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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- Flood risk can be measured in monetary terms, as a potential for loss of life or injury, as environmental impacts, or as combinations of all these indicators. Guiding proposed development to areas where the risks arising from inundation in terms of human impact, economic loss or potential environmental damage is minimised, is a key tenet of this guidance. The vulnerability of development to flooding depends on the nature of the development, its occupation and the construction methods used. For example, a sheltered housing complex would be more vulnerable than a retail unit. A broad classification of vulnerability has been developed (see chapter 3). The classification of different land uses and types of development as highly vulnerable, less vulnerable and water-compatible is influenced primarily by the ability to manage the safety of people in flood events and the long-term implications for recovery of the function and structure of buildings. Transport and strategic utilities infrastructure can be particularly vulnerable to flooding because interruption of their function can have widespread effects well beyond the area that is flooded. This reinforces the need for decisions to locate development in areas at risk of flooding to be fully justified with regard to wider proper planning and sustainable development considerations (see chapter 3). This was a key lesson learned from recent floods in Ireland when national primary roads and railway lines were flooded, and also the summer 2007 floods in the UK, during which water supply to large areas was cut off for a number of weeks. Flood risk assessment The assessment of flood risk requires an understanding of where the water comes from (i.e. the source), how and where it flows (i.e. the pathways) and the people and assets affected by it (i.e. the receptors). 2.15 2.16 2.17 2.18 The Source-Pathway- Receptor Model is described in greater detail in Appendix A. 13 Fig. 2.2: Source-Pathway-Receptor Model Guidelines for Planning Authorities The Planning System and Flood Risk Management river or sea e.g. flood defence people / housing Source Pathway Receptor Groundwater flooding Sewer flooding Overland flooding
- The principal sources are rainfall or higher than normal sea levels. The principal pathways are rivers, drains, sewers, overland flow and river and coastal floodplains and their defence assets. The receptors can include people, their property and the environment. All three elements must be examined as part of the flood risk assessment including the vulnerability and exposure of receptors to determine its potential consequences. Mitigation measures typically used in development management can reduce the impact on people and communities, for example, by blocking or impeding pathways but they have little or no effect on the sources of flooding. The planning process is primarily concerned with the location of receptors, taking appropriate account of potential sources and pathways that might put those receptors at risk. Risks to people, property and the environment should be assessed over the full range of probabilities, including extreme events. The flood risk assessment should cover all sources of flooding, including the effects of run-off from a development on flood risk both locally and beyond the development site. A staged approach should be adopted, carrying out only such appraisal and or assessment as is needed for the purposes of decision-making at the regional, development and local area plan levels, and also at the site specific level. The stages of appraisal and assessment are: Stage 1 Flood risk identification – to identify whether there may be any flooding or surface water management issues related to either the area of regional planning guidelines, development plans and LAP’s or a proposed development site that may warrant further investigation at the appropriate lower level plan or planning application levels; Stage 2 Initial flood risk assessment – to confirm sources of flooding that may affect a plan area or proposed development site, to appraise the adequacy of existing information and to scope the extent of the risk of flooding which may involve preparing indicative flood zone maps. Where hydraulic models exist the potential impact of a development on flooding elsewhere and of the scope of possible mitigation measures can be assessed. In addition, the requirements of the detailed assessment should be scoped; and Stage 3 Detailed flood risk assessment – to assess flood risk issues in sufficient detail and to provide a quantitative appraisal of potential flood risk to a proposed or existing development or land to be zoned, of its potential impact on flood risk elsewhere and of the effectiveness of any proposed mitigation measures. 2.19 2.20 2.21 The staged approach to flood risk assessment is outlined further in Appendix A. 14 2
- At regional level the focus will be on Stage 1 (identification of flood risk), where, in general, the need for more detailed flood risk assessment is flagged for city/county and local area plans. County development plans / LAPs may require all three stages depending on the nature of the flood risk and extent of development proposed. Further information on the hierarchy of regional, strategic and site specific flood risk assessments and their application within the planning system, along with sources of information to support the assessments, such as the flood mapping being produced under the CFRAMS programme, is contained in chapters 4 and 5 and Appendix A. Flood zones Flood zones are geographical areas within which the likelihood of flooding is in a particular range and they are a key tool in flood risk management within the planning process as well as in flood warning and emergency planning. There are three types or levels of flood zones defined for the purposes of these Guidelines:Flood Zone A – where the probability of flooding from rivers and the sea is highest (greater than 1% or 1 in 100 for river flooding or 0.5% or 1 in 200 for coastal flooding);Flood Zone B – where the probability of flooding from rivers and the sea is moderate (between 0.1% or 1 in 1000 and 1% or 1 in 100 for river flooding and between 0.1% or 1 in 1000 year and 0.5% or 1 in 200 for coastal flooding); andFlood Zone C – where the probability of flooding from rivers and the sea is low (less than 0.1% or 1 in 1000 for both river and coastal flooding). Flood Zone C covers all areas of the plan which are not in zones A or B. 2.22 2.23 15 Guidelines for Planning Authorities The Planning System and Flood Risk Management 17 298_FRM Publication Formatting Fig. 2.3: Indicative flood zone map extract Flood Zone A Flood Zone B Flood Zone C
- Further guidance on flood zones is available in chapter 3 and Appendix A. These flood zones are determined on the basis of the probability of river and coastal flooding only and should be prepared by suitably qualified experts with hydrological experience. The limits of the zones are broadly in line with those in common usage internationally. They are based on the current assessment of the 1% and the 0.1% fluvial events and the 0.5% and 0.1% tidal events, without the inclusion of climate change factors. Climate change impacts are discussed later. The lower probability for coastal flooding reflects the generally more dynamic nature of coastal flooding which often presents a greater risk to life than river flooding. The 0.1% limit is provided in order to guide highly vulnerable development away from areas where flooding is relatively rare but can occur. Floodplains will primarily be found in Zones A and B. In rivers with a well defined floodplain or where the coastal plain is well defined at its rear, the limits of Zones A and B will virtually coincide. Zone B will only be significantly different in spatial extent from Zone A where there is extensive land with a gentle gradient away from the river or the sea. It is in these extensive Zone B areas that less vulnerable development will be allowed to proceed without recourse to the Justification Test (see further discussion on floodplains at paragraph 2.34). The provision of flood protection measures in appropriate locations, such as in or adjacent to town centres, can significantly reduce flood risk. However, the presence of flood protection structures should be ignored in determining flood zones. This is because areas protected by flood defences still carry a residual risk of flooding from overtopping or breach of defences and the fact that there may be no guarantee that the defences will be maintained in perpetuity. The likelihood and extent of this residual risk needs to be considered, together with the potential impact on proposed uses, at both development plan and development management stages, as well as in emergency planning and applying the other requirements of these Guidelines in chapter 3. In particular, the finished floor levels within protected zones will need to take account of both urban design considerations and the residual risk remaining. The flood zones described above are indicative of river and coastal flooding only. They should not be used to suggest that any areas are free from flood risk, since they do not include the effects of other forms of flooding such as from groundwater or artificial drainage systems. Further guidance on flood zones is available in chapter 3 and Appendix A. 2.24 2.25 2.26 16 22 Construction of flood protection wall and demountable barriers in Clonmel
- Climate change and flood risk assessment The fourth assessment report of the IPCC concluded that warming of the climate is unequivocal1. There is a growing scientific consensus that these changes are expected to increase flood risk. In addition, climate change could also influence environmental and socio-economic factors such as national prosperity and social cohesion. The expected effects will most likely be felt first by the most vulnerable in society. While the different global emission scenarios and climate-change models may lead to substantially different positions by the end of this century, the trends for the next 20-30 years do not differ significantly. Rising sea levels and more frequent and more severe coastal storms will significantly increase the risk of coastal and estuarial flooding as well as that of coastal erosion. For example, the flooding that occurred in Dublin in 2002, when a 1 metre surge coincided with one of the highest spring tides of the year, could change from a relatively rare to a more common occurrence. Assuming a correlation between the rate of sea-level rise and the rate of coastal erosion, the current rate of coastal erosion and the consequential economic impacts of flooding are expected to multiply over the next century. Where the floodplain or coastal plain is well defined, climate change is expected to change the probability of flooding and the depth for a particular event with little change in spatial extent. Only where extensive areas of land rise gently from the river or the sea is climate change expected to significantly increase the area affected by flooding. Specific advice on the expected impacts of climate change and on appropriate allowances for sea-level rises, increased river flood flows etc., in relation to flood risk management, are provided on the OPW website (http://www.opw.ie). There is a great deal of uncertainty in relation to the potential effects of climate change, and therefore a precautionary approach should be adopted. Examples of the precautionary approach include: Recognising that significant changes in the flood extent may result from an increase in rainfall or tide events and accordingly adopting a cautious approach to zoning land in these potential transitional areas; Ensuring that the levels of structures designed to protect against flooding, such as flood defences, land-raising or raised floor levels are sufficient to cope with the effects of climate change over the lifetime of the development they are designed to protect; and Ensuring that structures to protect against flooding and the development protected are capable of adaptation to the effects of climate change when there is more certainty about the effects and still time for such adaptation to be effective. 2.27 2.28 2.29 2.30 1Inter-Governmental Panel on Climate Change (IPCC), 4th assessment report. “Climate Change 2007”. :: :: :: South Wall, Dublin Bay 17 Guidelines for Planning Authorities The Planning System and Flood Risk Management
- Flood risk management Parallel to flood risk assessment by regional and local authorities, flood risk management under the EU Floods Directive mentioned earlier and being overseen by the OPW, aims to minimise the risks arising from flooding to people, property and the environment. Minimising risk can be achieved through structural measures that block or restrict the pathways of floodwaters, such as river or coastal defences, or non-structural measures that are often aimed at reducing the vulnerability of people and communities, such as flood warning, effective flood emergency response, or resilience measures (e.g. public preparedness for flood events - see http://www.flooding.ie) for communities or individual properties. Under the Floods Directive catchment-based Flood Risk Management Plans (FRMPs) are currently being developed by the OPW in partnership with local authorities, the EPA and other relevant Departments, to provide the focal point and strategic direction for flood risk management in Ireland. These Plans, which will be co-ordinated with the Water Framework Directive (WFD) River Basin Management Plans (RBMPs), will be a key input into future regional planning guidelines and development plans, and will establish a prioritised set of flood risk management measures for their relevant areas, including the use of structural and non-structural responses as outlined above. Planning can have a significant role to play with respect to flood risk management, in particular in ensuring that future development needs avoid or minimise future increases in flood risk. The planning process therefore constitutes a parallel, but inter-dependent process to that of flood risk management. Floodplains A major function performed by the floodplain and wetlands is to hold excess water until it can be released slowly back into a river system or seep into the ground as a storm subsides. Floodplains have, therefore, a valuable function both in attenuating or storing floodwater and through their ability to convey floodwater in a relatively controlled and safe way. Areas of floodplain and wetlands should, therefore, be recognised and preserved to the extent possible as natural defences against flood risk. It is important to identify and, where possible, safeguard areas of floodplain against development in both urban and rural areas. By retaining open spaces for storage and conveyance of floodwater, flood risk to both upstream and downstream areas can be more effectively managed without reliance on flood defences. This is an important element of the now internationally accepted philosophy of “leaving space for water”. The Department has commenced the preparation of good practise guidance on constructed wetlands which will look at, inter alia, the use and performance of constructed wetlands in the attenuation of flood hazard. It is intended that this guidance will be published for consultation before the end of the year. 2.31 2.32 2.33 2.34 18 2 Clare Abbey flooding
- Sustainable Drainage Systems (SuDS) Development of previously ‘green’, or permeable, land within an urban area (such as gardens) increases the impermeable area. Rain falling on impervious surfaces is usually directed into a receiving watercourse through surface water drainage systems. Although such drainage systems are effective at transferring surface water quickly, they provide only limited attenuation causing the volume of water in the receiving watercourse to increase more rapidly and increasing flood risk. Sustainable Drainage Systems (SuDS) can play a role in reducing and managing run-off to surface water drainage systems as well as improving water quality and contributing to local amenity. SuDS comprise a wide range of techniques, including swales, basins, ponds and infiltration systems. In this context the Department is currently reviewing its document “Recommendations for Site development Works for Housing Areas” which traditionally was used to provide guidance on design standards for smaller drainage systems. The aim of this review is to set out best practice in regard to the use of Sustainable Drainage Systems in respect of surface water drainage. A small-scale yet practical example of a sustainable drainage system (SuDS) is the use of permeable pavements which can help to reduce run-off rates and flow volumes from parking areas as well as access roads. Permeable pavements allow rain-water to infiltrate through the paved surface and into underlying layers and sometimes storage containers allowing gradual release of water after periods of heavy rainfall. Permeable pavement techniques include the use of porous tarmac or solid block pavers with gaps to provide through flow of water. With regard to the increasingly frequent trend of paving over entire residential garden areas to provide off street car parking or hard landscaping under the exempted development provisions of planning legislation, the Department intends to review such provisions to ensure that only those complying with sustainable drainage principles will be exempted thus limiting the run-off which would otherwise contribute to overwhelming the drainage systems. In the interim, in considering planning applications for new or extensions to residential development, which include significant hard surfacing, planning authorities should seek to reduce the extent of hard surfacing and paving as well as requiring the use of sustainable drainage techniques, through discussions at the consultation stage, including in particular permeable paving or surfaces such as gravel or slate chippings. 2.35 2.36 2.37 2.38 Further guidance on the design and implementation of SuDS is provided in Appendix B. Permeable drainage channel Bioretention device used as part of a Sustainable Drainage System 19 Guidelines for Planning Authorities The Planning System and Flood Risk Management
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- Principles and Key Mechanisms Key Messages The key principles of a risk-based sequential approach to managing flood risk in the planning system are set out in this chapter. They are: Avoid development in areas at risk of flooding; If this is not possible, consider substituting a land use that is less vulnerable to flooding. Only when both avoidance and substitution cannot take place should consideration be given to mitigation and management of risks. Inappropriate types of development that would create unacceptable risks from flooding should not be planned for or permitted. Exceptions to the restriction of development due to potential flood risks are provided for through the use of a Justification Test, where the planning need and the sustainable management of flood risk to an acceptable level must be demonstrated. Planning principles The principle actions when considering flood risk management in the planning system are: Flood hazard and potential flood risk from all sources should be identified and considered at the earliest stage in the planning process and as part of an overall hierarchy of national responses coupled to regional appraisal and local and site-specific assessments of flood risk. Development should preferentially be located in areas with little or no flood hazard thereby avoiding or minimising the risk. Development in the context of these Guidelines includes all construction, such as transport and utility infrastructure as well as residential and other buildings. Development should only be permitted in areas at risk of flooding when there are no alternative, reasonable sites available in areas at lower risk that also meet the objectives of proper planning and sustainable development. Where development is necessary in areas at risk of flooding an appropriate land use should be selected. A precautionary approach should be applied, where necessary, to reflect uncertainties in flooding datasets and risk assessment techniques and the ability to predict the future climate and performance of existing flood defences. Development should be designed with careful consideration to possible future changes in flood risk, including the effects of climate change and / or coastal erosion so that future occupants are not subject to unacceptable risks. 3.1 2. 1. 3. 4. 5. 21 Guidelines for Planning Authorities The Planning System and Flood Risk Management
- Decisions on the location of development may be required before development plans have been fully reviewed in accordance with these Guidelines and prior to the availability of appropriate flood risk mapping. In such circumstances a planning authority may choose to: i) Vary the development plan to facilitate appropriate development provided that the variation is accompanied by a rigorous assessment of flood risk in accordance with these Guidelines and the application of the Justification Test where necessary; ii) Assess the proposal in accordance with the approach outlined in chapter 5 (5.27); and/or, iii) Await the review of the development plan in accordance with these Guidelines, where such as review is imminent. Land required for current and future flood management, e.g. conveyance and storage of flood water and flood protection schemes, should be proactively identified on development plan and LAP maps and safeguarded from development. Flood risk to, and arising from, new development should be managed through location, layout and design incorporating Sustainable Drainage Systems and compensation for any loss of floodplain as a precautionary response to the potential incremental impacts in the catchment. Strategic environmental assessment (SEA) of regional planning guidelines, development plans and local area plans should include flood risk as one of the key environmental criteria against which such plans are assessed where flood risk has been identified. The SEA process provides an opportunity to assess and consider flood risk with respect to other planning and environmental considerations and should be used to show how the sequential approach to managing flood risks has been executed. 6. 7. 8. 9. 22 3 Fig. 3.1: Sequential approach principles in flood risk management JUSTIFY MITIGATE PROCEED Ensure the type of development proposed is not especially vulnerable to the adverse impacts of flooding. Preferably choose lower risk flood zones for new development. Ensure that the development is being considered for strategic reasons. See Boxes 4.1 and 5.1. Ensure flood risk is reduced to acceptable levels. Only where Justification Test passed. Ensure emergency planning measures are in place. A V O I D SUBSTITUTE Fig. 3.1 sets out the broad philosophy underpinning the sequential approach in flood risk management, while Fig. 3.2 describes its mechanism for use in the planning process.
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