Experimental study of the early stages of a dam-break flow over fixed and mobile beds

The dam-break flow is an extreme example of a severe transient flow, often associate with catastrophic flood events. In a real case it is a very complex flow, involving not only many different time and space scales but also a complex fluid-bed interaction that will affect the propagating wave celerity. The need for studies on this kind of flow arises mainly from the risk assessment needed for existing, or to-be build, dams. The definition of the flooded areas and their evolution with time constitutes a valuable information for the authorities' evacuation plans conception. Numerical models are often used to obtain the required information about the dam-break flow propagation. However for an improved modelling it is important to know the physics of the flow. This can be acquired by means of small scale models in the laboratory, where the effect of the different variables can be isolated and analysed. The information thus obtained can be used to gain a better comprehension of the dam-break flow, but also to obtain an experimental results data base that can be used to calibrate and validate existing numerical and analytical models. The present study is devoted to the experimental analysis of the dam-break flow, with special emphasis on the initial stages of the dam-break flow. Rather than focus only in the macro-scale variables, i.e., water level evolution and wave celerity, the present study deals with the point-wise velocity measurements within the dam-break flow. For velocity measurements different techniques are used. Two of them based on imaging techniques (PIV and PTV), and a third one based on the acoustic Doppler Effect (UVP). These techniques allowed for a space and time description of the dam-break flow velocity field. The measurements were carried considering different flow heights and bed types. The dam-break flow over beds of sediments is here analysed and the velocity field of both water layer and sediment layer measured simultaneously. A two-layer numerical model is than used and tested against the obtained experimental results. The transient character of the dam-break flow motivated a statistical analysis of the dam-break flow. It consisted in a) repeating several times, for the same initial conditions, the dam-break experiments; b) obtaining images of the flow with an high acquisition rate (1000 Hz). An ensemble average procedure is presented and the results discussed. For fixed smooth beds, the obtained spatial and time resolved velocity profiles made it possible, under some conditions, to determine the shear stress in the bed. %This was made by identifying in some conditions the appearance of a log-layer. Finally the different measurement techniques are compared and discussed.(FSA 3) -- UCL, 201