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

86pressure in the embankment and geotechnical conditions occurring in the soilmatrix. Piping can sometimes be initiated by animals burrowing in the embankment.Planning GuidelinesMy site benefits from a high-standard flood defence, should itnot 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 issafe. A flood defence may be overtopped by a flood that is higher than that forwhich it was designed, or be breached and allow flood water to rapidly inundatethe area behind the defence. In addition, no guarantee can be given that flooddefences will be maintained in perpetuity. As well as the actual risk, which maybe reduced as a result of the flood defence, there will remain a residual risk thatmust be considered in determining the appropriateness of particular land uses anddevelopment. For these reasons, flooding will still remain a consideration behindflood 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 defencesagainst flooding. These may include custom-made flood defence structures orartificial (such as road or railway embankments) or natural features (such as naturallevees, sand dunes or other topographical features). These serve to reduce theactual risk of flooding since they reduce the frequency of flooding to that whichwill overtop the defence structure.What is residual risk?Residual risk is the risk that remains after all mitigation measures to reduce thefrequency of flooding have been taken. It can arise through overtopping or breachof the flood defences, usually by a flood that exceeds the design level of the defence.What is an adequate level of defence?The level of defence that is required for a particular area is generally commensuratewith the degree of damage that would occur in the event of the defence beingoverwhelmed.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, sluicegates, channel modification and diversion channels. These operate on a communitybasis but measures can also be taken to resist flood risk for individual developmentsor even properties. These include land-raising, raising floor levels or individualflood barriers, or ensuring that development at flood level is compatible withflooding. These are known as dry-proofing and aim to prevent water entering aproperty.What is flood resilience?The alternative to dry-proofing a property (because there are certainly economicaland possibly physical limits to this) is to wet-proof a property. This entails allowing