Spoiler baffle patch design for improved upstream passage of small-bodied fish

The cumulative effect of culverts in impeding upstream fish passage is similar to that of large-scale high-head instream structures. The common solution to overcome this impediment is to add ancillary elements, with the goal to lower water velocities and to create discrete low velocity zones (LVZs) for fish to rest. The addition of spoiler baffles is one example of retrofitting structures with ancillary elements, with their efficacy having been proven in various field studies. The scope of the present work is to conduct a detailed laboratory study to evaluate the arrangement of spoiler baffles of different sizes and spatial density. Chosen baffle arrangements are consistent with the present size and spacing recommendations of the New Zealand Fish Passage Guidelines. Flow fields for two different flow rates within different baffle arrangements were obtained with acoustic Doppler velocimeter (ADV) technology. We show that for the standard design, as recommended in the New Zealand Fish Passage Guidelines, the maximum value of mean streamwise velocity and turbulence parameters (turbulent kinetic energy (TKE) and Reynolds stresses) is less than for two other designs we tested. However, the distribution of flow parameters within the plane is significantly different for all tested designs. In general, a shorter size baffle arranged at medium spatial density exhibits highly heterogeneous flow, with numerous LVZs, as well as providing wider pathways compared to the standard design. Wider pathways can also accommodate various fish species, which generally prefer different water velocity and turbulence, with the generated flow field allowing a lower energy cost of fish swimming whilst resting or station holding. Thus, this study shows the viability of using shorter baffles as an alternative fish passage solution for culverts, which can subsequently be validated with different fish species.