The effect of hydrographs on bed load transport and bed sediment spatial arrangement

Field measurements of sediment transport in gravel bed rivers often reveal hysteretic effects due to differences in sediment availability between the rising and falling limbs of a flood hydrograph. However, only a small number of flume studies have analyzed the dynamics of sediment transport during hydrographs. Three types of stepped hydrographs with contrasting durations and magnitudes are simulated here under sediment recirculation conditions. Bed load transport rate and grain size have been measured continuously. The dynamic behavior of the surface armor layer has been explored by analyzing digital photographs and laser scanner surveys of the bed surface taken during hydrographs. The results indicate that sediment transport during the falling limb was lower than during the rising limb in all of the three types of hydrographs. This reduction is more evident for the low-magnitude hydrographs. The grain size of the bed remained virtually constant throughout the hydrographs but the grain size of transported sediments exhibited a counterclockwise hysteresis. Also, a significant increase in the reference shear stress for sediment entrainment was measured during the falling limb of hydrographs. Additionally, a detailed analysis of partial transport dynamics of the bed surface sediments reveals a reduction in sediment mobility during the falling limb of the hydrographs. The difference in sediment entrainment and transport before and after the peak of the hydrographs appears to be caused by a change in the organization of the surface sediments. An analysis of detailed laser scan bed surveys reveals a phase of bed restructuring (lower vertical roughness, clast rearrangement) during the falling limb of hydrographs. Consequently, changes in the degree of organization and complexity of the bed surface are likely the cause of the reduced mobility of sediments, and thus of the reduced sediment transport rate, during the falling limb of the hydrographs.