Scour holes

A data-driven risk-based analysis for scour holes in the Rhine-Meuse Delta

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In the Rhine-Meuse Delta over 100 scour holes are located. Many of these scour holes have dynamic behaviour and are growing in size and depth. A scour hole can lead to an increased probability of flooding. One of the potential hazards of the presence of scour holes is the occurrence of flow slides on the foreshore next to scour holes. Depending on the distance between the dike and the river, a flow slide can affect the water-retaining function of a dike. Flow slide is an indirect failure mechanism and can be used as a scenario in the safety assessment of direct failure mechanisms.

In the research, a data-driven method has been developed for the risk assessment of scour holes. The method is elaborated for flooding due to the failure mechanism overtopping. The development of a scour hole is coupled with the safety assessment regarding overtopping for the dike next to a scour hole. In the method, the development of a scour hole and the future dimensions are predicted in a probabilistic way by extrapolation of historically measured bed level data. With the predicted future scour hole dimensions, the probability of occurrence of a flow slide and the probability of several post flow slide profiles is calculated with the methods given in the Wettelijk Beoordelingsinstrumentarium (WBI). Based on the post flow slide profiles, the probability of exceeding the critical overtopping discharge is determined. Combining the probability of exceedance of the critical overtopping discharge with the consequences of a flood gives the flood risk due to the presence of a scour hole.

The developed data-driven method is applied to one case study, a scour hole in the river Spui. For this particular scour hole, the flood risk is assessed. A prediction is made for the future dimensions of the scour hole. The flood risk is determined with the probability of occurrence of flow slide, the probability of exceedance of the critical overtopping discharge and the expected consequences of a flood. The assessed flood risk is evaluated and risk mitigation measures are proposed. The economic feasibility of risk mitigation measures is analysed with a cost-benefit analysis.