Experimental Investigation of Physical Leaky Barrier Design Implications on Juvenile Rainbow Trout (Oncorhynchus mykiss) Movement

Abstract

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.

Influence of Channel‐Spanning Engineered Logjam Structures on Channel Hydrodynamics

“…Near‐wake ( x < 4 b 0 , x < 2Hs) and far wake ( x > 4 b 0 , x > 2Hs) are indicated in green and orange areas, respectively. Water surface profile is indicated in blue, with $${\overline{\mathbf{H}}}_{1}$$ and $${\overline{\mathbf{H}}}_{2}$$ depicting mean upstream and downstream flow depth, and Δ H being the backwater rise (Figure amended from Müller et al., 2021a).…”

“…Flow around a non‐porous logjam is analogous to flow passing a bluff body, such as a sluice, weir, and tidal gate, creating a zone of elevated pressure upstream of the logjam which causes the flow to diverge around the structure, leading to an increase in streamwise mean velocity beneath it. While a recirculation region forms immediately downstream of the structure, the high velocity region exiting the logjam act like a modified wall jet (Figure 1b in Müller et al., 2021a; Ead & Rajaratnam, 2002). This jet maintains its maximum velocity until a downstream distance of x / b 0 = (4 U 0 / U jet,max ) 2 before commencing a rapid decay (Bhuiyan et al., 2011; Ead & Rajaratnam, 2002).…”

“…In the case of the porous structure (Figure 1a in Müller et al., 2021a), the jet exiting the logjam is anticipated to decrease in strength due to the increased proportion of flow passing through the logjam. Depending on the logjam's physical structure and log arrangement, flow passing through the logjam creates smaller and weaker offset jets (Müller et al., 2021a), and a multiple jet configuration.…”

“…In the case of the porous structure (Figure 1a in Müller et al., 2021a), the jet exiting the logjam is anticipated to decrease in strength due to the increased proportion of flow passing through the logjam. Depending on the logjam's physical structure and log arrangement, flow passing through the logjam creates smaller and weaker offset jets (Müller et al., 2021a), and a multiple jet configuration. The interaction between a wall jet and an offset jet, or between two or more parallel jets, is characterized by three distinct regions (Figure 2; Daubner et al., 2018; Fujisawa et al., 2004; Wang & Tan, 2007).…”

“…These porous ELJs, also referred to as leaky barriers, consist of logs, fallen trees, or branches, sourced from the surrounding area. Installed perpendicular to the flow in the mid to upper catchment region (i.e., where channel width is smaller than key log length) (Linstead & Gurnell, 1999), they span the complete width of the river channel, allowing unhindered base flow through a vertical gap between the bottom of the structure and the riverbed (b 0 , Figure 2), as depicted in Müller et al (2021a) (Figure 1) for an idealized porous (a) and non-porous (b) logjam. Despite this vertical gap, current design guidelines only consider the requirements necessary to facilitate fish movement (Dodd et al, 2016) and disregard the impact of physical logjam design on channel hydrodynamics and alterations to the riverine habitat.…”