The Planning System and Flood Risk Management - Offaly County ...

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82Appendix C - Frequently AskedQuestionsPlanning GuidelinesWhat is flooding?Flooding is when land that is normally dry is covered by water. This can haveboth beneficial and adverse consequences. Deposition of sediments from riverfloods form the level floodplains alongside rivers and contribute to its agriculturalfertility and its biodiversity. When it affects human activities, it can threaten life,cause social and health impacts, damage buildings and their contents, interruptcommunications 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 floodingis due to intense and or prolonged rainfall which exceeds the capacity for it to sinkinto the ground. As a result, it runs off over land and into rivers and artificialdrainage systems. When the capacity of these is exceeded, the result is river andsewer flooding. The roofs of buildings, roads and hard standings prevent watersoaking into the ground thus increasing run-off and exacerbating the problems.Flooding can also occur when water that has soaked into the ground fills theunderground storage and overflows onto the surface.How will climate change influence flooding?Climate change could potentially have a very significant effect on flooding in thelonger term. Higher sea levels and wetter winters, with more intense rain storms,together with possible increases in storminess could significantly increase both thefrequency and intensity of flooding. For example, floods which currently have a1 in 100 chance (1% probability) of occurring in any one year could occur muchmore frequently.How is flood risk measured?Flood risk is a combination of the likelihood of occurrence and the consequencesof a flood occurring. This is normally expressed as:Flood risk = probability x consequencesProbability is difficult to estimate because it has to take account of the uncertaintyof 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 andproperty affected by the flood.What is a source-pathway-receptor model?A source-pathway-receptor model is a representation of the components of floodrisk. 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.) by which the flood reaches those receptors. For a risk to arise,all three components must be present and flood risk management is essentiallyconcerned with protecting the receptors and/or modifying the pathways.83What is a flood risk assessment?A flood risk assessment is the identification, quantification and communication offlood risk using the source-pathway-receptor model. It examines the sources offlooding and the pathways by which floodwaters might reach receptors, such aspeople, property and the environment to determine the likelihood of them beingaffected by flooding. It also examines the flood hazards that are likely to arise andthe vulnerability of receptors to such hazards. Flood risk can be assessed at differentscales, from the national, through regional and local to site-specific.What is a flood hazard assessment?A flood hazard assessment is the identification, quantification and communicationof 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 occurringboth 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 countyscale) of the risks of flooding to support spatial planning decisions such as the zoningof particular areas for development. It is an essential element in the adoption of thesequential approach to the consideration of flood risk in spatial planning.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 detailedas is necessary to obtain the information required to support the decision that is tobe made. A staged approach should be adopted, therefore, comprising a screeningassessment to identify whether there are flood risks to an area or site, a scopingassessment to confirm the sources of flooding and qualitatively appraise the risksand potential mitigation measures and an appropriate assessment to quantify therisks, appraise the effects of changes and the effectiveness of mitigation measures.At each stage the adequacy of the assessment to support the decision being madeshould be determined and the next stage should be initiated only if there is insufficientinformation to support the decision.What does the SFRA provide?The strategic flood risk assessment provides tools and methods to assist users inidentifying 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 mapsto help in assessing sites against flood risk criteria. Where development is or wouldbe at risk of flooding, it provides information on the mitigation measures considereddeliverable to reduce the actual risk to that development and on the residual risksthat would remain and how they might be managed.Planning Guidelines
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At the site-specific level, develop
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Planning Guidelines
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2regard to the guidelines in the pe
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4Planning GuidelinesReport of the F
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6Chapter 2:Flooding - Key ConceptsP
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82.5 Increase in flooding due to de
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10Flood risk2.13 Understanding floo
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122.21 In undertaking a flood risk
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142.30 Increases in rainfall intens
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16after periods of heavy rainfall.
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18that also meet the objectives of
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20more than 1 in 200). Development
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22Government under Section 28 of th
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24Planning GuidelinesDevelopment in
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26Chapter 4:Flooding and Spatial Pl
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28floodplain. Effective co-ordinati
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30Planning GuidelinesFig. 4: Develo
Page 42 and 43: 32• Include zoning objectives to
Page 44 and 45: 34specific areas, ensuring that flo
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Page 48 and 49: 38planning authority to ensure that
Page 50 and 51: 40• these guidelines, and in part
Page 52 and 53: 425.22 Where development proposals
Page 54 and 55: 44Chapter 6:ConclusionPlanning Guid
Page 56 and 57: 46TermsDefinitionPlanning Guideline
Page 64 and 65: 54Appendix A - Screening for andCar
Page 66 and 67: 56Climate changeincreased intensity
Page 68 and 69: 58Strategic assessments provide the
Page 70 and 71: 60As part of a FRA, information on
Page 72 and 73: 62A successful FRA is characterized
Page 74 and 75: 64• Suggested policies for sustai
Page 76 and 77: 66Planning Guidelines• The impact
Page 78 and 79: 68An additional document on interpr
Page 80 and 81: 70Table A2: The Flood Risk Balance
Page 82 and 83: 72Appendix B - Addressing FloodRisk
Page 84 and 85: 74Figure B2 below illustrates how t
Page 86 and 87: 76drainage processes and removing p
Page 88 and 89: 78• Signing of floodplain areas s
Page 90 and 91: 80flood level. Care must be taken t
Page 94 and 95: 84What does the SFRA not provide?Th
Page 96 and 97: 86pressure in the embankment and ge
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