Experiences with vessel borne ADCPs in shallow waters

Two ADCPs-narrow- and broadband-were used aboard different ships to measure fluxes through estuarine inlets. Water bias affecting the ship's velocity vector as obtained from bottom tracking necessitated the use of navigation systems when acoustically soft ground, layered by suspended sediment, was encountered. The alignment between the ADCP's direction and the gyro compass heading poses the same problem as in deep water. Vertical raw data profiles of the water velocity were fitted by the power law (1) for extrapolation and (2) for a check of the exponent 1/5 used with an existing moving boat system for flux measurement by one surface current sensor only. The fitted exponents proved to be Gaussian distributed around 1/6 to 1/4 . The flux measurements were subsequently used for the validation of hydrodynamic models of differing sophistication. Generally, ADCP and model results agreed within ca 10%.