FORM ROUGHNESS INDUCED BY TIDAL SAND WAVES: A MODELLING STUDY

Tidal sand waves are rhythmic bedforms usually found in tide-dominated shelf seas (e.g., the Dutch Continental Shelf, Figure 1). They are characterized by wavelengths of 100 - 1000 m, heights of 1 – 10 m and migration rates of 1 – 10 m/yr (van Dijk and Kleinhans, 2005). It is recognized that tidal sand waves generate form roughness that affects the hydrodynamics on a larger spatial scale. However, in basin-scale hydrodynamic models (that do not resolve the local sand waves), bed roughness is treated as a calibration parameter, and not directly linked to the local sand wave characteristics (Zijl et al., 2018). Here, we study the form roughness induced by sand waves in a tidal setting. To this end, we use two process- based hydrodynamic models. Firstly, a second order perturbation approach (based on the linear stability analysis of Hulscher, 1996) allows us to study the sand wave-averaged effects of sand waves on an oscillatory tidal flow. Secondly, Delft3D allows us to study the effects of sand waves on a propagating tidal wave.