Gravity currents moving over a sediment bed: suspension criteria and bed forms

Abstract In many natural situations, gravity currents flow over a mobile sediment bed as in riverine inflows into lakes and reservoirs or in submarine canyons in coastal regions. We present experimental results which clarify the near-bed physical processes of sediment suspension, bedform development, as well as the effect of the erodible bed on the mean flow structure of the current. Compared with the flow over a fixed bed, the vertical velocity is negative above the mobile bed, indicating therefore slip with an increase in horizontal flux close to the sediment bed. The sediment suspension criteria and suspension capacity established, allow prediction of the spatial change in sediment flux up to maximum suspension capacity, in good agreement with experimental results of [1] as well as with the present sediment flux measured toward the end of the sediment bed. Effects of local bursts, here intermittently caused by interfacial instability, tend to increase sediment suspension by related increase in local shear velocity. Concerning bed forms, near bed scaling criteria suggest that in the present study the bedform consists of ripples with the measured wave length being in agreement with criteria given by [2]. An expression for the ripple growth is given in relation with the theoretical criteria of [3].