Experimental study of gravity currents moving over a sediment bed: suspension criterion and bed forms

In many natural settings, gravity currents flow over a mobile sediment bed as in fluvial outflows 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, bed form 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 directed downward above the mobile bed, indicating therefore slip with an increase in horizontal flux close to the sediment bed. The sediment suspension model developed here allows to determine the spatial evolution of sediment suspension up to maximum suspension capacity, in good agreement with experimental results of Garcia and Parker (J Geophys Res: Oceans 98(C3):4793–4807, 1993) as well as with the present sediment flux measured toward the downstream end of the sediment bed. Effects of local bursts, here intermittently caused by interfacial instability, tend to increase sediment suspension through increases in local shear velocity. Concerning bed forms, near bed scaling criteria suggest that in the present study the bed form consists of ripples with the measured wave length being in agreement with criteria given by Lapotre et al. (Geology 45(3):243–246, 2017). An expression for the ripple growth is given in relation with the theoretical criteria of Charru et al. (Annu Rev Fluid Mech 45:469–493, 2013).