Acoustic determination of sediment concentrations, settling velocities, horizontal transports and vertical fluxes in estuaries

Acoustic Doppler Velocimeter (ADV) and Acoustic Doppler Profiler (ADP) backscatters were calibrated and used to estimate suspended particulate matter (SPM) concentrations, settling velocities, horizontal transports, vertical turbulent fluxes, and parameters related to estuarine particle trapping. Calibration of ADV acoustic backscatter (ABS) was carried out by comparison to material sampled by pump and via an Owen tube, using an optical backscatter (OBS) sensor as an intermediary. The ABS-SPM relationship was variable in time, even within a single tidal cycle. Temporal changes were, however, interpretable in terms of observed SPM characteristics. Estimated upward vertical turbulent SPM fluxes followed a temporal pattern similar to downward settling fluxes, but were much larger. SPM samples for calibration of ABS from a moored ADP (deployment duration approx. 7 mo) were available only during three cruises; an OBS was again used as an intermediary for calibration. As before, variations over time in the ABS-SPM relationship were found, and biofouling altered the ADP backscatter response during part of the deployment. Both profile and continuous wavelet transform time-series analyses were used to test the validity and physical meaning of the SPM data. Because of time variability in the ABS-SPM relationship, parameters were sought that were at least partially independent of this relationship; i.e., those involving ratios of concentrations within a single inferred SPM profile. Despite calibration difficulties, inferred SPM trapping efficiencies (ratio of maximum estuarine SPM to fluvial source SPM), settling velocities and horizontal transports contained valuable information concerning tidal to seasonal SPM processes. These techniques should be widely applicable in systems with SPM concentrations within the dynamic ranges of the ADV and ADP.