Effect of saltating sediment load on the determination of the mean velocity of overland flow

Where overland flow velocity is measured using dye or salt tracing, the mean velocity is often determined by multiplying the velocity of the leading edge of the tracer plume by a correction factor alpha. Flume experiments show that alpha varies with both Reynolds number and sediment load. For sediment-free flow over a sand-covered bed, alpha is less than the theoretical value of 0.67 in laminar flow and increases rapidly with Reynolds number in transitional flow and more slowly with Reynolds number in turbulent flow. For sediment-laden flow, alpha decreases as sediment load increases in transitional and turbulent flows. Saltating sediment extracts momentum from the flow, causing velocities near the bed to decrease and alpha to decrease. A multiple-regression equation is developed which can be used to predict the value of alpha from Reynolds number and sediment load in transitional and turbulent overland flows.