In the Rhine-Meuse delta many scour holes are present. These scour holes form a potential risk on the surrounding bridges, levees, weirs, dikes and tunnels. In general, scour holes can develop when there is a local change in hydrodynamic conditions or erosive capacity. Extensive research has been done in the past on scour holes close to hydraulic structures in a homogeneous subsoil, however, a large amount of scour holes are not related to hydraulic structures. Additionally, in deltas, the subsoil mostly consists of alternating layers of soil with different erosive capacities. This experimental study focused on a simplified situation of scour holes in heterogeneous subsoil with a three-dimensional character. The bed of the flume consisted of a concrete layer with a hole in the centre in which sand was exposed to the flow. Water depth, flow velocity and the size of the geometry were systematically varied to investigate their influence on scour hole development. Additionally, general observations were performed on the flow structures and scour development in and around the scour hole. It was observed that the scour development is different compared to the results of earlier investigations as a result of the non-erodible top layer. The process of undermining is a limiting factor for sediment transport out of the scour hole. The water depth showed a significantly smaller influence on the scour depth compared to existing empirical formulas. Additionally, the size of the geometry seems to be the most important varied parameter in this study. Relative to the size of the geometry, the scour depth remained approximately equal with different experimental conditions. Compared to two-dimensional experiments, the scour holes became deeper, which suggests importance of three-dimensional flow for scour hole development.

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.

Rapid hydropower development is taking place in the Mekong river, which is expected to change the natural river regime. Because of the ecological and economical importance of the Mekong delta, the effects of these changes could have large consequences in the delta area. This research focuses on the hydrodynamical processes and assesses potential risks of changes to these processes, caused by hydropower development, could have on the ecology and socio-economy. Furthermore, a number of mitigation measures in the delta have been proposed to reduce the potential risks.