Downstream ecological impacts of controlled sediment flushing in an alpine valley river: a case study

Sediment flushing may be effective to tackle the loss of reservoir storage as a result of siltation. When operationally possible, the impact of this practice on the downstream aquatic environment can be mitigated by limiting the sediment concentration of the discharged waters (controlled sediment flushing). However, this topic is poorly documented, and concerns arise when limits are discussed. We present the results of a 3-year field investigation concerning the controlled sediment flushing of a small reservoir on the Adda River, the main tributary of Lake Como-Italy. Two limits for suspended solid concentration (SSC) were adopted: 1.5gL-1, as average value throughout the whole working day, and 3.0gL-1, as alert threshold to adjust the ongoing activity. These constraints were essentially fulfilled in the course of the documented operations. The first year sediment flushing was more significant than the following year: 25000 tons of fines below 2mm in diameter were flushed in six non-consecutive days in summer 2010, while, one year earlier, 75000 tons were flushed in 16 non-consecutive days. In the third year of investigation (2011), no sediment evacuation took place. The benthic macroinvertebrate and the fish communities were surveyed a short distance below the reservoir, that is, in the potentially more affected river reach. Clear pieces of evidence of environmental quality degradation were not detected; the adopted strategies can therefore be considered to be appropriate when planning sediment flushing management in comparable contexts.