Passage Performance of Potamodromous Cyprinids over an Experimental Low-Head Ramped Weir: The Effect of Ramp Length and Slope

Abstract

Low-head ramped weirs are a common instream obstacle to fish movements. Fish passability of these structures, where water passes over but does not generate a waterfall, is primarily related to ramp length and slope, but their relative contribution has seldom been considered. This study aims to assess the passage performance of a potamodromous cyprinid, the Iberian barbel (Luciobarbus bocagei), negotiating an experimental ramped weir with varying ramp length (L) and slope (S). Four configurations were tested, with a constant discharge of 110 L∙s−1. Results suggest that both factors influenced passage performance of fish. Attraction efficiency (AE) increased with increasing L and S, whereas the number of successes (N) and passage efficiency (PE) decreased upon increasing L. For S, it was found that both N and PE peaked at the intermediate level (20%). These results suggest that configurations with the lowest slopes may not necessarily be the best option because they may be less attractive for the fish and their demand for space is higher. Higher slopes (but not excessive) could be more attractive to fish, less space-demanding, and therefore, more cost-effective. Future studies should investigate how discharge and boulder placement influence fish passage across ramped weirs, to improve habitat connectivity.

Assessment of Retrofitted Ramped Weirs to Improve Passage of Potamodromous Fish

“…No fish were sacrificed during the experiments, and efforts were made to minimize stress. Experiments were performed during late spring-early summer, reported by some authors as the main reproductive season for this species [5,44,45], following the previous findings by Amaral et al [41]. Four replicates were carried out for each configuration tested (6 configurations × 2 discharges × 4 replicates = 48 trials) using schools of 5 fish (20 fish tested per configuration) with similar size, which were not individualized.…”

“…Model performance was validated with values of water depth and velocity magnitude measured during experiments in 27 sampling points (Figure 3), located along the ramped weir and both up and downstream (more details in Amaral et al [41]). Velocity magnitude was measured with a flow probe (model FP 101, Global Water Instrumentation Inc., College Station, TX, USA), since water column was not deep, and there was excessive aeration and turbulence in most of the configurations tested, conditions that narrow the use of a 3D acoustic doppler velocimeter.…”

“…The experimental low-head ramped weir used in this study, with a length of 3.00 m and a slope of 10%, was selected based on the results of a previous study by Amaral et al [41], in which it was found that this combination of ramp length and slope may have adversely affected the successful negotiations for the same species. As in Amaral et al [41], the experimental ramped weir, made of maritime plywood, was fixed in the flume ( Figure 1A) at 2.50 m upstream of the acclimation area, a 0.60 m 2 area created by two mesh panels in the downstream zone of the flume, and immediately downstream of the ramp toe, a 0.50 m long zone was established as the approach area.…”

“…The experimental low-head ramped weir used in this study, with a length of 3.00 m and a slope of 10%, was selected based on the results of a previous study by Amaral et al [41], in which it was found that this combination of ramp length and slope may have adversely affected the successful negotiations for the same species. As in Amaral et al [41], the experimental ramped weir, made of maritime plywood, was fixed in the flume ( Figure 1A) at 2.50 m upstream of the acclimation area, a 0.60 m 2 area created by two mesh panels in the downstream zone of the flume, and immediately downstream of the ramp toe, a 0.50 m long zone was established as the approach area. Six different configurations ( Figure 1B) were tested, considering a Control ramp, without substrates, and five retrofitted designs contemplating: a Nature design, where cobbles of 10 to 30 cm were randomly distributed along the ramp; Small Boulders Aligned and Small Boulders Offset, using boulders 7.5 cm long × 7.5 cm wide × 15 cm high, spatially aligned and alternating respectively; and Large Boulders Aligned and Large Boulders Offset, where larger boulders (25 cm long × 7.5 cm wide × 30 cm high) were set in an aligned and alternating design respectively.…”

“…The lower discharge (Q = 55 L·s −1 ) was selected considering the recommendations by Baudoin et al [5] that suggests that the minimum water column over the ramp, referred to as non-limiting for the successful upstream passage of fish, should be between one and two times the body depth of the fish. Regarding the higher discharge (Q = 110 L·s −1 ; double the lower Q), which was previously tested by Amaral et al [41], it was selected to test the effect that higher water velocities and turbulence may have on hydrodynamics of different RD and, consequently, on the passage performance of fish. Discharge was measured by a flow meter installed in the supply pipe.…”