Field evaluation of ADCP refinements for profiling shallower waters

During recent years, RD Instruments (RDI) has received. increasing requests for the Acoustic Doppler Current Profiler (ADCP) to be able to operate in shallower water. Providing a well-resolved velocity profile in water depths of 0.6 m required several refinements of prior ADCP capabilities. These included creating an ADCP transducer (ZedHed/spl trade/) that has reduced ringing, bottom tracking at lower altitudes, increasing the pinging rate, and operating with smaller cell sizes in both slower and faster flows. This paper describes results from a field evaluation of these engineering developments. Data were collected by colocated 1200 kHz ADCPs during a flood/ebb cycle in a shallow, tidally affected, coastal lagoon north of San Diego, California. We installed in the lagoon bed a standard ADCP operating with a high ping rate (Mode 12). A ZedHed ADCP, set to work with high precision (Mode 11), was mounted in an Oceanscience's tri-hulled River Boat. Modes 11 and 12 were demonstrated to provide the capability to profile water depths of 0.6 m when used together with the ZedHed transducer (and the shallow bottom tracking capability for moving surveys). These modes were shown to meet noise specifications even for the small cell sizes required for profiling shallow water. An interesting possibility revealed in this work was the prospect of using Error Velocity in Mode 11 as a gauge of natural variability in turbulence and mixing studies.