Measurements of tidal flow around a headland with a shipboard acoustic Doppler current profiler

Abstract

A shipboard acoustic Doppler current profiler (ADCP) and moored current meters were used to obtain detailed measurements of the spatial structure of the tidal flow around a headland in Vineyard Sound, Massachusetts, where tidal currents typically range from 50 to 70 cm s−1. Eight shipboard surveys were conducted, each of which followed one of five trapezoidal tracks in the vicinity of the headland, completing 11–12 circuits in the course of the tidal cycle (12.4 hours). The measurements from the ADCP compare favorably with moored velocity measurements at two locations (rms deviations of ±3 cm s−1), but the comparison showed more scatter at two other sites (rms deviations of ±7 cm s−1). The ADCP measurements from separate cruises were merged to form a composite spatial representation of the tidal and residual currents, providing resolution of the spatial structure of the flow around the headland at scales from several hundred meters to 10 km. The semidiurnal tidal flow is relatively uniform in space, roughly following the bathymetry around the headland, while a residual current field consists of counterrotating eddies on either side of the headland, with spatial scales of 5–8 km and velocities as high as 25 cm s−1. The instantaneous current field indicates the formation of transient eddies on the downstream side of the headland during flood and ebb, with clear evidence of flow separation near the tip of the headland.