Monitoring Suspended Sediment Plumes Using an Acoustic Doppler Current Profiler

Monitoring suspended material plumes during dredging projects or undersea construction operations is often a fundamental requirement of state and federal Section 401/404 Water Quality Certificate regulations to protect water quality and minimize potential environmental impacts. Traditional monitoring techniques typically employ in-situ turbidity monitoring and collection of water quality samples at pre-defined locations (some distance down current of the activity) and at specified times. However, sediment plumes can have complex spatial and temporal structures, specifically small length scales (order ~10's of meters) and temporal scales (order -minutes). Applying traditional sampling techniques under these conditions can result in the underestimation of suspended sediment concentrations and the collection of water quality samples that are not representative of project related impacts. Recent environmental monitoring surveys used an acoustic Doppler current profiler (ADCP) to overcome the uncertainties associated with open-water plume tracking. As a current meter, the ADCP provides real-time current data to aid in predicting the path and arrival time of a plume at a given permit compliance distance downstream. Acoustic backscatter, or the signal strength received by the transducer, provides an indicator of suspended material concentrations in the water column, allowing the ADCP to provide continuous full water depth profiles of relative suspended sediment concentrations beneath the survey vessel. This backscatter display is an essential tool to provide a high-resolution visualization of the spatial and temporal variability of the suspended material plume. As a result, the ADCP allows for improved, or "targeted", sampling and in-situ data collection from a location and depth that accurately represents water quality conditions during a given project activity.