Quantifying thresholds for significant dune erosion along the Sefton Coast, Northwest England

Field and model hindcast data are used to establish a critical dune erosion threshold for the Sefton Coast (NW England). Events are classified as causing significant erosion if they result in: (a) a mean dune retreat along the entire study area of > 2 m; (b) a dune retreat of ≥ 5 m along a coastal segment ≥ 2 km in length; and (c) an eroded area ≥ 20,000 m2. For the period 1996 to 2008, individual storms were characterised using hindcast results from a POLCOMS-WAM model and measured data from the Liverpool Bay Coastal Observatory. Results show that combined extreme surge levels (> 1.5 m) and wave heights (> 4 m), or tidal water levels above 9.0 m Chart Datum (CD), do not always result in significant dune erosion. Evidence suggests that erosion is more likely to occur when wave heights are > 2.6 m, peak water level is > 10.2 m CD at Liverpool and when consecutive tidal cycles provide 10 h or more of water levels above 9.4 m CD. However, lower water levels and wave heights, and shorter events of sustained water levels, can cause significant erosion in the summer. While the return period for events giving rise to the most severe erosion in the winter is > 50 years, significant erosion in the summer can be caused by events with return periods < 1 year. It is suggested that this may be attributable to a known reduction in the mean dune toe elevation c. 30 cm. Although the study shows it might be possible to characterise objectively storm events based on oceanographic conditions, the resultant morphological change at the coast is demonstrated to depend on the time and duration of events, and on other variables which are not so easy to quantify. Further investigation is needed to understand the influence of alongshore and seasonal variability in beach/dune morphology in determining the response to the hydrodynamic and meteorological conditions causing significant erosion. Improved monitoring pre- and post-storm of changes in beach/dune morphology is required to develop reliable proxies that can be used to establish early warning systems to mitigate the impacts of erosion and flooding in the future.