Tuesday 1 November 2011

'Once-in-a-century' events have become distressingly familiar

OUR CONCEPT of “once-in-a-century” storms needs to be redefined following the latest flash floods in Dublin and the surrounding region, according to Ireland’s leading expert on climate change, Prof John Sweeney of NUI Maynooth.

After Dublin City Council categorised the latest floods as a “once-in-a century” event, he said this was now “a meaningless concept” based on standard trends, and we “no longer have such trends” because storms were becoming much more frequent.

“We can’t say this event is due to climate change, but the increase in frequency of intense rainfall events is on an upward curve – so we’re seeing nothing that contradicts the models [on climate change].

“It just so happened it was in Dublin this time, and not in Ballinasloe.”

Prof Sweeney said “much more statistical analysis” of rainfall patterns in Ireland was needed as a guide to future planning for events such as Monday evening’s rain, which led to the new record of 82.2mm for the greatest daily total for October at Casement Aerodrome, Baldonnel, since records began there in 1954.

Gerald Fleming, head of Met Éireann’s general forecasting division, said calculating “return periods” for such heavy rainfall was merely “playing with decimal points” and there was “certainly a question mark over whether they’re appropriate for the next 20 to 30 years”.

Dublin director of traffic Michael Phillips said it was not enough to be told by Met Éireann there would be heavy rain in Dublin.

“We need to develop a micro-system that will tell us if it’s going to be in Drumcondra or Ballsbridge so we know where to send out workforces.”

The last intense rainfall event in the Dublin area was on August 9th, 2008, when amounts in excess of 70mm were recorded in Celbridge, Lucan and Leixlip. Just three years earlier, on June 29th, 2005, 45mm fell in the Phoenix Park in less than an hour.

Coping with the consequences has presented real problems for the authorities.

After floods hit Drumcondra in November 2002 and then taoiseach Bertie Ahern was pictured standing up to his knees in water, priority was given to improving flood defences along the river Tolka.

There was, and still is, a risk that building up walls along one section of a river could result in it overflowing elsewhere.

Natural floodplains were also affected by urban and suburban development during the boom, simply by the spread of concrete and tarmacadam that allows water to gather in great quantities.

The controversial flood defences proposed for Clontarf would have been of no help on this occasion; they were designed to protect the area from inundation by the sea. Monday night’s flooding was caused by intense heavy rainfall over a period of several hours.

Some motorways and other major roads became impassable on Monday due to flooding, notably the M7 on the outskirts of Dublin, the N11 at the UCD Belfield underpass and the M50 between the Ballinteer and Firhouse junctions, triggering a six-hour closure.

Sean O’Neill, spokesman for the National Roads Authority, said the latter was due to a culvert backing up and flooding out onto the motorway. Six pumps were brought in by the operating company, M50 Concessions, to clear the water, which reached a depth of 300mm.

He explained all roads are designed to allow water to drain off their surface.

However, if the surrounding land and drainage is at or beyond capacity, the road is directly affected because “a domino effect takes place and the road becomes flooded”.

After the Dublin area emergency plan was activated, O’Neill said the authority lifted tolls on the port tunnel, allowing it to be used freely by motorists seeking to avoid flooded roads elsewhere. “The tunnel was fine and was used as a pressure reliever and exit route from the city.”

As for what happened in Dundrum shopping centre, where the entire ground floor was flooded, engineers have noted that the Dundrum Slang stream “is currently protected from a major flooding event which has a 2 per cent [1-in-50 year] probability of occurring in any given year”.

However, results from the river catchment flood risk assessment and management programme being carried out by the Office of Public Works (OPW) have shown that “there is no viable flood risk management option available for the entire Dundrum Slang”.

The programme derives from the 2007 EU “floods” directive, which requires all member states to undertake a preliminary flood risk assessment, based on available information, such as the OPW’s extensive flood mapping database.

This is available online at www.floodmaps.ie

Irish Times


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