Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013-Bruges , Belgium 169 Variation of flow separation over large bedforms during a tidal cycle

This study characterizes the shape of the flow sepa ration zone over natural compound bedforms during a tidal cycle and investigates how the flow separation zone depends on changing flow conditions, water levels and bathymetry. Field data collected during a full tida l cycle over large ebb-oriented bedforms provides h igh-resolution bathymetry and velocity measurements that are used to simulate the hydrodynamics structure during a ti d l cycle using the Delft3D numerical model. During the ebb, a large flow separation zone occurs on the steep lee side (14 to 23°) of each bedform. During the flood, no flow separation developed over the gentle slope of the flood lee side (3 to 5° on average). However, a small flow separation zone is often recognized near the crest, where the slope is locally up to 15°. T he shape of the FSZ is not influenced by changes in current velocit ies or water levels. On the contrary, it is largely influenced by changes in the bed morphology. In particular, varia tions in the shape of the crest during the tidal cy c e results in variations of the length of the FSZ.