Author(s): Dan Nilsson; Anders Andersson; Sofia Larsson; Robin Andersson; Mats Billstein
Linked Author(s): Robin Andersson
Keywords: Relative submergence; Turbulence; Experiments; Boulder; Fishway
Abstract: Regulated rivers often contain engineered structures, such as dams, that obstruct fish migration. To mitigate these impacts, a variety of fish-passage solutions have been developed, including bypass channels and technical, semi-technical, and nature-like fishways. Rock-ramp fishways are a type of nature-like fishway, consisting of an inclined channel with boulders arranged to mimic natural riffles. These boulder arrangements create preferential migration corridors and low-velocity resting areas for fish. Reservoir operations can cause substantial fluctuations in flow depth within rock-ramp fishways, thereby changing the relative submergence of the boulders. The relative submergence, RS = h/H, is defined as the ratio of flow depth h to boulder height H. Variations in RS have been shown to modify the local hydraulic conditions, including the mean velocity distribution and turbulence characteristics (Nilsson et al., 2025). Silva et al. (2012) showed that fish altered their spatial use of the flow and tail-beat frequency in response to changes in turbulence intensity (TI), preferentially occupying regions with moderate TI while avoiding highly turbulent cores. They also identified Reynolds shear stress (RSS) as a key turbulence parameter, with variations in RSS being strongly linked to fish passage performance. Motivated by these findings, the present study examines how changes in RS, at approximately constant bulk velocity, modify the spatial distribution of TI and RSS in a laboratory-scale rock-ramp fishway.
Year: 2026