Formation of Refuge Areas Behind Alternative Gravel Mounts for Fishes During Flood Stages

Pietro Beretta Piccoli, and Youichi Yasuda

Department of Civil Engineering, College of Science and Technology, Nihon University, Tokyo, Japan

Abstract: The alternative construction of gravel mounts in channelized rivers is expected to successfully increase the flow heterogeneity and create refuge areas where aquatic animals may hide during floods stages. Five physical models were investigated using three different gravel mount’s heights h_gm above two slope steepness. Comparisons were made with two further experiments above flat gravel bed conditions. Both the discharge Q = 0.0588 m³/s (simulation of medium-sized flood) and the distance between gravel mounts L = 1.60 m were kept constant throughout the research. Water surface profile and velocity field characterize the flow conditions, where two thresholds are used to define refuge areas: mean velocity 10 cm/s and standard deviation of the velocity 7 cm/s. After the installation of gravel mounts, oscillations are formed in the center of the channel, which amplitude increased above steeper channel slopes. Water surface rise is confirmed, but the impact on flood protection could no be quantify outside model conditions. The expected hydraulic conditions for refuge areas are generally met close to the gravel bed behind each gravel mount, while the relative size of the refuge volume per gravel volume is the largest for h_gm/L=0.03. The flow stability inside refuge areas is confirmed with a fast Fourier transform.

Key words: gravel mounts, alternative installation, local flow, refuge area