MODELLING DUNE EROSION OF THE SEFTON COAST, LIVERPOOL BAY, UK

Abstract

Dune erosion at sandy coastal systems poses significant threat to the hinterland areas due to flooding and subsequent damage to life and property. The Sefton Coast (Fig.1), bounded by Mersey (South) and Ribble (North) estuaries in the Liverpool Bay, represents about 20% of the UK's dune system, has a high recreational value, form an effective coastal flood defense and consists of a significant number of conservation areas of national and international interest. This area is however susceptible to erosion during storm events where landward retreat occurs leading to dune breach and coastal flooding. Present study focused on an area around the Formby Point of the Sefton Coast (dashed-line polygon in Fig.1), which is a highly dynamic sedimentary system. The dune system at Formby Point beach showed continuous accretion (max ~4.7 m/year) during the last century but has become an eroding beach since early twentieth century (max. ~6.2 m/year). However, beach accretion can still be seen in far south and north of Fomby Point. The Sefton Coast experiences a semi-diurnal macrotidal regime with a mean spring tidal range reaching 8.22 m (Esteves et al., 2012). Mean annual significant wave height (Hs) is about 0.53 m while extremes exceed 5 m (Brown et al., 2010). The aim of this study is to investigate the response of the Formby Point dune system to extreme storm conditions based on the March 2010 storm event (max. Hs>3.5 m, see Brown, 2010). We use the 2DH process-based morphological model XBeach, which has been developed to investigate dune erosion under extreme conditions (Roelvink et al., 2009), in combination with the SWAN spectral wave model (Booij et al., 1999). The model set-up consists of a coarse-grid model (Sefton) and a fine-grid model (Formby Point) which encloses the area of interest. The Sefton model is driven by the Liverpool Bay WaveNet buoy wave data (Esteves et al., 2012). The outputs of the Sefton model provide wave and water level boundary conditions to the Formby Point morphodynamic model to investigate the beach response to the selected storm event. The model setup described above is then used to simulate nearshore hydrodynamics and morphodynamics. Fig. 1shows a snapshot of simulated Hs corresponding to the storm event occurred in the Liverpool Bay during 30 March and 02 April 2010, at the mean tidal level. Offshore waves are significantly attenuated when travelled from offshore to nearshore. Fig. 2 shows the model simulated exposure of the dune system and the beach during Low Water (LW) (a) and High Water (HW) (b). At LW, the beach exposes up to about 3 km while at HW, the entire beach drowns, exposing the dune system to incoming waves. Therefore, a storm event combined with HW may lead to damage the dune system and hence flooding of the hinterland of the River Alte area, due to the low dune height in this area. Relatively high morphological changes are observed around the Crosby channel area (see location in Fig. 1). The storm driven dune erosion occurred during the HW. Large storm waves propagate along the Crosby channel towards the River Alte area. The River Alte area is susceptible to coastal flooding during storms, as a result of the low dune crest in this area. Work is currently in progress to predict the dune evolution and flooding under the future climate change and sea level rise scenarios.