INVESTIGATION:THE NATIONAL Roads Authority (NRA) and Fingal County Council are investigating what caused the flooding that closed the N3 at Blanchardstown and a section of the M50 north of Finglas in recent days.
The N3 in particular was deluged following torrential rainfall at the weekend and again on Tuesday evening, causing widespread traffic chaos. Its dual-carriageway was so waterlogged that it resembled two parallel canals.
Hugh Cregan, a senior engineer at the NRA, said cameras were being put into road drainage pipes in the affected area to find out if they were blocked.
"There was a significant flooding problem and we want to find out what caused it and what needs to be done to ameliorate this situation in future," he told The Irish Times yesterday, adding that the NRA would be reviewing its drainage standards.
All motorway drainage in Ireland is designed for one in 50-year floods, in line with standards set by the Highways Agency in Britain.
But Mr Cregan said if flooding was going to become more frequent, these standards may have to be changed.
Motorway drainage generally depends on concrete pipes and gulleys as well as "French drains" - half-metre strips of gravel on either side of a roadway through which excess surface water seeps into drainage pipes underneath.
But the NRA, like roads authorities in other countries, also had to bear in mind where the water is discharged and whether nearby streams have the capacity to take it; these judgments are based on calculations made at the design stage.
Drainage issues would also be covered by the environmental impact statement on any new road scheme. This would cover all watercourses in the vicinity of the road and whether any of them would need to be culverted for its construction.
According to Mr Cregan, the drainage pipes need to be "self- cleansing" - in other words, their diameter cannot be so large that they become clogged with debris. Thus, making the pipes bigger "could cause more problems than it solves", he said.
"What we're doing here [to deal with flooding on motorways] is no different from any state-of-the-art road schemes in other countries.
"We also take account of the latest rainfall figures and developments elsewhere, particularly in the UK," he added.
However, Mr Cregan emphasised that the NRA "can't design what's beyond the roadways", such as new housing schemes or other developments. "Others who subsequently develop land in the vicinity of motorways need to take account of what we're doing".
The weakest point is likely to be the maintenance of motorways, according to another engineering source. This is generally the responsibility of local authorities or, in the case of public-private partnership (PPP) projects, the private operators.
"Blocked drains or gulleys could be the real problem, rather than the design of drainage systems," this source said. "Another issue is whether money should be spent upgrading drains, if heavy rain is only going to cause flooding once in 20 years or so."
David Burrows, managing director of StepGreen, a carbon and energy consultancy, warned yesterday that the "freak weather" that caused the heavy rainfall could be a foretaste of things to come as a result of worldwide climate change.
