vulnerable, less vulnerable or water-compatible, depending largely on the risks to people who will use the development, the effects of damage to buildings and structures that might be caused by flooding, and the potential environmental damage that could be caused arising from pollution caused by the development were it to flood. What is the definition of the flood zones in these Guidelines? The flood zones are defined on the basis of the probability of flooding from rivers and the sea. Because of the generally more dynamic nature of coastal flooding compared to river flooding, a lower probability of coastal flooding is used to define the highest-risk zone. Zone A is at highest risk and has a 1 in 100 (or 1%) chance of flooding in any one year from rivers and a 1 in 200 (or 0.5%) chance of flooding from the sea. Zone B is at moderate risk of flooding from rivers and the sea and its outer limit is defined by a 1 in 1000 (or 0.1%) chance of flooding in any one year. Zone C is the low risk area, with a less than 1 in 1000 (<0.1%) chance of flooding from rivers, estuaries or the sea in any one year. The definition of these zones does not, however, take account of the potential for flooding from other sources, such as ground water or artificial drainage systems. Flooding from these sources could occur in any of the zones and as such should always be considered, regardless of zone. My development is classed as appropriate for the flood zone in which it is located, do I need to carry out a flood risk assessment? Yes. While the combination of risk and vulnerability is such that the development is generally acceptable, the risk remains and it may change during the lifetime of the development. The flood risk assessment will enable consideration of mitigation measures, such as flood-resistant and flood-resilient construction, that will reduce that risk. In addition, the flood zones are based on river and coastal flooding only and other causes of flooding may give rise to a higher risk than that from rivers or the sea. 69 Guidelines for Planning Authorities The Planning System and Flood Risk Management
- What is a flood defence? Flood defences comprise man-made structures designed to reduce the likelihood of flooding in an area. They can include embankments, bunds, walls, sluice gates, storage reservoirs or diversion channels. An area is being considered for zoning but is within Flood Zone A, as is the majority of the urban centre. The town is defended by a substantial coastal flood embankment. How does the sequential approach apply in this situation? The graduation of the residual risks across the town should be established with the SFRA, so that the scale and nature of the residual risks are understood. When alternative lands are being considered within the development plan the land with the lower residual risk should be given priority. What is the standard of protection? The standard of protection is the flood event against which the flood defences are designed to protect an area. It is usually expressed as the annual probability of exceedance of a particular flood level, such as the 1% (or 1 in 100) flood. What is freeboard? Freeboard is a safety margin to account for uncertainties in water-level prediction and/or structural performance. It is the difference between the height of the flood defence or floor level and the design flood level. Freeboard should account for uncertainty in hydrological predictions, wave action, modelling accuracy, topographical accuracy and the quality of digital elevation models. What is meant by overtopping? Overtopping is a mechanism of failure of flood defences. Flood water reaches levels that are higher than the flood defence level and flows over the crest of the structure. While the structure may remain stable, erosion of the landward face could lead to collapse of the defence. 70
- What is a breach? A breach is a structural failure of a defence, which allows water to flow through. Breach failure can be caused by internal piping failure and failure due to erosion of the landward face as a result of overtopping of the defence crest. Piping failure is due to small seepage paths through an embankment carrying away fine sediment and rapidly expanding due to a combination of external water level, internal water pressure in the embankment and geotechnical conditions occurring in the soil matrix. Piping can sometimes be initiated by animals burrowing in the embankment. My site benefits from a high-standard flood defence, should it not be in Zone C? No. Flood defences can only reduce the risk of flooding, they cannot eliminate it. Therefore, the presence of a flood defence indicates an area at risk, not one that is safe. A flood defence may be overtopped by a flood that is higher than that for which it was designed, or be breached and allow flood water to rapidly inundate the area behind the defence. In addition, no guarantee can be given that flood defences will be maintained in perpetuity. As well as the actual risk, which may be reduced as a result of the flood defence, there will remain a residual risk that must be considered in determining the appropriateness of particular land uses and development. For these reasons, flooding will still remain a consideration behind flood defences and the flood zones deliberately ignore the presence of flood defences. What is actual risk? Actual risk is the risk of flooding taking account of all features that act as defences against flooding. These may include custom-made flood defence structures or artificial (such as road or railway embankments) or natural features (such as natural levees, sand dunes or other topographical features). These serve to reduce the actual risk of flooding since they reduce the frequency of flooding to that which will overtop the defence structure. What is residual risk? Residual risk is the risk that remains after all mitigation measures to reduce the frequency of flooding have been taken. It can arise through overtopping or breach of the flood defences, usually by a flood that exceeds the design level of the defence. 71 Guidelines for Planning Authorities The Planning System and Flood Risk Management
- What is an adequate level of defence? The level of defence that is required for a particular area is generally commensurate with the degree scale of damage consequences that would occur in the event of the defence being overwhelmed. It is often an economic decision, but also considers social and environmental factors. What measures can be taken to protect against flooding? Primary flood-defence measures include barriers to flow, such as embankments, bunds and walls, and measures to control flow, such as storage reservoirs, sluice gates, channel modification and diversion channels. These operate on a community basis but measures can also be taken to resist flood risk for individual developments or even properties. These include land-raising, raising floor levels or individual flood barriers, or ensuring that development at flood level is compatible with flooding. These are known as dry-proofing and aim to prevent water entering a property. What is flood resilience? The alternative to dry-proofing a property (because there are certainly economical and possibly physical limits to this) is to wet-proof a property. This entails allowing water to enter the property when flooding occurs but ensuring that the design and materials are such that more rapid drying out and reoccupation is possible than using conventional construction methods. This is flood-resilience and, while it does not eliminate damage, it does reduce it to a more acceptable level. What should minimum floor levels be to mitigate flood risk? The minimum floor levels for new development should be set above the 1 in 100 river flood level (1 in 200 coastal flood level) including an allowance for climate change, with appropriate freeboard. However, where the consequences of flooding are significant, a higher standard should be considered. In setting floor levels it is important to ensure that access facilities for the disabled are not compromised. These Guidelines adopt a risk-based approach and therefore, where it can be shown that the consequences of inundation can be managed and recovery delivered quickly (for example, at an industrial or commercial premises), a degree of the residual risk could be accepted and floor levels set at a lower level. These would be exceptional circumstances, supported by good urban design reasons, and described clearly within the FRA. 72
Source Department of the Environment, Heritage and Local Government and the Office of Public Works: The Planning System and Flood Risk Management: Guidelines for Planning Authorities (November 2009) · Licence the publisher's termsText read from the publisher’s PDF; tables, maps and drawings do not survive as text, and page numbers are the PDF’s (its first page is 1). Check the original before relying on a passage.