Effect of storm clustering on beach/dune erosion

Pushpa Dissanayake, Harshinie Karunarathna

Thursday 2 july 2015

16:00 - 16:15h at North America (level 0)

Themes: (T) Extreme events, natural variability and climate change, (ST) Learning from disasters

Parallel session: 13I. Extreme events - Lessons Disaster

A cluster of small storms can impose more devastating effects on beach/dune systems compared to a single large event. An exceptional series of storms, which occurred at close intervals, was experienced in UK during the last winter (2013/2014). No individual storm event occurred during this period can be regarded as exceptionally severe. The maximum storm wave height during December 2013 – January 2014 period was only 4.6 m and had a return period of about 1.5 years. However, there were seven storms occurred at very close intervals during this period and that is very unique. The present study investigates the morphological changes of a beach/dune system under the impact of these clustered storms. The study area is the Sefton coast, which is about 36 km long, bounded by Mersey (South) and Ribble (North) estuaries in the Liverpool Bay, UK. This study used the XBeach coastal morphological model to investigate the response of the Sefton coast beach/dune system to the clustered storms through modelling cross-shore profile change (1D). The cumulative impact of the each storm in the cluster as opposed to individual storm events was analysed using model outputs. If the first storm in a cluster is strong enough to interact with the nearshore ridge-runnel system along the Sefton coast where significant bar erosion can take place. Then, the beach will be exposed to subsequent, less severe storms thus leading to significant erosion. The largest beach profile change, and therefore the strongest clustering effect was found in the central part of the Sefton coast where exposure to incident waves is maximum. Storm clustering impacts however depend on the local morphological characteristics along the Sefton coast. The results found in this study will be very useful for the future management of this highly dynamic beach/dune system.