Bed shear stress estimation for gravity currents performed in laboratory

Gravity currents are caused by density differences between two fluids which may be due to temperature, dissolved substances or the presence of particles in suspension. In this study saline currents, in which the higher density is produced by dissolved salt, are reproduced in laboratory with the aim to characterize the bed shear stress. Saline currents can in fact be responsible for high erosion rates and the bed shear stress is a quantification of this erosive capacity. The dynamics of buoyancy driven flows are complex and the effect of the initial density gravity current on the bed shear stress is not explored yet. The results herein showed confirm the importance of detailed velocity profile measurements for the determination of the friction velocity which is a key parameter for the currents propagation and for characterizing the momentum and mass exchanges between the current and the bed. The spatial evolution of the bed shear stress caused by the passage of a gravity current is here estimated using the logarithmic velocity profile method for, as a first attempt, a value of the von Kármán constant of k 0.405. The use of this constant is then verified and discussed

An experimental and numerical study on the spatial and temporal evolution of a scour hole downstream of a rigid bed

The present study consists of a new mathematical-experimental investigation on local scour downstream of a rigid bed. New scouring experiments, under both steady and unsteady flow conditions, were carried out and the scour patterns were measured with a 3D Laser Scanner. The initial bed shear stress induced by the turbulent flow over the sand bed was estimated by analysing velocity profiles. A mathematical model simulating the local scour downstream of a rigid bed and using both information relating to the turbulent flow and the physical and mechanical properties of the sand was developed. The mathematical structure of the model includes a second order partial differential parabolic equation with the shape of the mobile bed as unknown parameter. The sediment transport was computed by the Einstein approach. The space-time evolution of the simulated scour holes are in good agreement with the measured ones for both steady and unsteady flow tests. © 2014 Taylor & Francis Group, London