Scour hole behaviour: A field study of the Dutch Rhine Delta

Scour holes are common features in river deltas formed by a combination of hydrodynamic and geotechnical conditions. While previous studies have primarily focused on their long-term evolution toward equilibrium, their response to a single flood wave remains relatively underexplored. This study makes use of a unique, extensive dataset of frequently measured bed levels in the Dutch Rhine Delta to investigate the short-term behaviour of scour holes.

The analyses reveal that scour hole behaviour can be divided into two categories: dynamic and stable. We propose a conceptual model to describe this behaviour. Dynamic scour holes show large fluctuations in area, depth and volume caused by seasonal discharge variations. These temporal fluctuations exceed annual growth rates. By contrast, stable scour holes show virtually no variations in shape and dimensions. They are generally smaller in area but almost twice as deep as dynamic scour holes. Their large depths and steep slopes prevent sediment from being transported up the slopes, showing a stabilisation in depth.

Dynamic scour holes are typically located in upstream river reaches, where the sandy beds and engineering works allow for significant erosion and aggradation. Stable scour holes are found in tidal river reaches, where peak discharges are attenuated. Due to the absence of these peak discharges, combined with a strong heterogeneous river bed composed of easily and hardly erodible layers, growth in area is limited.