Modeling Form Roughness Induced by Tidal Sand Waves.

Tide‐dominated sandy shelf seas, such as the Dutch North Sea, are covered by sand waves. Yet, basin‐scale hydrodynamic models do not include any sand wave information because their grid sizes are too coarse to resolve sand waves individually. We explore the possibility of parametrizing the effects of sand waves on the larger‐scale tidal flow by means of a form roughness. Specifically, our aim is to see to what extent the flow over a sand wave field can be reproduced by that over a flat seabed with an increased effective roughness (accounting for both grain and form roughness). To do so, we use two process‐based hydrodynamic models: a second order perturbation approach, and Delft3D. Both models demonstrate that the presence of sand waves causes amplitude decrease and phase shift of the tidal flow. We explore the dependencies of form roughness on different sand wave characteristics (wavelength, height and asymmetry). Shorter and higher sand waves cause a higher form roughness, while our analysis does not reveal any dependency on sand wave asymmetry. Notably, the consideration of a tidal flow, characterized by several tidal constituents, each represented by an amplitude and a phase, results in a more complex form roughness analysis than in a fluvial setting, where the flow is unidirectional and steady. We thus obtain an amplitude‐based form roughness and a phase‐based form roughness, each yielding a different value, yet displaying the same qualitative dependencies. Plain Language Summary: Tide‐dominated seas, such as the Dutch North Sea, are often covered by tidal sand waves, that is, seabed undulations characterized by wavelengths of hundreds of meters, heights of several meters and migration rates of several meters per year. The presence of sand waves causes an increase in the roughness experienced by the tidal flow. As a result, the tidal flow may for example, slow down or lower in amplitude. Yet, the effects of sand waves are generally not considered in larger‐scale hydrodynamic models. We explore the possibility of representing the effects of sand waves through an increase in bed roughness on a flat seabed, that is, a form roughness. To do so, we use two process‐based hydrodynamic models. Results show that higher and shorter sand waves result in a higher form roughness. Furthermore, the unsteady nature of the tidal flow gives rise to multiple criteria to determine form roughness. These criteria yield different values for form roughness, highlighting the complexities of considering form roughness in a tidal setting. Key Points: The characteristics of tidal flow (unsteady, multiple tidal constituents) make studying form roughness more complex than in a river settingThe presence of sand waves causes amplitude decrease and phase shift of the tidal flow through an increased effective roughnessHigher and shorter sand waves result in an increased form roughness [ABSTRACT FROM AUTHOR]