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Cork flood defence: Campaign against ‘experiment that is likely to fail’ continues

Cork flood defence: Campaign against ‘experiment that is likely to fail’ continues
The OPW flood protection plans include direct defences such as embankments and walls, as well as pumping stations and better forecasting. Picture: Larry Cummins

Newly published research has described the OPW’s controversial €140m flood defence plan for Cork as an “experiment which is likely to fail”.

The work by consultant geologist Anthony Beese, which has been ongoing for several years, warns that elements of the largest flood defence scheme of its kind in the history of the State could increase flood risk in places, could put buildings in the city centre at risk of structural damage, and may reduce groundwater quality in a regionally important aquifer.

The research was published thanks to Save Cork City (SCC), the main opponents of the OPW’s Lower Lee Flood Relief Plan, and is the latest report commissioned by the group to cast doubt on the OPW’s approach to managing flood risk in the city.

The OPW scheme is designed to protect over 2,100 properties, including 900 homes and 1,200 businesses, against tidal and river flooding, through a suite of measures including:

  • Direct defences places along the north and south channel, comprising embankments, railings, ground level changes, and, in some locations, 1.2m-high walls;
  • A network of pumping stations behind the defences to prevent “back of wall” surface water flooding. The pumping stations will require deep excavations up to 5m deep;
  • A detailed flood forecasting system;
  • Revised dam operating procedures at Inniscarra and Carrigadrohid dams;
  • The designation of upstream washlands in the floodplain to facilitate greater advance discharges from the dams; and
  • A flow-regulation structure on the south channel of the Lee to reduce flow in its south channel during extreme rainfall events.

Amid vocal opposition over the early proposals, the OPW amended its design proposals in several areas, to include the introduction of demountable barriers along historically sensitive areas like the North Mall, and the blending of flood defences into public realm improvements in other areas like the Lee Fields.

Save Cork City has already commissioned three major reports on the OPW’s approach and figures.

They include a report from HR Wallingford in 2017, which suggested a tidal barrier could be built in the lower harbour for €140m. The OPW says the actual cost could be as high as €1bn.

A report by flooding experts at Delft University (TU Delft) in the Netherlands suggested that the OPW has overestimated the cost of building a tidal barrier by at least €800m and that such an approach should not be discounted on cost grounds.

The third report, by MMC Quantity Surveyors, suggested that the actual cost of the proposed walls and embankments could be €76m higher that OPW estimates.

This latest report will raise fresh questions about the OPW’s fundamental approach to tackling flood risk in Cork.

Mr Beese said the OPW and its consultants have failed to adequately consider the city’s complex ground and groundwater conditions, and the impact the flood defence scheme would have on the groundwater levels in these underground gravels.

The city is built over an aquifer filled with glaciofluvial gravels and water lying some 140m under the full width of the river valley, and over a network of buried historic channels that flow under its streets.

The water levels in the aquifer rise and fall daily with the tide. Water flows between the aquifer and the buried channels. They are connected by water.

This complex interaction, the professor argues, was not fully assessed, nor has the impact the proposed walls, parapets and pumping stations would have on these groundwater levels.

He warns that the risk of flooding from these underground sources would remain “unacceptably high” even after flood-wall defences are installed.

And he said the OPW scheme could actually impede underground water flows, deplete the water table levels which could pose a “high risk of settlements and consequent structural damage” to city centre buildings, and could also result in a reduction in the groundwater quality in the gravel, which has been classed as a regionally important aquifer.

Cork flood defence: Campaign against ‘experiment that is likely to fail’ continues

“In summary, installing cut-off walls and a groundwater dewatering system at Cork is an experiment in separating two interdependent surface water systems which is likely to fail,” said Mr Beese.

His report also raises concerns about a groundwater report prepared for the OPW, describing its findings as “incomplete” and suggesting that the modelling used in the report is based on an unsuitable software package.

He makes a number of recommendations, including that the OPW undertake a more detailed assessment of the geological and hydrogeological impacts, issues and risks across the full width of the River Lee floodplain before making any decision to proceed with the current flood defence plan.

Crucially, he said the OPW should assess the scale of deep basements in the city, and examine how they inhibit groundwater flows.

“In conclusion, based on this overview of the ground and groundwater conditions at the city of Cork, the proposed ‘walls’ scheme, which is designed to control the complexly interactive flood sources that are present in the River Lee valley, is not viable.

“It is to be expected that the residual risks associated with any implementation of the scheme would result in excessive costs and long-term disruption, especially given the urban context,” he said.

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