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Perceived as environmental-friendly hydraulic structures, leaky barriers used for natural flood management are introduced into rivers, potentially creating migration barriers for fish. Using sustainable, local materials to construct wooden barriers across river channels in upper catchments, these barriers aim to slow down the flow, reduce flood peaks and attenuate the flow reaching downstream communities. Yet little is known about their impact on hydrodynamics and fish passage. Here, we examined two model barrier designs under 100% and 80% bankfull flow conditions in an open channel flume. These barriers included a porous and a non-porous design, with the latter emulating the natural accumulation of brush, sediment and leaf material between logs over time. Flow visualization and velocity measurements recorded with acoustic Doppler velocimetry characterized the flow field upstream and downstream of the barriers. Our fish behavioural studies revealed that juvenile salmon ( Salmo salar ) movement between downstream and upstream sections of the flume was inhibited by barrier design rather than discharge, influencing upstream fish passage and their spatial preference, indicating the importance of barrier design criteria to facilitate fish movement.

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Rivers have been subject to the construction of numerous small-scale anthropogenic structures, causing alteration and fragmentation of habitats. Despite their impact on fish habitat selection, migration and swimming performance, more hydraulic structures are being added to riverine systems. These mainly have the purpose of harnessing renewable energy or mitigating the impact of flooding, as in the case of leaky barriers that are widely used for natural flood management. By providing a sustainable and cost-effective supplement to traditional hard engineering flood risk management methods, these channelspanning wooden barriers are constructed using sustainable, local materials, intended to slow down surface water and groundwater flow, reduce flood peaks, and attenuate the flow reaching downstream communities. Despite their increasing popularity, little is known about the design implications on fish movement or hydrodynamics. Using scaled laboratory flume experiments we investigate how the physical design of four leaky barriers varying in porosity, length, provision of overhead cover, and color, impacts on fish movement and spatial usage, and the channel hydrodynamics. Our fish behavioral analysis reveals that juvenile rainbow trout (Oncorhynchus mykiss) movement reduces with barrier presence. Upstream passage increases with barrier color but not cover, for shorter rather than longer leaky barriers, and for a non-porous barrier compared to its porous counterpart. Barrier specific flow alterations appear to play a secondary role compared to barrier color. Our study showed that physical barrier design and leaky barrier presence alter fish movement, and therefore care needs to be taken during the design of such natural flood management structures.

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