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

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10Flood risk2.13 Understanding flood risk is a key step in managing the impacts of flooding.Flood risk is a combination of the likelihood of flooding and the potentialconsequences arising. These guidelines recommend a staged approach toflood risk assessment that covers both the likelihood of flooding and thepotential consequences.2.14 Firstly, it is important to define the two components of flood risk in order toapply this guidance in a consistent manner.Planning Guidelines• Likelihood of flooding is normally defined as the percentage probabilityof a flood of a given magnitude or severity occurring or being exceededin any given year. For example, a 1% probability indicates the severityof a flood that is expected to be exceeded on average once in 100 years,i.e. it has a 1 in 100 (1%) chance of occurring in any one year.• Consequences of flooding depend on the hazards associated with theflooding (e.g. depth of water, speed of flow, rate of onset, duration,wave-action effects, water quality), and the vulnerability of people,property and the environment potentially affected by a flood (e.g. theage profile of the population, the type of development, presence andreliability of mitigation measures etc.)2.15 Flood risk is then normally expressed in terms of the following relationship:Flood risk = Likelihood of flooding x Consequences of flooding2.16 Flood risk can be measured in monetary terms, as a potential for loss of lifeor injury, as environmental impacts, or as combinations of all these indicators.Guiding proposed development to areas, where the risks arising from inundationin terms of human impact, economic loss or potential environmental damageis minimised, is a key tenet of this guidance.2.17 The vulnerability of development to flooding depends on the nature of thedevelopment, its occupation and the construction methods used. For example,a sheltered housing complex would be more vulnerable than a retail unit. Abroad classification of vulnerability has been developed as shown in theGlossary of Terms. The classification of different land uses and types ofdevelopment as highly vulnerable, less vulnerable and water-compatible isinfluenced primarily by the ability to manage the safety of people in floodevents and the long-term implications for recovery of the function and structureof buildings (see Glossary of Terms).2.18 Transport and strategic utilities infrastructure can be particularly vulnerableto flooding because interruption of their function can have widespread effectswell beyond the area that is flooded. For example, flooding of primary roadsor railways can deny access to large areas beyond those directly affected by
the flooding for the duration of the flood event, as well as causing damage tothe road or railway itself. Flooding of water distribution infrastructure suchas pumping stations or of electricity sub-stations can result in loss of wateror power supply over large areas. This can magnify the impact of floodingwell beyond the immediate community and reinforces why decisions to locatedevelopment in floodplain should be taken very carefully (see paragraphs2.24 and 2.36 below). This was a key lesson learnt from recent floods in Irelandwhen national primary roads and railway lines were flooded, and also theSummer 2007 floods in the UK, during which water supply to large areas wascut off for a number of weeks.Flood risk assessment2.19 The assessment of flood risk requires an understanding of where the watercomes from (i.e. the source), how and where it flows (i.e. the pathways) andthe people and assets affected by it (i.e. the receptors). The source – pathway– receptor model is described in greater detail in Appendix A.Overland runoffand muddyflooding due tointensive rainfallClimate changeincreased intensityof summer stormsGroundwaterflooding due toraised water tableSurcharged sewercauses basementfloodingUrban creep:increased pavingSewerexceedancefloodingDirect overlandflow and pondingin low spots (sinks)Floodingthrough thealluvialsBlockage or sewer collapseImpervious paved areaFig. 2: Sources, pathways and receptors of FloodingReservoir orcanal breach2.20 The principal sources are rainfall or higher than normal sea levels , the principalpathways are rivers, drains, sewers, overland flow and river and coastalfloodplains and their defence assets and the receptors can include people,their property and the environment. All three elements must be examinedas part of the flood risk assessment. Mitigation measures typically used indevelopment management can reduce the impact on people and communities,for example, by blocking or impeding pathways but they have little or noeffect on the sources of flooding. The planning process is primarily concernedwith the location of receptors, taking appropriate account of potential sourcesand pathways that might put those receptors at risk.11Planning Guidelines
Page 7: At the site-specific level, develop
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Page 16 and 17: 6Chapter 2:Flooding - Key ConceptsP
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Page 28 and 29: 18that also meet the objectives of
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Page 32 and 33: 22Government under Section 28 of th
Page 34 and 35: 24Planning GuidelinesDevelopment in
Page 36 and 37: 26Chapter 4:Flooding and Spatial Pl
Page 38 and 39: 28floodplain. Effective co-ordinati
Page 40 and 41: 30Planning GuidelinesFig. 4: Develo
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Page 44 and 45: 34specific areas, ensuring that flo
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Page 50 and 51: 40• these guidelines, and in part
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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
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60As part of a FRA, information on
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62A successful FRA is characterized
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64• Suggested policies for sustai
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66Planning Guidelines• The impact
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68An additional document on interpr
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70Table A2: The Flood Risk Balance
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72Appendix B - Addressing FloodRisk
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74Figure B2 below illustrates how t
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76drainage processes and removing p
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78• Signing of floodplain areas s
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80flood level. Care must be taken t
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82Appendix C - Frequently AskedQues
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84What does the SFRA not provide?Th
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86pressure in the embankment and ge
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Printed on recycled paper containin
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