Monitoring and modeling nearshore morphodynamic behaviour on storm time scales

Abstract

Coastal monitoring: Bathymetric surveys of an area of 1 square kilometer of the shoreface near Ter Heijde have been perfomed using the NEMO Jetski. Instruments installed are a Single Beam Echo Sounder (SBES) for depth measurements and accurate GPS for positioning (RTK-GPS). The quality of the Jetski as a platform for bathymetric surveys has been assessed. The vertical (depth) accuracy is of order 0.1m; the horizontal position accuracy is of order 0.5m. The local morphology shows a shoreparallel subtidal bar, situated 500m offshore and around –4m NAP. The position of the bar is monitored within weeks before and after storm events between September and December 2008. The bar has migrated approximately 10m onshore during a calm weather period. In a stormy month the bar migrated 25m - 30m offshore. Coastal modeling: The numerical area model Delft3D is set up to reproduce the morphological changes as monitored in the field. Wave conditions from the offshore Europlatform buoy were used. Model results show offshore bar migration during storm as well as some flattening of the bar crest. Other results from a simulated storm show that where subtidal bar is higher, the longshore and cross-shore flow velocity are also higher and more sediment is transported in cross-shore direction. Synthesis: Theory and observations described in literature show agreement with the field observations at Ter Heijde. They show onshore bar migration during fair weather conditions, whereas the bar moves offshore during storm conditions. The main onshore driving component is wave asymmetry whereas the main offshore driving component is undertow. Also the model results look very similar to the resulting patterns from the field measurements. One dissimilarity is that the bar crest in the model flattens more during storm than it should be considering the field observations.