Results from the first deployment of the BASS Rake field prototype

The BASS Rake is an acoustic travel time current meter designed to make spatially and temporally dense velocity profile measurements in the continental shelf wave bottom boundary layer. The thinness of the layer is responsible for high levels of bottom shear stress which are important contributors to the sediment entrainment process and which enhance turbulent dissipation of flow energy. The BASS Rake is a modification of BASS, the Benthic Acoustic Stress Sensor, using a new geometry to image flow in the WBBL. A laboratory prototype has previously demonstrated the features and near bed capabilities of the new design. The field prototype described was constructed with standard BASS components to evaluate the measurement technique and the performance of the support frame in the near shore zone. The field prototype measures the horizontal velocity vector at ten heights from the bottom up to 30 cm above the bottom. The results of tow tank calibration and cosine response measurements are presented. After calibration the field prototype was deployed in 3 m of water immediately outside the surf zone of a local beach. Profiles were recorded continuously at 1 Hz for approximately 3.5 weeks in December of 1996. This period includes both calm and storm conditions. Selected data from this deployment are presented.