Role of turbidity currents in the siltation process of an artificial lake

The path and the rate of sediment deposition in a reservoir are indispensable to identify a strategy suited for its rehabilitation. In fact deposition patterns reflect, not only, transport processes, but can also provide useful information as to inflowing sediment load in a reservoir. In particular the paper reports the results obtained by way of a numerical model applied to the Rendina reservoir, an artificial lake located in Basilicata (the South of Italy), to simulate the path and the entity of turbidity currents. The object is to investigate possible strategy to reduce the trapping sediment in this lake the history of which is particularly interesting from the point of view of erosion processes of catchment areas and sedimentation rates. The simulation performed by means of the SMS (Surfacewater Modelling System) to reproduce turbidity currents was carried out bearing in mind the real flow rate and sediment load data observed in the inlet section in a typical year. The simulation started in October, a month when, due to the irrigation use of the reservoir, the normal water level is at its lowest and the specific sediment load also reaches its highest rate. Model results compared against data from bathymetric surveys showed that the path of the turbidity currents is strongly influenced by the incoming concentration values in the lake during the water-filling period which corresponds to greater erosion and transport in the catchment area. Moreover, the presence of several metres in depth of sediment near the dam confirmed the possibility that the high sediment load that arrives in this area in the early months of the rainy season, after a typically dry summer, creates an equilibrium that is not very sensitive to the changed incoming solid concentration.