Long-term dynamics of tidal sand waves

Abstract

Tidal sand waves are bedforms that have been observed at the sandy bed of shallow seas all over the world. They have heights (vertical distance between crest and trough) of several meters and lengths (crest-to-crest distance) of hundreds of meters. Sand waves can migrate with several meters per year. Sometimes they move into navigation channels or uncover cables and pipelines, causing them to be at risk to be damaged. Therefore, understanding sand wave dynamics is crucial. Currently, decisions are being made based on the extrapolation of historical trends, which has large uncertainties. In this study, it was investigated whether an existing sand wave model was able to accurately reproduce historical sand wave behaviour. This model gave good results when calibrated with site-specific data. Sand waves are often removed by dredging and it remained unclear whether they can recover after this. The analysis of dredged sand waves in the North Sea revealed that in some areas, they recover within a few decades. In other areas, there was no sign of recovery during the measurement period. This was likely due large water depths, strong currents and/or a lack of sand. Results from a new 3D sand wave model demonstrate that large-scale sand banks influence sand wave patterns, as sand waves on top of sand banks have straight crests, whereas they have curvy crests when banks are not present. Finally, model results show that sand wave patterns are also influenced by the availability of sand: a limited availability results in intermittent crests that are spaced further apart. All of these model results are consistent with observations.