Definition of storm thresholds for significant morphological change of the sandy beaches along the Belgian coastline

In this paper a storm threshold for significant morphological change along the Belgian coastline is presented. Using a dataset covering topography, bathymetry and hydrodynamic conditions for the period 1983–2007, the main erosive years on the one hand, and the most severe storm events on the other hand have been selected. The topographic data were processed into erosion and accretion trends (corrected for nourished volumes) for both the supratidal beach and the dune. In total 42 storms were recognised and for each of these storms, the maximum water level, the storm duration, the wave energy and the main wind direction have been determined. To link the eroded volumes to the storm events, the total erosion of all the affected areas per year (i.e. winter season) has been compared with the total wave energy of all the storm events that occurred during the same period. Based on the five most erosive winter seasons (1984, 1987, 1990, 1994 and 1995), it is concluded that in order to cause a significant morphological impact along major parts of the Belgian coastline, as a first indication an individual storm should be characterised by the following criteria: maximum significant offshore wave height higher than 4 m, maximum water level above + 5 m TAW, storm duration longer than 12 h, an induced wave energy above 6.5 E + 05 J/m² and wind direction between W and NW. Although the applied methodology has its limitations due to the wide time-spacing of the topographic measurements and the lack of details on beach morphological changes, the severest storm periods from the last 25 years clearly left significant marks in the time series of recorded volume evolution. Therefore the proposed thresholds provide a valuable first estimate of the threshold for significant morphological change along the entire Belgian coastline. If in the future pre- and post storm related beach morphology measurements would be available, the above defined thresholds could be refined or even thresholds for different sectors along the Belgian coastline could be established taken into account the beach morphology.

► Definition of threshold for significant morphological change at the Flemish coast.
► Long-term (1983–2007) topographic and hydrodynamic datasets are used.
► Comparison of the total eroded volume each winter and the total wave energy.
► Maximum water level, wave height, storm duration and direction are determining.