A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering of the Asian Institute of Technology, Bangkok, Thailand

Thesis (M.Eng.) - Asian Institute of Technology, 1977

The purpose of this study is to investigate the effect of non-uniformity of grain size on sediment discharge in a steady flow. Laboratory experiments have been conducted by using a recirculating flume, in which the graded sediments with a medium diameter of 0.50 mm and a geometric standard deviation of 2.23 have been placed. The suspended load, total load, flow discharge and the water surface slope have been measured for several different flow conditions. The grain-size distributions of transported sediments have also been measured by mechanical sieving method. The values of critical shear stress evaluated by two different methods based on the experimental data are compared with the previous studies by Straub and Shields, and the interaction effect between sediment fractions of different grain size is discussed. The values of sediment transport evaluated by the DuBoys' as well as Engelund and Hansen's formula show quite good agreements with the measured ones, while the Einstein's bed load function is concluded to give considerably smaller values. The sediment load is also evaluated for each grain size fraction, into which the total load is divided, by using the DuBoys ' formula, and is compared with the measured value. It is suggested that in the sediment mixture of non-uniform size, the coarse grains might tend to move more easily as compared with the uniform sediment of the same size, while the fine sediment might be obstructed to move and show smaller discharge.