A new method for the measurement of bedload transport using time sequenced bathymetric data

Vessel mounted Multibeam depth sounders can be used to map the bathymetric features of rivers, including sand waves. This research has shown that it is possible to use two sequential bathymetric surfaces which contain sand wave profiles to determine bed load transport. Starting with the well known bed-load equation for translating bed forms, analytic and geometric considerations are presented which establish a relationship between the rate of scour on the eroding side of a sand dune and the average transport in the same dune, for certain ideal conditions. It is further shown that the rate of scour is twice the average transport. Using these two pieces of information, bedload transport can be computed from the scour rate obtained from the difference of two surfaces acquired over a finite time interval. To test the new method, three data sets were acquired. Two of these sets were obtained in flumes under controlled conditions. This provided a measured value of a bedload to which the value computed by the new method could be compared. Computed results show very good agreement with measured data, having average values that were 30% and 8% greater than the measured value for sets 1 and 2 respectively. The third data set was from Mississippi River bathymetry. Computations were made on 5 large sand waves for which sequential data were available over a range of time intervals. Although no measured value was available to compare to, the results were consistent within a given wave. All indications are that the method is repeatable and consistent when reliable bathymetric data are available. Instrument resolution was considered, and will play an important role in determining a minimum time interval between sequential profiles. At this time the method is limited to use on large sand bed rivers in which lower regime sand waves (no antidunes) are present and in which measurements can be made. For optimal data collection efficiency, some knowledge of dune size and wave speed is necessary.