BED-LOAD SAMPLING IN SAND-BED RIVERS

Bed-load sampling in sand-bed rivers was studied. A mechanical sampler called the Delft Nile sampler was designed and operated in flume and field conditions. The flume data obtained for different bed material sizes in the range of 280-1,070 mu m were used to determine the sampling efficiency factor of the bedload sampler. The sampling efficiency factor was found to be about 1.0 for bed material sizes larger than about 400 mu m and about 1.5 for finer bed material sizes. Field measurements in the Nile river (Egypt) and in the Rhine river (The Netherlands) were taken to determine the proper number of samples to be taken in a cross section in relation to the errors involved. The bed-load sampler was equipped with an underwater video camera to facilitate visual observations of the bed-load sampling process. Based on the visual observations, five types of bed-load samplings were distinguished and a computation method was proposed to determine the proper mean bed-load transport rate from the collected samples at a certain sampling station. The error of the mean transport rate in a sampling station was determined as a function of the number of samples. The samples should be taken at different locations along a bed form (dune). Finally, the overall error of the cross-section integrated bed-load transport rate was determined as a function of the number of samples in a sampling station and the number of sampling stations in a cross section.