Laboratory study on fish behavioral response to meandering flow and riffle-pool sequence driven by deflectors in straight concrete flood channels

Sectional channelization of natural rivers for flood mitigation causes discontinuity of the riverine corridor, leading to unfavorable conditions for fish movement in the longitudinal direction. To minimize the negative effects, hydraulic structures (e.g. deflectors) are proposed as potential restoration measures in concrete flood channels. In this study, indoor flume experiments were carried out to assess the variation of flow field (velocity magnitude, turbulent kinetic energy, and Reynolds stress) and bed morphology with the inclusion of deflectors. The flow and sediment regimes were then compared in the presence or absence of deflectors. The experimental results indicated that deflectors triggered a non-uniform flow pattern and stimulated riffle-pool sequence formation through local scouring and subsequent deposition. To better understand the response of predaceous chub (Parazacco spilurus) and mud carp (Cirrhinus molitorella) to deflectors, fish trajectories, distribution, and number of transits were analyzed using a video monitoring system. The results suggested that fish responded differently to the baseline (without deflector) and deflector scenarios: (1) fish struggled against the oncoming current when crossing the physical barrier in the channelized section; (2) fish moved and rested in the diverse habitats created by deflectors. Additionally, the presence of sediments in the channel bed was essential in attracting fish to the channelized section. Therefore, it was concluded that using deflectors at appropriate intervals and allowing the existence of sediments in flood channel sections can benefit fish movement and enhance channel naturalization.