In the Rhine-Meuse Delta in the Netherlands, deep scour holes can develop very suddenly. The holes may form a potential risk for the stability of surrounding riverbanks, dikes, bridges, tunnels and buildings. A scour hole can mostly be the result of two conditions: local changes in hydrodynamical conditions or local changes in erosive capacity of the bed. Heterogeneity of the subsoil in the Rhine-Meuse Delta causes differences in erosive capacity, resulting the formation of deep scour holes.

A distinct method to predict the development of scour holes in heterogeneous subsoil is not found yet. In this research a numerical model that simulates the hydrodynamics in a scour hole is used to gain more insight in the flow processes that are present in such a hole. The dependencies of the flow processes on the upstream water depth, upstream flow velocity and depth of the scour hole are investigated by changing these parameters systematically in the model simulations. The processes that are included in the analysis are the flow separation and recirculation zone, the contraction of flow, the horseshoe vortex and the vortices downstream of the hole.

Changes in the depth of the scour hole showed some interesting phenomena. It is found that three-dimensional processes suppress the formation of the recirculation zone, as flow separation happened for milder slopes and shallower holes in two-dimensional simulations. The contraction of flow is not necessarily stronger for larger holes, which was hypothesized.

The analysis of the bed shear stress in the model results shows a constant upstream slope angle, as the critical value for bed shear stress is not reached. In the downstream half of the hole, this value is reached and therefore erosion and sedimentation are expected. The bed shear stress in the model results shows strips of zero bed shear stress with large gradients at both sides of the hole. These strips have an angle of 1:8, which is also found in the laboratory experiments by Koopmans (2017). The downstream expansion of the scour hole with a poorly erodible top layer had the same widening angle.