Modeling sediment dynamics of a tidal sand bank

Richard P. Signell and Erin R. Twomey (USGS, Woods Hole, MA, USA)

Using a combination of repeated swath bathymetry surveys and sediment transport modeling, we describe several aspects of the sediment dynamics on Middle Ground, a long linear sand bank in Vineyard Sound, Massachusetts. Middle Ground is approximately 6 km long, 800 m wide 16 m high in a water depth of about 20 m. It is comprised of coarse sand, and sits on a bed of gravel. The bank has existed for at least 400 years in dynamic equilibrium with the tidal currents, which are in the range of 1.2-1.8 m/s. Repeated high-resolution (1-m pixel) swath bathymetry surveys show that the bank is covered with large sand waves of up to 4 m height moving in a counterclockwise direction around the bank of up to 10 m/month. Remote sensing imagery suggests the tidal flow crosses the bank obliquely at a small angle such that counterclockwise residual tidal circulation should be generated through a frictional bottom torque mechanism.

Using the freely-available, open-source hydrodynamic model ROMS, with sediment transport capabilities developed under the Community Sediment Transport Modeling project, we have run 3D sand transport simulations on the bank using both suspended and bed load transport. The results confirm that tidal currents cross the bank at a small oblique angle, and the resulting residual counterclockwise circulation is consistent with the pattern inferred from the sand waves. The model results show a strong focusing of sediment transport on the bank, providing a mechanism for maintenance. Modeling work is currently underway to understand observed changes that occur to the bank during the winter, possibly due to strong mean currents and waves associated with storms.