Experimental study on sediment deposition and water level surge under unsteady sediment supply

Storms often cause serious rainfall runoff in mountain river areas, which results large amounts of sediment form upstream hills to downstream channels, leading to a reconstruction of the riverbed and finally water and sediment disasters. High concentration sediment transport may exist during flash floods, and performs unsteady supply process in channels. Based on laboratory experiments, this paper analyzed the responses of riverbed elevation and water level to unsteady sediment supply. The unsteady sediment supply is described as a single triangular sediment process. The sediment supply rate of all tests is greater than the sediment transport capacity of the flow. Results show that the riverbed deposits and water level rises continuously during sediment supply, while the flow depth decreases correspondingly. The greater the rate of sediment supply, the faster the rising of riverbed elevation and water level. After the sediment supply ended, the deposited bed degraded and the rising water level decreased. Compared with the constant sediment supply, the riverbed elevation and water level under unsteady sediment supply rise greatly. In addition, it is found that the flow discharge with saturated sediment supply is much less than that without sediment supply in the same water level. Because the concentration sediment transport increases the flow resistance and then makes the water level sharply rise. The study highlighted the important effects of the unsteady sediment supply on bed morphology and water level surge in water and sediment disasters, and enhanced the understanding of the mechanism caused by the sharply rise of water level in flash floods.