H. Friedrich

Sand (i.e., < 2 mm) infiltration into a gravel bed is recognized to affect sediment dynamics of coarser fractions (i.e., entrainment and transport rate), near‐bed velocities, bed morphology, and to pose significant engineering and ecological issues. Local hydraulics, sediment dynamics, and sand infiltration into the gravel matrix are related and mutually affect each other in gravel‐bed rivers. However, relatively little research has been done to explore sediment dynamics and sand infiltration in a mobile gravel‐bed under unsteady flow conditions. In the present study, we experimentally simulated three hydrographs of different durations of rising and falling limbs, and monitored sand infiltration over beds with a coarse and a fine grain size distribution. Results confirmed that the addition of sand enhances sediment dynamics. Moreover, the shape of the rising limb of the hydrograph was responsible for altering identified sand infiltration processes and bedload rate. Short time‐to‐peak floods (i.e., short time from the start to the peak of the hydrograph) experienced higher sediment transport rates during the falling limb than the rising limb of the hydrograph, favoring sand infiltration. In contrast, long time‐to‐peak floods (i.e., long time from the start to the peak of the hydrograph) experienced higher sediment transport rates during the rising limb than the falling limb of the hydrograph, resulting in reduced sand infiltration.