Replenishment of sediment downstream of dams: Erosion and transport processes

Large dams in Alpine valleys retain most of the sediments, especially coarse material, which is deposited in the delta of the reservoirs. The common sediment evacuation measures allow to flush only the fine sediments for example by turbidity current venting. Since the steep rivers below alpine dams are situated in coarse alluvial containing glacial material, their bed armors when the supply of bed load is interrupted. Spawning grounds for fish reproduction, which require appropriate bed forms, are consequently becoming rare. This can be mitigated by the replenishment of adequate gravel material downstream of large dam combined with artificial flood release for mobilizing these deposits. According to the revised Swiss Water Protection Act, flow and sediment transport regime have to be restored downstream of dams in the frame of river revitalization measures. Mrs. Elena Battisacco carried out for the first time, systematic physical experiments in a flume in order to study which arrangement of sediment deposits are suitable for recreating morphological variability downstream. Using innovative image processing analysis and measurement techniques, Dr. Battisacco could give new recommendations regarding the placement of the artificial gravel deposits and the required discharge to mobilize them, with the purpose that the deposits can recreate morphological bed forms when transported in the river reach downstream. Also for the first time, the influence of the hydrograph of the artificial flood on the transport mechanism of the replenishment deposits was studied systematically. With her research, Dr. Battisacco could give a new insight on the replenishment technique in general and on the erosion and transportation mechanism of artificial deposits placed in different arrangements in a river downstream of dams in particular.