Experimental observations on sediment motion under weak one-dimensional bed load

Some preliminary results of measurements of one-dimensional sediment transport on a flat bed are presented. Image processing was applied to measure the time evolution of the areal concentration and of the velocity of the transported grains in a series of bed load tests with values of the bed shear stress up to twice the threshold for incipient motion. The concentration and velocity data were used to compute the time evolution of the unit width solid discharge. It was found, as expected, that the sediment transport is an episodic phenomenon, particularly at low shear stress. The automatic measurement method enabled obtaining the instantaneous values of the solid discharge for a statistical analysis. The frequency distribution of the sediment rate values was analyzed; the shape of the distribution is not self-similar as the shear stress varies. Finally, it was found that up to 30% of the transport events have no significant contribution to the total transport; about 70% of the transported volume is related to only the 40% higher transport events.