Hydrodynamics of Tidal Flow Across A Submarine Sand Ridge

Middle Ground is a sand ridge that runs along the channel of Vineyard Sound just west of West Chop headland on Martha’s Vineyard.The ridge is situated in the channel at a slight angle to reversing tidal flow along the channel.This tidal flow is asymmetrical with respect to the ridge, with the ebb cycle stronger to the north of the ridge and the opposite true on the south side, resulting in a counterclockwise mean flow around the ridge.The goal of this study was to resolve the small-scale processes on and around the ridge that are responsible for its maintenance and shape.This was done by collection and analysis of in situ water velocity measurements and comparison of the data to a numerical model.Harmonic analysis was performed on a moored ADCP time series, which confirmed the ebb dominance on the north side of the ridge.A mean flow of 5-10 cm/s was found to flow in the direction of the ebb tide and was confirmed by the model.In order to extend the in situ observations to locations closer to the ridge, spatial transects across the ridge were obtained using the Autonomous Underwater Vehicle REMUS over a complete tidal cycle.Analysis of REMUS data showed a strong localized shearing of velocity over the ridge crest during the strongest currents.Comparison to the model indicated some spatial variability in the tidal asymmetry relative to the ridge.More field observations from both mooring and REMUS data are necessary to fully resolve the small-scale processes most relevant to Middle Ground.

Abstract by Tess Brandon.