Evaluating fish passage effectiveness through a sequence of modified
vortex rock weirs
Abstract
Vortex rock weirs (VRW) are often used in natural channel design
applications to maintain channel form and function, provide physical
channel stability, and contribute to habitat enhancement. A balanced
approach is required to achieve conflicting goals of VRWs, which include
providing erosion protection while allowing fish passage under various
water level conditions. This research evaluated a sequence of
asymmetrical rock weirs with 3-dimensional flow. Field assessments
completed between June and September 2018 monitored water level, water
temperature, and surveyed channel features at 10 rock weirs and 11
adjacent pools under different water level conditions. The structural
dimensions and local velocity at each rock weir were compared to the
swimming characteristics of local fish species to determine fish
‘passability’ and suggest best practices for rock weir design and
construction. Results concluded fish passage occurs through gap and
over-weir flow pathways and was most effective under low water level
conditions. Further, appropriate design considerations based on rock
weir gradient, rock weir width, keystone size, and pool length
contributed to 100% fish passage effectiveness under all water level
conditions. To address conflicting goals and the impact on fish passage
for small-bodied fish species, methodology is provided for predicting
local velocity and fish passage effectiveness through rock weir systems,
inform best practices for rock weir design and construction while
balancing the requirements for channel stability and fish passage, and
contribute to fish population management strategies.