Shelf-wide analyses of sand wave migration using GIS: A case study on the Netherlands Continental Shelf

Shelf seas often display rhythmic bedforms, of which tidal sand waves are the most relevant to study from an engineering point of view due to their dimensions and dynamic nature. Environmental conditions such as water depth, flow velocities and grain size vary across entire shelf regions, consequently influencing the characteristics of tidal sand waves, including migration rates. However, as opposed to site-specific analyses, shelf sea-wide analyses on sand wave migration are scarce. Through the use of timeseries of bathymetric data, we investigate the spatial variability of sand wave migration on the scale of the Netherlands Continental Shelf (NCS). Two existing data-driven techniques, Pairs of Source and Target Points (PSTP) and Spatial Cross Correlation (SCC), were implemented in a Geographic Information System (GIS). Through testing at four sites on the NCS, PSTP was deemed the preferred technique for a shelf-wide analysis and was hence used to quantify the NCS-wide migration rates and directions. To illustrate the spatial variability, the quantified migration rates and directions were averaged per km². For the NCS in general, migration to the south-west or north-east at typical rates between 0 and 3 m/year was found. The south-western region of the NCS demonstrated migration rates below 0.5 m/year with migration directions alternating over short distances. The north-eastern region exhibited uniform migration to the north-east at rates which gradually increased when in closer proximity to the Wadden Islands. Typical rates in this region were found to be between 2 and 8 m/year. The highest migration rates were observed near coastal regions and on top of sandbanks. The presence of these sandbanks strongly deflects the migration directions of superimposed sand waves at a number of locations. The analysis performed within this study demonstrates valuable insights into methods that can be used to evaluate shelf-wide regions of interest. A deeper understanding of the dynamic nature of the North Sea bed assists in optimising hydrographic re-survey policies, critical infrastructure planning and subsequent monitoring. Furthermore, this study may offer a wider set of tools that could be used for correlating sand wave migration to its governing physical mechanisms and environmental parameters.