3D Modelling of nearshore morphodynamics in a topographically challenging environment
Though the models of sediment transport and morphodynamics are reaching a mature stage, applications in different environments still show limitations and complications, which are of interest for the developers as well as for the coastal and ocean modelling community. Two 3D hydrodynamic models, ELCOM (Estuary, Lake and Coastal Ocean Model: ) and Delft3D (), were first employed to simulate barotropic- and baroclinic-hydrodynamics in Lake Constance . Main river discharges with temperatures were imposed as point-sources. The model setup had initially a stratified lake with zero current velocities, and was forced by the heterogeneous wind field COSMO (Consortium for Small Scale Modelling, ) from National Swiss Weather Service. Heat exchange at the free surface was estimated using the meteorological data from German Weather Service. Model predicted temperatures and currents were compared with the field data (thermistors, ADCP and drifters). The calibrated model of Delft3D was then used to investigate morphodynamics. Simulating nearshore morphodynamics was challenging  due to the steep bed gradient (>1:10) of the lake shore compared to the coastal environments (e.g. German North Sea coast, Sefton coast < 1:100). Therefore, different model setups were tested to transform offshore hydrodynamics into the high resolution nearshore model grid. Forcing with a measured storm event (Hs,max ~ 0.5 m), nearshore morphodynamics of Kressbronn Bay , was finally simulated (Fig. 1).
Fig. 1: Location of Kressbronn Bay in Lake Constance, and simulated erosion (m, negative values) and sedimentation (m, positive values) areas along the nearshore
Predicted stratification of both models reasonably agreed with the measured temperatures. Currents at deep layers always had a better agreement with the data than currents at surface layers in both models. Results further indicated that the wave propagation into the bay is independent from the direction of wind approach. The eastern shore area experienced about 0.1 m erosion and sedimentation within the simulated storm period (Fig. 1). This study concludes, both ELCOM and Delft3D have almost similar model skill in generating baroclinic dynamics in Lake Constance. In contrast to the coastal/ocean environments, the model-nesting is a challenging approach to transform offshore hydrodynamics up to the nearshore areas of lakes. This study finally established that a numerical model can be used to investigate the neasrhore morphodynamics of lakes, which is quite novel for the morphodynamic modelling.
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