Estimating bedform migration rates from large scale models and repeat bathymetry

Determining sand wave migration rates is particularly important for the design and consenting of subsea cabling because it enables understanding of required cable burial depths, seabed preparation measures and their impact on the environment. This report summarises the activity of a knowledge exchange programme between Natural Resources Wales and Bangor University, supported by HR Wallingford. The programme, called ‘Translating sand waves,’ aimed to explore whether sand wave migration rates can be inferred from operational oceanographic models and sand wave dimensions, in the absence of repeat multibeam surveys. This could help with timely consenting in cases where historical data is unavailable.
Two areas were assessed: the area around Milford Haven, particularly Turbot Bank and St. Gowans Shoal; and, the North Wales coast, particularly Constable Bank. Both these areas are areas where subsea cabling exists or is planned and where repeat multibeam surveys are available.
Oceanographic models with available outputs were compared and the Scottish Shelf Model selected based on spatio-temporal and vertical resolution. However, the model was still relatively coarse spatial resolution (1 km grid) and so the detailed coast or sand bank bathymetry was not well resolved. The model did not include waves and so data from a lower resolution NEMO model was interpolated onto the Scottish Shelf Model grid. Sediment transport was calculated using the Soulsby – Van Rijn model.
Bedload transport rates were also calculated by tracking sand wave migration in the areas of overlapping measured bathymetry.
Comparison of the two approaches showed some differences which were attributed to the presence of secondary bedforms in the measurements and lack of resolution in the model. The differences were large enough to suggest that data from large scale operational models would struggle to predict sand wave migration rates. However, the data can be used to indicate sensible timescales for repeat bathymetry to be taken in order to assess sand wave migration through measurements. For Turbot Bank, surveys should be taken at frequencies of 1-10 years to ensure no aliasing of bedforms (sand waves travelling less than half a wavelength between surveys). At Constable Bank, annual or higher frequency surveys are required due to the smaller bedforms.