Sediment erosion by Gortler vortices: the scour-hole problem

Experimental results on sediment erosion (scour) by a plane turbulent wall Jet, issuing from a sluice gate, are presented which show clearly - it seems for the first time - that the turbulent wall layer is destabilized by the concave curvature of the water/sediment interface. The streamwise Gortler vortices which emerge create sediment streaks or longitudinal sediment ridges. The analysis of the results in terms of Gortler instability of the wall layer indicates that the strength of these curvature-excited streamwise vortices is such that the sediment transport is primarily due to turbulence created by these vortices. Their contribution to the wall shear stress is taken to be of the same form as the normal turbulent wall shear stress. For this reason, the model developed by Hogg et al. (J. Fluid Mech. Vol. 338, 1997, p. 317) remains valid; only the numerical coefficients are affected. The logarithmic dependency of the time evolution of the scour-hole depth predicted by this model is shown to be in good agreement with experiments. New scaling laws for the quasi-steady state depth and the associated time, inspired by the Hogg et al. (1997) model are proposed. Furthermore, it is emphasized that at least two scouring regimes must be distinguished: a short-time regime after which a quasi-steady state is reached, followed by a long-time regime, leading to an asymptotic state of virtually no sediment transport.