Effect of engineered logjams on hydrodynamics and fish response

Due to human interventions such as river channelization, the diversity of the flow, sediment, and wood regimes in rivers has decreased. A common measure to locally reestablish flow heterogeneity are nature-based solutions such as logjams with the aim to create or increase habitats for aquatic organisms such as fish. To optimize the design of nature-based solutions and to leverage the habitat creation for fish, we need to create a better understanding of the underlying flow and turbulence characteristics due to nature-based solutions. Laboratory experiments were conducted to investigate how different logjams affect the flow and turbulence properties. High-speed imaging was used to characterize the flow field at the surface and at a vertical plane at the channel centerline. The experiments investigated logjams differing in solid volume fraction, submergence level, as well as log alignment. All tested parameters altered the wake region. The results of the log alignment indicate that a random arrangement can lead to an evenly reduced velocity in the wake and lower turbulence levels. In contrast, a regular arrangement can lead to jets going through the structure and entering the wake unblocked, resulting in higher turbulence levels. The different turbulence levels may have implications for fish response. As a next step, field measurements are planned to complement laboratory experiments. Selected engineered logjams will be investigated at a restored river reach at the Emme River in Switzerland. Specifically, flow measurements will be obtained through drone images and Acoustic Doppler Velocimetry and compared to results of fish abundance.