LOCAL AUTHORITIES must stop rezoning land for housing on flood plains to avoid future flooding disasters, NUI Maynooth climatologist Prof John Sweeney has said.
Climate research indicated that extreme weather events, such as the heavy rainfalls which have caused flooding in recent days, were likely to become more common, Prof Sweeney said, and better planning was needed to avoid severe flooding.
The equivalent of a month's rain fell in parts of Dublin and other east coast areas on Saturday, resulting in flash flooding and travel chaos. More rain is expected in the coming days, with a risk of further flooding in already waterlogged areas, according to Met Éireanns.
Flood plains are areas beside rivers and streams which are covered in fine alluvial sediment which is laid down over years as a result of flooding. This sediment acts as a type of storage area for flood waters and helps to prevent floods spreading. However, if flood plains are built on, that water storage capacity is lost.
"Once you start urbanisation on flood plains, you remove the storage capability for flooding and the water ends up going into the main channel down stream," Prof Sweeney said. "Low-lying, more vulnerable areas should not be built on, we should not be putting people in there, and you have to be very careful about putting infrastructure such as roads and rail in."
Measures to protect buildings in areas prone to flooding can make matters worse as man-made barriers cause the water to move more quickly and flood other areas.
"When walls are put in around a development this can affect other areas and make the flood peak higher and the flood peak quicker than it otherwise would."
In recent decades there had been a huge amount of construction of flood plains, but householders may not be aware that their property was at risk.
"In the short term, a house could be built and sold and 10 years down the line, that's when the flooding happens, when the person who built and sold the house is long since departed leaving long-term flooding problems behind."
However, Prof Sweeney said developers were not necessarily the ones to blame.
"The problem starts earlier than with the developers. Local authorities rezone agricultural land for residential development and people are entitled to expect that if the land has been zoned for housing and they've bought a house built on it, that there isn't a problem."
Climate research was showing that rainfall amounts were increasing. While there was not yet sufficient evidence to attribute this to climate change, Prof Sweeney said, warmer conditions were likely to lead to increased rainfall because warm air holds more water vapour.
"The ultimate bottom line is that we shouldn't be rezoning in areas that are known to have a vulnerability to flooding. More conservative planning is what's needed, the precautionary principle needs to be applied more ruthlessly in order so that you don't put people at risk. You have to take a long-term view."
Earlier this year the Department of the Environment issued new planning guidelines to local authorities which recommended land in areas at the highest-risk from flooding should be used for amenity purposes and parking, ahead of housing, however it did not propose a ban on building on flood plains.
The department, in conjunction with the Office of Public Works, is finalising further guidelines in relation to flood risk management. However, these will not prohibit residential development on flood plains, a department official said.
Rain will spread eastwards tonight and tomorrow, but will also remain heavy in Connacht, Munster and Ulster, according to Met Éireann.
The Irish Times