HP scenarios
The HP scenarios consisted of the combined increase of flow amplitude
and up-ramping rate (Figure 2c and Table 1). To reduce impacts on
hydropower flexibility and production, in each river all field
experiments were carried out over two consecutive days. The first day,
we targeted a combination of three flow amplitudes and three up-ramping
rates. The same three scenarios were repeated the next day but the drift
nets were placed further towards the river center to achieve higher peak
flow velocities as at day one (Figure 2a and Table 1). The selection of
the targeted up-ramping rates (1, 2 and 3 cm/min) and peak flow
velocities (0.4 and 0.8 m/s) was based on the results of previous flume
experiments, which showed distinct drift responses in these ranges
(Schülting et al., 2018b). In terms to achieve the targeted HP scenarios
and to define appropriate sampling sites, for each HP reach we set up
and calibrated 2D unsteady hydrodynamic-numerical models to pre-define
the operation mode of each hydropower plant (KUB) or retention basin
(KWO, KLL).
In contrast to KUB and KWO, at KLL the maximum turbinable peak flow had
to be kept constant to avoid flow velocities > 1 m/s and
thus ensure wadable conditions. With this limited discharge setting, for
the Linth, it was also necessary to define an alternative HP scenario
with a reduced up-ramping rate (0.5 cm/min) as for the Sitter and
Hasliaare. Moreover, due to different hydrological conditions (e.g.,
melting snow, rain) and operational characteristics of the hydropower
plants and/or retention basins (e.g., turbine type, concessional
limitations), base flow and down-ramping rates varied between the HP
reaches. These constraints resulted in some differences between the
targeted (modeled) and the measured flow amplitudes, peak flow
velocities and up-ramping rates (Appendix B in Data S1).
Nevertheless,
measured flow amplitudes in the
Sitter and Hasliaare increased approximately twofold respectively
threefold from HP scenario 1 (SC1) to SC2 and ca. fourfold respectively
five-sixfold from SC1 to SC3, whereas it remained almost constant in the
Linth (Figure 2c and Table 1). An increase in mean and maximum
up-ramping rate from SC1 to SC2 and to SC3 in addition to a higher peak
flow velocity at day two was also achieved but varied between HP
reaches. Mean up-ramping rate between SC1 and SC3 increased 2.5 times in
the Sitter, 1.5 to 4 times in the Hasliaare and 1.5 to 2 times in the
Linth. From day one to day two, peak flow velocity over all scenarios
increased in average 4 times in the Sitter, 0.25 times in the Hasliaare
and 0.15 times in the Linth (Table 1).