effect of managed hydropower peaking on the physical habitat, benthos and fish fauna in the River Bregenzerach in Austria

The lower reaches of the Bregenzerach, a river in western Austria, have been affected by heavy hydro‐peaking discharges (up to 60 m³ s⁻¹) for several decades. In conjunction with a plan to construct a new power plant, mitigation measures were developed to reduce the adverse effects of hydro‐peaking in this river reach. The impact assessment examined the effects of a new flow management system introduced in 1992. The study concentrated on three main components: abiotics (morphology, hydrology, hydraulics), fish and benthic fauna. Prior to mitigation, both the fish and invertebrate fauna were heavily affected by the peaking. Benthic biomass was less than 15% of that predicted by an altitude model. After mitigation benthic biomass recovered to about 60% of the model prediction. No post‐mitigation improvement was found with respect to fish biomass. Flow velocity distribution data showed that rising surge releases had two distinct phases, a bed‐filling and a flow acceleration phase. The present flow management system, based on a dual‐flow logistic, resulted in increased base flows and reduced peak flows, but did not alter the ramping rates. The reduction of the adverse effects of the bed‐filling phase was apparently responsible for the recovery of the benthic fauna. However, the unaltered ramping rate of the acceleration phase may have inhibited the development of the fish community. An analysis of the hydrograph demonstrated the possibility of adjusting the present management strategy to reduce the ramping rate of the acceleration phase, smoothing out the peak duration curve.