Turning the tide of opinion over city’s flood plan

Rob McNamara examines the plans and alternative proposals to tackle Cork city's flooding issues. Are they water tight?
Turning the tide of opinion over city’s flood plan
Image from the report by HR Wallingford on the Cork City Tidal Barrier. The report includes a cost estimate for the tidal barrier proposed by Save Cork City.

Cork city desperately needs flood defences. Of that, there is no doubt. Flooding events in recent years have taken their toll on traders, business people and the general public.

At the moment, the debate on the best way to move forward comes down to what will be more effective; a tidal barrier downstream or raised quay walls in the city centre.

Other questions include how much are people willing to put up with to get adequate flood defences and what solution is the fastest, most effective and least obtrusive on daily life in Cork city.

At the moment there are two very different plans to tackle the problem in circulation.

One is a Government led Office of Public Works initiative, supported by Cork City Council, to spend €140m on raising quay walls along the River Lee to protect the city from flooding. This project could take seven to 10 years to complete.

The other is a tidal barrier championed by the Save Cork City (SCC) group. SCC say a tidal barrier will save the city from years of disruption and will cost a similar amount to the OPW plans but will be completed within two years.

The publication of an independent report,commisioned by SCC, into the efficacy and cost of a tidal barrier by UK-based independent civil engineering and environmental hydraulics organisation HR Wallingford has added weight to this idea.

Under OPW plans, the city and its rivers areas would be dramatically changed. With the River Lee central to the aesthetics of Cork, it's important that the solution is explored in its minutiae.

The Lower Lee Flood Relief Scheme proposals devised by engineers at the OPW plans new walls along the River Lee from the Lee Fields into the city centre and walls being raised on the north and south channels of the river. Floodgates and barriers will also be constructed at various parts of the river.at various parts of the river.

A fluvial flood forecasting system and a new dam discharge system will alert authorities to emergencies at Carrigadrohid and Inniscarra dams and over 2,000 properties are said to be protected in the scenario of large-scale flooding. The first phase of construction is expected to start later this year.

The OPW have drawn criticism for their potential to distance the people of Cork from their long-standing relationship with the river.

SCC, group of architects, engineers and academics have been at the forefront of campaigning for an alternative solution that doesn't impact as much on the city and tackles the problem of flooding downstream - some 10km away from the city centre at Lough Mahon.group of architects, engineers and academics have been at the forefront of campaigning for an alternative solution that doesn't impact as much on the city and tackles the problem of flooding downstream - some 10km away from the city centre at Lough Mahon.

A report they have commissioned by HR Wallingford states that a tidal barrier will “avoid the disruption that construction of the walls would cause and also avoid the impacts that walls would have on the city landscape”.

The report also says that tidal barrier will protect more of the city of Cork as it will include areas to the east of the proposed flood defences and the flood relief scheme.

HR Wallingford has been involved in numerous engineering and environmental hydraulics projects around the world, including the Bruney Tenburong Bridge, the Bristol Deep Sea Container terminal, the Mubarak Al-Kabeer port project in Kuwait, the London Gateway Port and assessed flood risk on the Thames Estuary.

Their report on a Cork tidal barrier states: “Options available for tidal flood mitigation include a tidal barrier, which is open under normal conditions, and a tidal barrage, which prevents tidal flows upstream. A tidal barrier is the preferred choice for Cork because the present tidal regime is maintained and the impacts on navigation, drainage, morphology, saline intrusion, water quality, fish passage and the environment are minimised by a barrier.

It adds that a tidal barrier will reduce flood risk from tidal flooding; will allow for the storage of fluvial water from upstream during surge tide events when the barrier is closed; permit free discharge of fluvial flows to avoid increasing fluvial flood risk and will not worsen groundwater flooding.

No negative environmental impacts for water circulation and fish passage have been identified by the report but it is stated that these would have to be further considered in the final design.

According to the report, €94m of the cost estimate will be taken up by construction.

“Uncertainty in the estimate means that the estimated cost could range from €110m and the lower end of the range to €170m at the higher end. The price of €140m contains a contingency of about €30m,” the report states.

It is suggested that maintenance costs would amount to €1m per annum, up to 25 years.

The report concludes: “The alternative scheme has the major benefit of not requiring the construction of walls along the quays in the city, thus helping to maintain the historic landscape of the city and avoiding the disruption that the construction of walls would cause. The alternative scheme would also avoid the scenario of overtopping or failure of flood walls in the city, which could cause a serious risk to life as well as damage to the city.” 

The OPW are currently finalising their own report on the costs of a tidal barrier with Irish engineering consulting firm ARUP as part of their public consultation process. However, their findings are expected to closely tally with the OPW's previous stance.

The OPW tidal barrier cost estimate will be released into the public domain upon its publication. They did not comment on the HR Wallingford report but said that previous figures for the cost of a tidal barrier being put forward by SCC (€135M) are likely to be much more costly.

A spokesperson for the OPW said: “As part of the public consultation process on the Lower Lee Flood Relief Scheme, the OPW committed to producing a report on the costs surrounding the potential for a tidal barrier in Cork and this report is currently being finalised by our consultants. Its preliminary findings confirm the previously advised position of the OPW that a tidal barrier is not currently viable and would, in fact, cost many times the figures being put forward by the SCC group.

“In addition, any tidal barrier, assuming it was to overcome the significant environmental hurdles which it would present, would only resolve the tidal flooding problem in the City as it would not deal with the fluvial problem and, so, some defences would still be required in the city.”

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