Background Topography Affects the Degree of Three-Dimensionality of Tidal Sand Waves

Offshore tidal sand waves on the sandy bed of shallow continental shelf seas are more three-dimensional (3D) in some places than others, where 3D refers to a pattern that shows variations in three spatial directions. Such sand waves have crestlines that meander, split or merge. The degree of three-dimensionality seems to vary especially when large-scale bedforms, such as tidal sand banks, are present underneath the sand waves. Understanding this behavior is important for offshore activities, such as offshore windfarm construction or the maintenance of navigation channels. In this study, the degree of three-dimensionality of sand waves at five sites in the North Sea is quantified with a new measure. Results show that tidal sand waves on top of tidal sand banks are more two-dimensional (2D) than those on bank slopes or in open areas. These differences in sand wave pattern are supported by numerical simulations performed with a new long-term sand wave model. The primary cause of these differences is attributed to the deflection of tidal flow over a sand bank, which causes sand wave crests to be more aligned with the bank at its top than at its slopes. It is subsequently made plausible that the different patterns result from the competition between two known mechanisms. These mechanisms are nonlinear interactions between sand waves themselves (SW-SW interactions) and nonlinear interactions between sand banks and sand waves (SB-SW interactions). On bank tops, SB-SW interactions favor a 2D pattern, while SW-SW interactions, which elsewhere produce a 3D pattern, are less effective.