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 s izes in the range of 280-1,070 mu m were used to determine the samplin g efficiency factor of the bedload sampler. The sampling efficiency fa ctor was found to be about 1.0 for bed material sizes larger than abou t 400 mu m and about 1.5 for finer bed material sizes. Field measureme nts 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 sample r was equipped with an underwater video camera to facilitate visual ob servations of the bed-load sampling process. Based on the visual obser vations, five types of bed-load samplings were distinguished and a com putation method was proposed to determine the proper mean bed-load tra nsport rate from the collected samples at a certain sampling station. The error of the mean transport rate in a sampling station was determi ned as a function of the number of samples. The samples should be take n at different locations along a bed form (dune). Finally, the overall error of the cross-section integrated bed-load transport rate was det ermined as a function of the number of samples in a sampling station a nd the number of sampling stations in a cross section.