A group campaigning to maintain access to the River Lee have unveiled plans for a €135 million tidal barrier which they say will remove the need for building flood defences in the city centre as proposed by the OPW in its €140 million flood relief plan for Cork.
The Save Cork City Group say their proposal for a tidal barrier at the eastern end of Lough Mahon downstream of the city would obviate the need for many of the flood defences in the city centre proposed by the OPW which they say will make the river less accessible to Corkonians.
The group unveiled its plans at a public meeting attended by over 100 people who heard engineer and resident of the Cork city quays, Michael Ryan, explain that a tidal barrier at Lough Mahon/Little Island could be built for a lot less than the OPW's estimated cost of €450 million to €1 billion.
Mr Ryan said that flooding in Cork city involved a complex of factors including upriver flows, tidal surges, a series of historic culverts and pipes under the city and the fact that the city is built on an extensive aquifer which is supplied and affected by both river flows and tidal surges.
However building a tidal barrier downstream of the city with sufficient capacity to store water coming down the Lee valley and through the city during periods of heavy rain would remove one of the main drivers behind flooding in the city centre and offered a viable solution, he said.
Three options
Mr Ryan said that the Save Cork City group had looked at three possible options for a tidal barrier - near the Jack Lynch Tunnel at the western end of Lough Mahon, at Little Island at the eastern end of Lough Mahon and at Great Island further down the harbour.
The Save Cork City group estimated that any barrier would have to be sufficiently downstream of the city to allow a storage capacity of around 8 million cubic metres for flood waters coming down the River Lee.
A tidal barrier at the Jack Lynch Tunnel only offered capacity of 6 million cubic metres.
A barrier at Great Island was also discounted as the channel was quite deep and the river was quite fast flowing which would make the cost of a tidal barrier prohibitively expensive.
This left the middle option of a tidal barrier at the eastern end of Lough Mahon at Little Island said Mr Ryan.
Locating a tidal barrier here offered a storage capacity of 26.5 million cubic metres - just over three times what is required.
This had the advantage of not being prohibitively expensive at €135 million, roughly the equivalent in today’s prices of what the €70 million Jack Lynch Tunnel cost in 1999.
The tidal barrier that they were proposing for Little Island would be 910 metres long with a 60 metre wide navigation channel with a tidal gate comprising of six sluice gates for ships to pass through and an 850-metre long wave wall to prevent tidal surges.
The barrier would also involve the use of 910 metres of sheet piling to prevent seepage underneath.
The barrier would be built of earthen dam embankments protected by rock armour which would be sufficient given the barrier is in a sheltered area and not in the open Atlantic, said Mr Ryan.
“Our design is earth and rock embankments - we are not talking about new technology or something that is very difficult to construct ..... it is a simple design with no tunnels, no bridges, no walkways in order to keep the cost down so it can be done for a reasonable budget.
“And it uses a known technology and can be built in 24 months, it involves minimum disruption in the city centre without the need for walls that restrict access to the river and it protects more parts of the city such as the Docklands and it creates a potential amenity in Lough Mahon.”
