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

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6Chapter 2:Flooding - Key ConceptsPlanning Guidelines• Flooding is a natural process and can happen at any time in a wide varietyof locations. It constitutes a temporary covering of land by water and presentsa risk only when people and human assets are present in the area whichfloods. Assets at risk from flooding can include housing, transport and publicservice infrastructure, commercial and industrial enterprises, agriculturalland and the environmental and cultural heritage.• Understanding flood risk is an essential step in managing the associatedimpacts and in making informed decisions. The science is complex, anddatasets are currently being enhanced by detailed studies. Therefore, theguidance recommends a staged approach to flood risk assessment, whichcovers both the likelihood of flooding and the potential consequences.• Screening, scoping and appropriate flood risk assessments are key steps andare described in this chapter and supporting appendices. Planning authoritieswill need to commission assessments of flood risk and ensure that the impactsof climate change are accounted for.• The management of potential future flood risk aims to reduce the potentialrisks to people, property and the environment through a hierarchy ofavoidance, followed by substitution of lower vulnerability uses (whereavoidance is not possible) and, only if avoidance and substitution are notpossible, reduction and management of the risks through a variety oftechniques.Types and causes of floodingDifferent forms of flooding present different forms and degrees of danger to people,property and the environment, due to varying depth, velocity, duration, rate ofonset and other hazards associated with flooding. With climate change, the frequency,pattern and severity of flooding are expected to change and become more damaging.2.2 There are essentially two major causes of flooding:• Coastal flooding which is caused by higher sea levels than normal,largely as a result of storm surges, resulting in the sea overflowing ontothe land. Coastal flooding is influenced by the following three factors,which often work in combination:- High tide level;- Storm surges caused by low barometric pressure exacerbated byhigh winds; the highest surges can develop from hurricanes; and
- Wave action which is dependent on wind speed and direction,local topography and exposure.7• Inland flooding which is caused by prolonged and/or intense rainfall.Inland flooding can include a number of different types:- Overland flow occurs when the amount of rainfall exceeds theinfiltration capacity of the ground to absorb it. This excess waterflows overland, ponding in natural hollows and low-lying areasor behind obstructions.- River flooding occurs when the capacity of a watercourse isexceeded or the channel is blocked or restricted, and excess waterspills out from the channel onto adjacent low-lying areas (thefloodplain).- Flooding from artificial drainage systems results when flow enteringa system, such as an urban storm water drainage system, exceedsits discharge capacity, the system becomes blocked, and / or cannotdischarge due to a high water level in the receiving watercourse.Flooding arising from a lack of capacity in the urban drainagenetwork has become an important source of flood risk, as evidencedduring the Summers of 2007 and 2008.- Groundwater flooding occurs when the level of water stored inthe ground rises as a result of prolonged rainfall to meet the groundsurface and flows out over it, i.e. when the capacity of thisunderground reservoir is exceeded. Groundwater flooding tendsto be very local and results from interactions of site-specific factors.Such flooding may often result in significant damage to propertyrather than be a potential risk to life.- Estuarial flooding may occur due to a combination of tidal andfluvial flows, i.e interaction between rivers and the sea, with tidallevels being dominant in most cases. A combination of high flowin rivers and a high tide will force water back up the river channeltending to increase water levels inland, which may flood overriver banks.2.3 A less frequent form of flooding arises from the failure of infrastructuredesigned to store or carry water (e.g. the breach of a dam, a leaking canal, ora burst water main), or to protect an area against flooding (e.g. breach of aflood defence, failure of a flap valve or pumping station or blockage of a pipeor culvert). Because of the sudden onset, the impacts of this form of floodingcan be severe (see Para 2.7).2.4 Coastal erosion of both the foreshore and the shoreline itself is intimatelylinked with coastal flooding. The loss of natural coastal defences, such as sanddunes, due to erosion can increase the risk of flooding in coastal areas. Coastalerosion should therefore be considered in coastal areas within the planningprocess, although this issue is not within the focus of these guidelines.Planning Guidelines
Page 7: At the site-specific level, develop
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Page 12 and 13: 2regard to the guidelines in the pe
Page 14 and 15: 4Planning GuidelinesReport of the F
Page 18 and 19: 82.5 Increase in flooding due to de
Page 20 and 21: 10Flood risk2.13 Understanding floo
Page 22 and 23: 122.21 In undertaking a flood risk
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Page 28 and 29: 18that also meet the objectives of
Page 30 and 31: 20more than 1 in 200). Development
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
Page 42 and 43: 32• Include zoning objectives to
Page 44 and 45: 34specific areas, ensuring that flo
Page 46 and 47: 36should use the flood risk informa
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
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56Climate changeincreased intensity
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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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