Proposed Cork Tidal Barrier and Flood Control Works for Cork City

Alistair G. L. Borthwick

I am Professor of Applied Hydrodynamics at The University of Edinburgh, an Emeritus Fellow at St Edmund Hall, Oxford, and hold Adjunct Professorships at National University Galway, University College Cork, Peking University, and Shanghai Jiao Tong University. I was previously Professor of Engineering Science at the University of Oxford, where he worked for 21 years from 1990-2011. I was Head of Civil & Environmental Engineering at University College Cork from 2011-13, where I was the Founding Director of the SFI Centre for Marine and Renewable Energy Ireland. My research interests include environmental fluid mechanics, river basin management, river flooding, coastal and offshore processes, and marine renewable energy. My methods for modelling free surface flows have been applied worldwide to floods (China, Mexico), water quality (Brazil, Morocco), wave-induced currents (Spain, Mexico), lake mixing (Hungary), fish passes (Japan), sea defences (UK), and offshore structures on the continental shelf (UK, USA). For more than 20 years, I have collaborated with colleagues in Hungary on rivers and lakes, and in Peking University on water resources and large rivers. In 2016, I was awarded Dr honoris causa by Budapest Műegyetem for contributions to Civil Engineering. I am a Fellow of the Royal Academy of Engineering, and a Fellow of the Royal Society of Edinburgh. Cork City is built on land through which the River Lee flows. The city is of vital socio- economic importance to Ireland, and has proved historically vulnerable to flood events. Upstream of Cork are two hydropower dams at Inniscarra and Carrigadrohid. The associated reservoirs have large capacity exceeding 45 million m3 (according to the Lee Catchment Flood Risk Assessment Study, 2010 1), and so should be capable of preventing flood flows after heavy rainfall events, exceeding the 1 in 100 year return period (noting that this is a moveable figure given climate change). The River Lee is tidal, and so an understanding of highest tide levels and inflow velocities from the sea and the interaction between tide and river flow components is of the utmost importance when making an assessment as to whether there is a need for additional flood protection works. 1 http://www.lee.cfram.com/reservoirs.html

In my opinion, the correct way forward would be to employ experts, such as Prof. Michael Hartnett of NUI Galway and perhaps Emeritus Prof. Roger Falconer of Cardiff University, to carry out a detailed numerical modelling study of the River Lee, with upstream inputs adjusted to account for future projections of extreme runoff events, with the dams modelled, and with the open sea boundary adjusted for sea level rise in accordance with IPCC projections. The effect of sea level rise on the sea boundary should also be modelled, and the highest water level determined using a suitable probabilistic method, such as that devised by Sobey (2005) for dealing with data series of monthly maximum and minimum sea levels. See Appendix A, extracted from Borthwick (2009). Using a shallow flow computational model, it would then be possible to add in a tidal barrier, and then carry out a detailed feasibility study as to whether a tidal barrier, water level control at Lough Mahon, and dam operation control measures might be a better overall solution to flood risk in Cork than merely raising the crest elevation of flood walls along the Lee. The City planners and engineers should of course carry out a proper sustainability analysis of any proposed scheme, noting the societal, economic, institutional, and environmental benefits and drawbacks of the various options.

Please note that this opinion has been given freely as a private citizen, with strong family ties to Cork, and not as a representative of Edinburgh University.

Yours faithfully, Alistair Borthwick, FREng, FRSE Appendix A

Sobey (2005) provides the following methodology for dealing with data series of monthly maximum and minimum sea levels. First, a least-squares procedure is used to subtract the mean sea level trend and major (19-year period) astronomical tidal forcing from the data series. The remaining surge data are shifted according to a datum corresponding to the mean higher high water or mean lower low water level depending on whether maximum or minimum levels are under consideration. The shifted data are then fitted to an appropriate extreme value distribution (Extreme Value II or III if maxima, or log- normal if minima) by maximizing the log likelihood function, and checking the best fit against 95% confidence intervals. It is then possible to estimate the magnitude of the storm surge sea level event for a given return period, Tr years. The datum shift is then reversed, and finally the extreme sea level estimated by adding back the mean sea level trend, and astronomical forcing, such that

where z(Tr) is the extreme sea level associated with a surge of return period Tr years, m is the rate of mean sea level rise, t is time, and a, Ω, and f are the amplitude, wave number, and phase of the astronomical tidal forcing. Where the tidal forcing is less simple, then the final term in equation (1) can be obtained using a shallow water numerical model.

A further problem is that tides and surges interact with each other. A solution is to use a joint probability method to evaluate the combination of surge and tide levels. Heffernan and Tawn (2004) propose a conditional method for multivariate extremes that lends itself to a much improved approach for assessing risk related to sea levels and wave heights. Coles and Tawn (2005a, b) have developed a Bayesian framework aimed at spatial and seasonal analyses of extreme sea levels.

References

Borthwick, A.G.L. (2009) Coastal risk: advance or retreat. In New Perspectives on Risk Analysis and Crisis Response, Eds. C. Huang, J.B. Weiner, and J. Ni, Atlantis Press, Amsterdam, 15-26.

Coles S. and Tawn J. (2005a) Bayesian modelling of extreme surges on the UK east coast,

Phil. Trans. Roy. Soc. A, 363: 1387-1406.

Coles S. and Tawn J. (2005b) Seasonal effects of extreme surges,” Stoch. Environ. Res. Risk. Assess., 19: 417-427.

Heffernan J.A. and Tawn J. (2004) A conditional approach for multivariate extreme values, Journal of Royal Statistical Society Series B, 66: 497-546.

Sobey R. (2005) Extreme low and high water levels, Coastal Engineering, 52: 63-77.