The impact of tidal lagoons on future flood risk on the North Wirral and Conwy coastline, UK

Lyddon, Chartlotte; Plater, A.J.; Brown, J.M. ORCID: https://orcid.org/0000-0002-3894-4651; Prime, T.; Wolf, J. ORCID: https://orcid.org/0000-0003-4129-8221. 2015 The impact of tidal lagoons on future flood risk on the North Wirral and Conwy coastline, UK. Southampton, National Oceanography Centre, 85pp. (National Oceanography Centre Internal Document, No. 16)

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Abstract/Summary

This report considers the viability of tidal lagoons in the North Wirral and Conwy coastlines, to mitigate future flood risk and reduce the cost of damage in these areas. The report is aims to provide information on the feasibility and benefits of tidal lagoons as mitigation and adaptation strategies to future sea-level rise, as part of the RISES-AM project. Sea-level has been rising since instrumental records began in the 1700s, and has been rising at a rate of 3.0 ± 0.7 mm / yr-1 since 1990 (Hay et al., 2015). Low probability, plausible high-end sea-level rise scenarios, where global average warming exceeds 2oC in respect to the pre-industrial level, estimate up to 0.98 m sea-level rise (SLR) by 2100 (Church et al., 2013). There is a move away from hard defences in favour of strategies which can mitigate flood risk benefit and allow coastal communities to adapt to and benefit from high-end SLR scenarios (Linham and Nicholls, 2010). Tidal lagoons could be one such innovative option. The report aims to assess the impact of the construction of tidal lagoons on flood risk on the North Wirral and Conwy coastline, under future high-end sea-level rise scenarios. Computer simulations of extreme flood events, using a 2D hydrodynamic model called LISFLOOD, will estimate changes in the extent and depth of flooding following the construction of a lagoon under both present day and future extreme climate conditions. The results of LISFLOOD suggest that: • Colwyn Bay and the North Wirral coastline are not areas at increased flood risk under baseline future high-end SLR, due to steep topography and existing defences. • Infrastructure at Stanlow oil refinery and Connah’s Quay in the North Wirral domain and residential areas in the Colwyn Bay domain at Llandudno, Rhyl and Prestatyn experience increased flood risk under RCP 4.5 (0.72 m SLR) and RCP 8.5 (0.98 m SLR) with no tidal lagoon. This is due to low-lying topography. • The presence of a tidal lagoon on the North Wirral provides flood risk benefit to infrastructure at Stanlow and Connah’s Quay as the magnitude of tidal currents is limited through the Dee and Mersey Estuary. However the size of the lagoon and the bathymetry of Liverpool Bay may mean the lagoon in this study may not be financially feasible. • The construction of a tidal lagoon at Colwyn Bay increases extent and depth of inundation at Llandudno, Rhyl and Prestatyn under all sea-level rise scenarios. Increased flood risk in these areas following the construction of a tidal lagoon is reason enough not to build a lagoon in this location. Tidal lagoons have the potential to offer flood risk benefit and become part of integrated strategies to minimise flood risk in coastal areas. The benefits of tidal lagoons are dependent on their shape, size and location, and feasibility studies should consider impacts in the near- and far-field.