The definitive statement came as the OPW and Arup engineers, who are overseeing the €140m Cork City flood relief scheme, briefed city councillors on the status of the project behind closed doors on Monday.
In documents seen by thethey said they have conducted a detailed consideration of two tidal barrier alternatives — one at Little Island and the other, a double-barrier option either side of Great Island.
Both options were considered under several headings, including navigational impact and safety, environmental impact on designated sites, resilience, reliability and risks, climate change adaptability, and cost.
The engineers said a tidal barrier at Little Island would increase peak velocity flows by a factor of three resulting in potentially unsafe conditions, with these increased flows also impacting negatively on nearby environmentally sensitive areas.
They said it would require the raising of a 500m stretch of the N25 and the construction of embankments along the Cork to Midleton railway, would be extremely unlikely to be acceptable to the Port of Cork and other harbour users, and would increase the potential for ongoing dredging.
This option could cost at least €656m to build, with the price tag rising to just over €1bn to build, maintain, and deliver all the additional fluvial flood defences.
They said while the Great Island tidal barrier option did not have the same environmental or safety impacts, it would cost €1.2bn to build with the price tag soaring to €1.86bn with the maintenance and ancillary costs added in.
Neither option is financially viable and walls would still be needed to protect the city from fluvial flooding, they insisted.
They told councillors that they have also examined several upstream storage alternatives proposed by Save Cork City, the campaign group leading the opposition to the OPW scheme’s reliance on raised quay walls, and which has argued that a tidal barrier could be built at Little Island for just €140m.
The OPW and Arups said that computer modelling showed that storing flood waters upstream will not remove the need for direct defences, and could delay peak flows on the Shournagh river, which would actually increase flood risk in the city.
They said submissions which suggest that the existing dams at Inniscarra have sufficient storage to avoid defences entirely are flawed and are premised on a number of incorrect assumptions, based on a proposed dam operating regime which would actually jeopardise dam safety by significantly increasing the risk of overtopping of the dams.
They said technically viable options to raise the dams would all impact on the protected Gearagh region, would require the displacement of 80 properties, the raising or relocation of a minimum of 8km of national or regional roads, would sterilise at least 5 sq km of land, and would not tackle tidal flooding risk in the city.
The engineers also confirmed that they are examining groundwater concerns in the city, raised by Save Cork City and its experts.
“This issue is recognised and well understood and is being assessed in detail. Groundwater monitoring, pump testing and seepage analysis is ongoing,” they said.
“Preliminary assessment to date confirms that groundwater flood risk will be manageable and that the scheme will not significantly impact the normal groundwater regime.”
The briefing came as the OPW confirmed its intention to seek planning permission for the first phase of the overall flood-defence scheme — the €6m Morrison’s Island public realm project — by the end of the month.