OPW defends €450m estimate for Cork city tidal barrier

The Office of Public Works has defended its estimate that building a tidal barrier for Cork city would cost at least €450 million. This follows claims by a campaign group that a barrier could be built for €135 million and would remove the need for flood defences in the city centre that would reduce access to the river.

According to chartered engineer Ezra MacManamon of the OPW, the agency stands over its estimate that a tidal barrier in Cork Harbour would cost €450 million-€1 billion to construct and would do nothing to prevent fluvial or river flooding of Cork city centre.

Last week the Save Cork City campaign group revealed proposals in a document called Potential Cork in which it said a 910m-long earth and rock armour tidal barrier could be built at the eastern end of Lough Mahon to prevent tidal flooding of Cork city.

But Mr MacManamon said that while it was theoretically possible to use such defences against tidal flooding, as happens in parts of the Netherlands, the OPW was satisfied that its estimate for the cost of a tidal barrier in Cork Harbour was accurate.

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"Our figures are based on the Lee Catchment Flood Risk Assessment and Management study, which estimated the cost of a tidal barrier at €450 million and more recently our consulting engineers, Arup, have put the cost of a tidal barrier at €1 billion," he said.

"They based that figure on a review of the Delft University of Technology report, which is a case study done on flood defences in the Netherlands, New Orleans and Vietnam, so again it's a very thorough and rigorous review of the cost of providing flood defences."

No detail

According to Mr MacManamon, the Save Cork City estimate of €135 million includes a figure of €27 million for maintenance of the barrier, which incorporates a 60m navigational channel, but it provides no detail on the cost of site exploration or the actual detail design of the barrier.

“There could well be technical difficulties at that specific location; it’s sitting on top of a gravel aquifer, which could prove a very technical issue that needs to be addressed following substantial site investigation, which could be very costly, but the Save Cork City group don’t address that.”

Mr MacManamon also pointed out studies by the OPW that found building a tidal barrier would prevent €40 million worth of flood damage over the 50-year lifespan of the scheme, so any tidal barrier would have to cost less than €40 million to be cost-effective.

“Even if you were to accept the Save Cork City estimate of €135 million as the cost of the barrier, it would only prevent damage of €40 million, so it’s not cost-effective or economically viable. Plus, of course, it doesn’t address the issue of fluvial flooding which is a huge problem in Cork.”

He said the only plan that would prevent fluvial flooding in Cork city was the €140 million OPW flood defence scheme involving the building of flood defences in the city centre. Save Cork City proposals for upstream flood-management measures in conjunction with a tidal barrier were just not feasible, he added.

He said the Save Cork City group advocated the use of flood-mitigation measures upstream but it was again vague on detail, and the OPW estimated that such measures would require the sacrificing of some 40sq km of land in flood-mitigation measures.

He said building flood defences in the city centre as proposed by the OPW to deal with a one-in-100-years flood would also allow the ESB to change its discharge regime at Inniscarra Dam to allow discharges to increase safely to 550 cubic metres per second without leading to flooding of the city centre, as happened in 2009.