An improved method for determining dredging source terms

The determination of sediment loss rates from dredging activities is important for quantifying the potential impact on the surrounding physical and biological environment. This paper describes an improved method for measuring and analysing data to determine measurable dredging ‘source terms’ using Sediview software and data from two vessel-mounted acoustic Doppler current profiler devices, one of which is mounted with a vertical beam. The methodology is described in detail and an example data set is presented based on measurements undertaken around a working back hoe dredger. The technique described in this paper offers an improvement over the traditional application of the software used to measure dredger source terms (i.e. the use of one vessel-mounted acoustic Doppler current profiler in a Janus configuration). The use of a vertically orientated acoustic beam allows for a greater range (depth) of acoustic backscatter measurements to be acquired, as the data are not contaminated by side-lobe reflections. Although partially dependent upon the dredging methodology, this improvement is important as high suspended sediment concentrations tend to occur near to the bed. Subject to robust calibration, an improved source term can be collected using two acoustic Doppler current profiler instruments and the software allows the determination of suspended sediment flux (kg/s) across an entire survey transect, by using discharge data from one instrument and calibrated concentration data from the other, rather than the use of a series of water column turbidity profiles and subsequent spatial interpolation (an alternative method to the use of the software).