Preliminary tow tank and flume tests of a prototype BASS Rake wave bottom boundary layer sensor

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 vertical extent of the WBBL is typically one to several centimeters, varying with water depth and wave conditions. 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. An analysis of the flow distortion due to the sensor is presented suggesting some dependence of the gain on flow speed. Tow tank tests demonstrate the suggested dependence and are used to calibrate the sensor. In flume tests, the horizontal velocity vector is measured at 0.5 cm, 2.4 cm, and 5.0 cm above a sand bottom at nominal flow speeds of 10 cm/s, 20 cm/s, and 34 cm/s and a depth of 8.6 cm. The 34 cm/s test included a significant bedload with no observed degradation of sensor performance. Velocity profiles are acquired within a 4 ms window at a 1 Hz rate. The flume measurements are compared to concurrent measurements made with an LDV