Processes of initiation of motion leading to bedload transport in gravel-bed rivers

Transport processes that lead to the initiation of bedload motion in gravel-bed rivers have not yet been clarified. We report patch- and grain-scale processes involved in the initiation of bedload motion in a natural gravel-bed stream as observed through a series of video experiments. With increasing flow strength, the phases of initiation of motion that have been identified are ( 1) within-patch grain instability ( grain vibration, pivoting, and grain-scale rolling), ( 2) within-patch gyratory step-and-rest motion, and ( 3) general sediment motion involving downstream transport from an individual patch and the throughput of grains inherited from upstream.