Dam-break over mobile bed : determination of bed shear stress based on velocity profile measurements

Dam breaks are disasters that involve the release of large volumes of water and sediment downstream. Studying these phenomena is important to better predict the risks, the flood risk areas as well as the topographical consequences.The final objective of this Master’s thesis is to try to better characterize the shear stress of water on sediments, based on velocity profiles, which is linked to all these events. A physical representation of this phenomenon is essential to understand these interactions between water and sediments. Thanks to the hydraulic laboratory (in the LEMSC) at our disposal, we were able to reproduce, on a reduced scale, a dam break on a moving bed made of coarse sand. These experiments were recorded with a high-speed camera enabling the detection of the different interfaces like the free surface, the interface between clear water and upper limit of the mobile bed, and the interface between the mobile bed and the fixed bed. Once all these images were recorded, we used a non-intrusive technique called "Particle Image Velocimetry" (PIV) thanks to the software DaVis8 (LaVision) in order to determine the velocity profiles of the flow for each time and at each position of the flume. Two general methods were then used to calculate the shear stress, and then compared: the Clauser method, using the vertical profile of the horizontal velocity u(z), and the Slope method, using the longitudinal profile of the average horizontal velocity U(x). The results of the Slope method were also compared with a numerical simulation free of experimental errors. Following our experimental observations, we determined that a decisive part of the velocity profile in the portion which develops inside the moving sediment layer has a logarithmic shape and that these different methods leads to relevant results that are of the same order of magnitude. Other prospects and ideas for future research are also proposed.