Mitigation of hydropeaking in an Alpine stream: the Noce River case study

The upper course of the Noce Stream (3rd order gravel-bed stream, 1200-950 m asl, NE Italy) had been regulated since the mid ‘30s by storage hydropower production (Cogolo HPP) and associated hydropeaking (1 to 7 m3/s). Within the recent boom in run-of-the-river (RoRPP) power plants in Alpine streams, in 2014- 2015 three RoRPPs (total about 9 MW) were built in the hydropeaking reach. This has been taken as an opportunity to mitigate hydropeaking by the local Environment Agency (APPA Trento), because the three RoRPPs run in series, and the upstream intake is located in the release channel of the storage power plant. As a result, hydropeaking in an 8-km stretch was substantially eliminated. The ecological impacts of hydropeaking on the hyporheic zone had been assessed in 2006-2008, and were due to the deposition of fine sediment transported by the water released by the storage HPP, hardly removed within the regulated flow regime. The resulting clogging caused a reduction of the interstitial space available to invertebrates. We repeated the same hyporheic monitoring scheme in 2016, and recorded relevant changes in the hyporheic communities. Ecological data recorded by APPA allowed detecting the positive (reduced clogging) and negative (increased pollutants concentration) effects of these mitigation measures. Application of different sets of hydrological and hydrodynamic indicators allowed to quantify hydropeaking changes in the entire river reach. This work illustrates an interesting option for hydropeaking mitigation coupled with further hydropower production and highlights unexpected side effects that may suggest further environmental measures.