Sediment-induced buoyancy destruction and drag reduction in estuaries

Abstract

This paper presents an analysis of drag reduction by buoyancy destruction in sediment-laden open channel flow. We start from the log-linear profile proposed by Barenblatt (Prikladnaja Matematika i Mekhanika, 17:261-274, 1953), extended with a second length scale to account for free surface effects. Upon analytical integration over the water depth, an expression for sediment-induced drag reduction is found in terms of an effective Chézy number, water depth, bulk Richardson number, and Rouse number. This relation contains one empirical/experimental coefficient, which was obtained from a large series of numerical experiments with a 1DV point model. Upon calibration of this model against field and laboratory observations, we tuned the turbulent Prandtl-Schmidt number and found an optimal value of σ T=2, consistent to observations by Cellino and Graf (ASCE, J Hydraulic Engineering, 125:456-462, 1999). All numerical results could be correlated with the simple relation Ceff = C0 + 4\ g hRi2\beta, which is valid for fine sediment suspensions under conditions typical in open channel flow.