An experimental study of the transport pathways of individual bedload clasts over a gravel-bed substrate

The movement of individual clasts over a water-lain gravel substrate is investigated in a series of flume experiments. The effects of three experimental variables (mobile clast size, flow condition and release pocket) on the characteristics of extracted transport parameters (step length, step velocity, pathway sinuosity and rest period) are examined using a novel facsimilie of a real gravel bed. As the mobile clast size is increased the mean step length and mean step velocity of individual steps increases. The sinuosity of individual pathways increases with mobile clast size. The release pocket, which determines the region of the substrate over which the clast can move, was also found to influence the transport parameters. In general, mobile clasts exhibit shorter step lengths and more sinuous pathways over substrates that have a number of large clasts present in the bed surface. The mean rest period was found to be dependent on the mobile clast size and the release pocket. Mobile clasts equal to and greater in size than the median (of the gravel substrate) exhibited longer rest periods when their pathways coincided with areas in which large obstacle clasts were present. In contrast, mobile clasts smaller than the median exhibited longer rest periods when their pathways coincided with areas devoid of the influence of large obstacle clasts. Examination of the tracer pathways identified a number of areas that operate as trap areas or transport routes for mobile clasts. Trap areas were identified in a number of different topographical locations, in both the of and upstream, on the stoss side of, large clasts. In general, compared to the rest of the bed, the trap areas exhibit lower than average streamwise velocities, mean vertical velocities away from the boundary and a dominance of turbulent structures away from the boundary. The ability of the trap areas to trap mobile clasts was found to decrease with increasing mobile clast size. Furthermore, this effect was found to be greater at stronger flows. In general, compared to the rest of the bed, the transport routes exhibit higher than average streamwise velocities, mean vertical velocities towards the boundary and a dominance of turbulent structures towards from the boundary. In total, these observations contribute to an understanding of the elementary parameters of transport and are therefore of substantial value to the future development of models that deal with the prediction of bedload transport rates and simulations of bedform development.