Laura, Pieter, Johan and Suzanne have published a research article on sand wave-induced form roughness. We extend the concept of form roughness to a tidal setting and study how sand wave-induced form roughness depends on bedform characteristics (i.e., height, wavelength, asymmetry).
We explain here the main insights described in the article. If you want more details, you can read the full open-access article here.
Form roughness in a tidal setting
Numerical models can sometimes have too coarse grids such that sand waves cannot be resolved individually. Yet, sand waves do affect the tidal flow by modifying its amplitude and phase. If we want to take these effects into account in models where sand waves are not explicitly considered, we can parametrise them. Bedforms are usually parametrised by an increased roughness over a flat seabed (i.e., form roughness). Previous work on form roughness focused on river dunes or ripples, where the flow considered was unidirectional. However, regions where sand waves may be found are characterised by an unsteady tidal flow. We thus extend the concept of form roughness to a setting characterised by an unsteady tidal flow, described by several tidal constituents, each with an amplitude and a phase.
We find that sand waves affect both the amplitude and phase of the tidal flow, yet in a different manner. As a result, we cannot represent the entire effect of sand waves on the tidal flow. Thus, we can represent the effects of sand waves on the flow amplitude, but not on the flow phase, or viceversa for example. This yields different criteria to compute form roughness, highlighting the complexity of considering a tidal setting with respect to a river setting (where the steady nature of the flow yields a unique form roughness value).
