This study characterizes the shape of the flow separation 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 tidal cycle over large ebb-oriented bedforms provides high-resolution bathymetry and velocity measurements that are used to simulate the hydrodynamics structure during a tidal 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°. The shape of the FSZ is not influenced by changes in current velocities or water levels. On the contrary, it is largely influenced by changes in the bed morphology. In particular, variations in the shape of the crest during the tidal cycle results in variations of the length of the FSZ.