Sediment flushing from an Alpine reservoir: Hydraulic monitoring and biological impact

The sediment flushing from artificial reservoirs is often necessary for both economical and technical reasons; on the other hand, the increase of suspended solids concentration (SSC) generally impacts negatively on effluents biota. In order to define ecologically-based limits for SSC, the presented paper provides experimental evidence of the effects of two sediment flushing events on the aquatic biota of a creek downstream of an Alpine reservoir. Sampled data concern both fish and macro-benthos communities. The reservoir is located in Valtellina (Italian Alps) and at the end of August 2006 and August 2007 it was subjected, for about two weeks, to controlled procedures to remove volumes of sediment roughly estimated in 20×103 m3 (2006) and 35×103 m3 (2007). The reservoir is quite small (1.3×106 m3 storage) but possesses strategic importance for hydro-power production, collecting water from a system of higher-level reservoirs. A simple literature model for predicting biological effects of suspended solid loads is tested against the collected data. This model gives an index of the impact (Severity Effect Value) as a function of overall duration and average SSC: if properly evaluated, it might support the planning phase of analogous operations.