Al Plueddemann, Physical Oceanography
Rich Signell, USGS
Rocky Geyer, Applied Ocean Physics & Engineering
Project Title: Hydrodynamics of Tidal Flow Across A Submarine Sand Ridge
The Middle Ground sand ridge runs along Vineyard Sound at a slight angle to the reversing tidal flow along the channel. This study sought to understand small-scale processes associated with the ridge by collection and analysis of in situ velocity data and comparison to a numerical model. Velocity time series from a mooring confirmed the expected dominance of the ebb tide on the north side of the ridge, and an observed mean flow of 5-10 cm/s in the direction of the ebb tide was confirmed by the model. The mean of the cube of the velocity, a proxy for sediment transport, was directed towards the ridge. Multiple spatial transects across the ridge at different stages of the tidal cycle were obtained using the Autonomous Underwater Vehicle (AUV) REMUS. The REMUS data showed strong, localized velocity shear near the ridge crest during the strongest currents. Cross-ridge shear in the model (which used relatively smooth bathymetry) was similar, but weaker and broader. Four potential processes for ridge formation and maintenance were evaluated based on the observations and model results.
This project was particularly worthwhile for me because of its combination of field and computer work. Collecting the REMUS transects involved planning field missions around several variables, including scientists’ busy schedules and the tidal cycle in Vineyard Sound. Each mission was a learning experience in working with AUVs and in the realities of field work. The raw data then had to be processed, analyzed and visualized, which I did primarily using Matlab software. Although there is still work to be done on this project, it has taught me a great deal about the scientific process in physical oceanography.