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Planning

Flood risk and development




Introduction
The underlying causes of flooding, heavy rain and high sea levels, are, essentially, uncontrollable. However, the factors affecting the extent and severity of the flood can be addressed. The most influential of these factors is development, in particular development in flood plains i.e. areas adjacent to rivers that tend to become flooded following periods of heavy rain. Historic records will help to indicate which areas might be prone to flooding, although it is always possible that areas not known to have flooded in the past or for which no records of flooding are available, might flood in the future due to changes in upstream or downstream conditions or the occurrence of a more extreme rainfall event.

 

The risk of flooding should be taken into account in all cases where development is being considered. This is in the interests of individuals proposing the development and of the public in general.

 

Impact of flood risk on development
Locating development in an area at risk from flooding can lead to property damage, human stress and hardship, problems obtaining property insurance and consequential demands for the expenditure of local authority or central government resources on flood protection works. The construction of protection works either at the time of the development, or at a later date, will incur additional costs, may not provide absolute immunity from the risk of flooding and can, if not appropriately designed, have detrimental effects on flood risk elsewhere.

 

Impact of development on flood risk upstream
In times of flood, the river flows not only through its normal channel but also along the flood plains. Any constriction of the natural flow path can 'back-up' the river and lead to increased flood levels upstream. The construction of buildings or houses, and particularly embankments for infrastructure or protection, in or across a floodplain can therefore not only put the development itself at risk of flooding, but can also increase the flood risk for land and properties upstream. The same is obviously true of any construction in, or encroachment into, the normal river channel.

 

Impact of development of flood risk downstream
Natural or agricultural land, such as forests, woodland, pastures or crop fields, is normally able to absorb a considerable proportion of any rain that falls onto it. Covering such land with buildings, tarmac (such as for parking areas or roads), or other impermeable materials significantly reduces this ability to absorb rainfall, and will lead to increased land runoff. As a result, large developments, including those away from major rivers, can increase river flows and the risk of flooding to land and property downstream. A number of smaller developments built up over a period of time can have the same effect.

 

Impact of tidal flood risk on development
A combination of high tides, waves, high winds and surges developed from low-pressure systems can lead to extensive flooding. Current predictions of climate change indicate that the risk of flooding from the sea will increase in the future. Any area below current or predicted future peak sea levels, including areas behind existing defences that offer only a certain level of protection, are at risk from flooding in the future.

 

Impact of possible climate change
Current predictions of climate change in Ireland indicate that winters will become wetter and the rainfall distribution 'stormier', and that sea levels will rise. This would mean that areas not currently prone to flooding may be at risk from flooding in the future.