The Behaviour of Deep Scour Holes in Rivers: Stable States or Dynamic Fluctuations?

Deep scour holes in rivers, resulting from local flow variability, can pose a serious threat to nearby infrastructure, like pipelines, bridge pillars and the stability of embankments. Previous studies have revealed that in rivers with a stratified river bed, scour holes even evolve without the presence of an obstruction. Peak discharges and corresponding high flow velocities can break the hardly erodible top layer, exposing the underlying easy erodible bed materials to the flow. For instance, during the 2021 summer flood in the Maas River, the Netherlands, 15 scour holes, with a depth varying between 3 and 15 meters were formed within days. One of these scour holes threatened a ferry landing. Another scour hole uncovered a pipeline.

This research investigates the behaviour of scour holes to make well-considered choices to fill deep holes after their formation or to remain them open. We analysed the behaviour of eight different scour holes in the Rhine-Meuse Estuary and the River Waal over the period 2018-2024 using multibeam bed level measurements. We expressed the behaviour of a scour hole in terms of variations in area, volume and depth and tried to relate this to discharge variations. We can classify this behaviour into two categories: stable and dynamic. Stable scour holes, often located in sand bodies from former channel belts, show neglectable variations in area, volume, and depth within the monitored period. In contrast, dynamic scour holes respond to discharge fluctuations, expanding during peak flows and contracting during low discharges. On the long-term, over a period of more than five years, the dynamic scour holes shows a continuous growth or decrease in scour hole characteristics. This group of scour holes deforms and migrates which, at some point in time, may make them a threat to nearby infrastructure.

Our findings highlight the importance of understanding scour hole dynamics for effective river management, emphasizing that not all scour holes are in a stable state after their formation. While some recently formed scour holes pose immediately a threat to infrastructure, others may become a threat after repeated peak discharges that lead to significant growth in their dimensions.