A race-track recirculating flume for cohesive sediment research

We investigate a procedure for carrying out experiments on the erosion of a deposited mud bed by a steady flow in a race-track recirculating flume. This is a large-scale flume 0.5 m wide consisting of two straight sections 16 m long connected by two circular bends. The currents are produced by a vertical disc pump. It is shown that experimental conditions producing uniform erosion over the whole surface of the mud bed are met when the bed is restricted in a limited straight part of the flume. Measurements of the flow kinematics by a Laser Doppler Anemometer performed in clear water confirm that the flow applies a quasi-uniform bottom shear stress over the bed. Stratification effects are suppressed in this facility due to the strong mixing of suspended sediment concentration by the disc pump. A theoretical analysis of stratification effects however indicates that they remain small as long as the depth-averaged concentration is below 13.5 g l(-1). A procedure to determine quasi-instantaneous erosion rates from depth-averaged suspended sediment concentration is finally presented.