Macroinvertebrate drift response to hydropeaking: An experimental approach to assess the effect of varying ramping velocities

Intermittent storage hydropower production is the only renewable source offering both a sufficient storage potential and a high temporal flexibility in production to ensure grid stability. However, hydropeaking (generation of peak discharges by hydroelectric operation) causes fluctuations in the wetted width, water depth, flow velocity, and bottom shear stress downstream of a hydroelectric facility. River biota are known to be affected by these changes, which leads for example to increased drift. Correspondingly, abundance and biomass of macroinvertebrates are frequently found to be reduced in impacted river stretches. Although there is sufficient evidence for increasing drift proportions due to hydropeaking, only few studies have highlighted the role of the rapidity of flow increase before and flow decrease after a peak event (ramping velocity). Here, we present the outcome of experimental hydropeaking in artificial flumes, mimicking two ramping velocity treatments (T1: 0.5 cm/min, T2: 1 cm/min water table change) in early and late spring (June '14, March '15). Macroinvertebrate drift was significantly higher in treatments as compared with control flumes. Drift proportions peaked during the up-ramping phase and were slightly lower during the peak discharge phase. Drift proportions of T2 treatments were significantly higher than those of T1 in June but not in March. Our findings suggest that hydropeaking requires thoughtful ramping management, to allow macroinvertebrates to seek for refugia on the stream bottom during the peak event. However, the evidence of daytime-dependent and seasonal drift patterns also plea for adaptive management, to account for the temporal variability of macroinvertebrate drift.