There is a strong tidal energy potential in France, especially in the Alderney Race. In the area of study, where turbines will be installed, bathymetry variations are causing velocity fluctuations with a high turbulence rate in the water column: large coherent turbulent structures can be observed at the sea surface. Such events can have a major impact on the marine tidal turbines behaviour and structural fatigue. To reproduce and analyse these turbulent events, tests are carried out in the wave and current circulating flume tank of IFREMER in Boulogne-sur-Mer. Before trying to reproduce a complex bathymetry, we chose to introduce the topic by studying elementary obstacles representative of real seabed elements (with an aspect ratio of the magnitude of the mean bathymetry variations): a wide square cylinder and an inclined floor. Experiments are carried out with Reynolds number as high as achievable in Froude similitude: Re = 2:5 _ 105 and Fr = 0:23. The impact of the aspect ratio is studied by comparing results obtained with PIV and LDV measurements on the cube and cylinder cases. The addition of an inclined floor is also investigated. Results show a significant increase of the wake with the aspect ratio. The inclined floor induces a reduction of the shear layer created by the obstacle and modifications on the shedding frequency.