Author(s): Roman Weichert

Linked Author(s): Roman Weichert

Keywords: Ecohydraulics; Vertical slot fishway; Upstream migration; Velocity fluctuations; Small fish

Abstract: Vertical slot fishways are often constructed to provide migration pathways for fish obstructed by a dam or weir. German design criteria for fishways consider fish related requirements, as e.g. minimum slot width (fish thickness), minimum flow depth (fish height) or maximum flow ve-locity (swimming ability). Nevertheless, a design parameter always acts as a substitute for a complex reality, i.e. do not adequately reflect the variability of hydraulics and fish biology in a vertical slot fishway. Design velocities of vertical slot fishways at German waterways typically range between 1.5 m/s and 1.8 m/s. Comparing these values with results from fish swimming capacity tests suggest that the physiological needs of small-bodied fish with weak swimming capacity are not well acknowledged. However, anecdotical observations of small fish passing such fishways indicate knowledge gaps of the interplay between swimming ability, fish behavior and hydraulics of ver-tical slot fishways. In this context, it is not clear whether swimming performance is underesti-mated or flow conditions in slots are more favorable than assumed. Focusing on flow, recent velocity measurements suggest that rather velocity fluctuations than near-wall low velocity zones may be beneficial to passage success. The Federal Waterways Engineering and Research Institute (BAW) and the Federal Institute of Hydrology (BfG) are planning ecohydraulic tests to study whether velocity fluctuations can af-fect swimming behavior and passage success of small fish or not. A major challenge is to measure flow velocities and fish passages simultaneously. Velocity measurement instruments cannot be used as they would disturb fish behavior. Additionally, due to the spatially and temporally ran-dom character of the fluctuations, it is not possible to exactly determine the relevant velocities in the moment when the fish is passing the slot. To overcome these challenges, we developed flume experiments with three different flow scenarios at a single slot: A) quasi constant flow without significant fluctuations as a baseline, B) randomly fluctuating flow with natural velocity-fluctuation spectra comparable to those in vertical slot fishways and C) artificially varied flow with defined repeated large-scale fluctuations representing the largest fluctuations of scenario B generated using rotatable fins. In our contribution, we focus on the results of the hydraulic pre-study. We present turbulent characteristics of flow in a typical vertical slot fishway using ADV-measurements based on the IPOS framework. We also show results of PIV measurements in a scale model that have been performed to create the hydraulic setup for scenarios A-C. As a conclusion, we demonstrate that it is possible to specifically influence velocity fluctuations in a way that the initial research ques-tions can be addressed in ecohydraulic experiments.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221043

Year: 2022