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. Tidal sand waves are rhythmic bedforms that are found at the bottom of shallow seas such as the North Sea. These bedforms have heights of several meters, spacings of hundreds of meters and they can be very mobile. As such, they are important to offshore activities, such as constructing wind farms. Field data indicate that sand waves mainly display a three‐dimensional (3D) pattern, with bed level variations in three directions. However, sand waves on tops of sand banks appear to be more 2D (variations in one horizontal and in the vertical direction) than those on slopes. In this study, we analyze bed level data of five areas in the North Sea. The application of a new measure to quantify how 3D a sand wavefield is, confirms that sand waves on bank tops are mostly 2D, while they are 3D on bank slopes or in open areas. The relationship between sand banks and sand wave patterns is investigated with a new sand wave model. Results show that sand banks alter the tidal flow, thereby changing the orientation of the sand waves on top. The emergence of 2D or 3D sand waves is interpreted in terms of two competing nonlinear mechanisms. Analysis of field data shows that offshore sand waves on the tops of sand banks are more two‐dimensional than sand waves on bank slopesThe differences in sand wave pattern depending on the background topography are supported by simulations with a new sand wave modelThese differences are hypothesized to result from nonlinear sand wave‐sand wave (SW‐SW) and sand bank‐sand wave interactions (SB‐SW) Analysis of field data shows that offshore sand waves on the tops of sand banks are more two‐dimensional than sand waves on bank slopes The differences in sand wave pattern depending on the background topography are supported by simulations with a new sand wave model These differences are hypothesized to result from nonlinear sand wave‐sand wave (SW‐SW) and sand bank‐sand wave interactions (SB‐SW)