Statement: storm surge barriers and application potential to Cork

Updated: Feb 9

Date 12 November 2018

Faculty of Civil Engineering and Geosciences

Department of Hydraulic Engineering

Stevinweg 1

2826 CN Delft

Postal address

P.O. Box 5048

The Netherlands

As a response to major floods in 1953, the Dutch have constructed a system of dams and barriers to protect coastal areas against flooding. This Delta Works system includes well known storm surge barriers with large movable gates such as the Eastern Scheldt and Maeslant barriers. Over the past decades these barriers have operated successfully. In other parts of the world, e.g. in New Orleans, Venice and London, barriers have been built after major storm surge events as well. At this moment barriers are in the screening, planning or design phase in many other regions (e.g. Shanghai, Nieuwpoort in Belgium, New York, Texas) for the purposes of coastal protection and adaptation to sea level rise.

A storm surge barrier (also called a tidal barrier in the studies for Cork) is a fully or partly movable barrier that can be closed to temporarily close off bays and estuaries during storm surges to provide coastal flood protection. It can also be closed to exert a level of downstream control to help alleviate fluvial (river) flooding. Storm surge barriers can significantly reduce the length of coastline that is directly exposed to coastal flooding and thereby reduce (or prevent) raising of embankments behind the barrier. This is particularly beneficial in densely populated areas i.e. in the case of well-established and historic cities. A barrier solution can therefore facilitate additional benefits for waterfront development.

Aspects that require attention are the management and maintenance of barriers, the effect on tidal flows, tidal ecosystems and navigation. Also, construction costs of barriers are generally fairly high and maintenance operations and budgets are required. Based on data from 15 existing barriers a rule of thumb for cost estimation has been developed. We found that the average unit costs of existing barriers are around 2.3 million Euros per meter width of the gated area, considering barriers with large and complex gates and 2013 price levels. This rule of thumb gives an indicative value, and it is noted that unit costs for actual barriers show considerable variation. The presented unit cost could be used in the screening and early planning stages, i.e. mainly for prefeasibility cost estimates.

Review of cost estimate for the Little Island barrier, Cork

A team of my university has performed a review of previous cost estimates of the Little Island Tidal Barrier (OPW, 2017). For further details reference is made to the original review document (Molenaar and Jonkman, 2018). Some main findings are summarized below.

The Office of Public Works (2017) has provided a capital cost estimate of 340 M€ in their 2017 report for the Little Island Tidal Barrier. A review of this cost estimates reveals that the above mentioned unit rate for complex tidal sector gates has also been applied to the simpler and smaller tidal sluice gates. This is considered to be less appropriate, as the proposed gates for discharge openings in the Little Island barrier are less complex structures than used in existing barriers. The OPW cost estimate seems also conservative with respect to the cost estimate of the connecting embankments that are part of the barrier system (see review report for details).

An alternative cost estimate has been provided in our review (Molenaar and Jonkman, 2018). A first order capital cost estimate for the Little Island Tidal Barrier based on the unit costs from existing barriers (Mooyaart and Jonkman, 2017) would result in an approximation cost below 200 M€. Using the unit costs for sector and lift gates, basically derived from subsets, the resulting cost of the Little Island Tidal Barrier would be about 186 M€ (60 m at 2.2 M€/m and 90 m at 0.6 M€/m for navigation and sluice gates respectively).

Save the City of Cork (SCC), as an alternative to the OPW scheme, has proposed to construct the tidal or storm surge barrier right away, mainly to avoid the construction of all the walls in Cork’s city centre. The SCC proposed barrier would be crossing Lough Mahon from Little Island to Horse Head, see Appendix 1 for a short description; it shall be referred to as Little Island Tidal barrier from hereon. On behalf of SCC, HR Wallingford has looked into the (pre-feasibility) design of the barrier and prepared a cost estimate for it.

However, it is also noted that the there is a large variation in unit costs between projects, resulting in a considerable uncertainty in the estimate. Project construction prices are heavily dependent on factors such as market circumstances, prices of raw materials and labour, the adopted design and the considered scope of the project (e.g. the inclusion of mitigation measures within the bay etc.). Therefore, the development of a more detailed, and project specific cost estimate for the Little Island Tidal Barrier is highly recommended. Further development of the present pre-feasibility design would be required.

Overall, given the flood risk issues in Cork and the geography of the area, I consider the proposed tidal barrier at Little Island as an interesting and attractive option which certainly warrants further investigation. The Little Island Tidal Barrier option should not be discounted on the grounds of the preliminary and uncertain cost estimate only. A broader and more rigorous assessment of costs, risk reduction benefits and other effects (e.g. social, heritage, environmental and economic impacts) of various strategic alternatives is recommended to provide a basis for decision making.

Prof. dr. ir. S.N. (Bas) Jonkman

Professor of hydraulic engineering

Delft University of Technology, Delft, the Netherlands

References / background

Mooyaart L.F., Jonkman S.N. (2017) Overview and design considerations of storm surge.

ASCE Journal of Waterway, Port, Coastal, and Ocean Engineering Vol. 143 Issue 4

Molenaar W.F., Jonkman S.N. (2018) Review of the OPW cost estimate Little Island Tidal

Barrier Cork. TU Delft report April 10, 2018.

OPW - Office of Public Works, Lower Lee (Cork City) Flood Relief Scheme (2017).

Supplementary Report – Option of Tidal Barrier, Job no. 230436-00 by Arup, 5

December 2017.