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

- 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