Leaky barriers: leaky enough for fish to pass?

Abstract

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.

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

“…Our present results show that the high momentum flow of the primary jet, caused by the barriers blockage effect, did not prevent upstream passage in line with the findings in (Müller et al, 2021), however, only strong swimmers (i.e. Atlantic salmon (Salmo salar, Linnaeus, 1758) and rainbow trout (Oncorhynchus mykiss,) were tested.…”

“…More fish moved upstream when comparing a colored, orange barrier (LB1-orange) against a design sealed with colorless, transparent wrapping (LB1-natural), with the latter emulating a more natural design as the wooden dowels account for the barrier color. The decreased number of upstream passing fish under the natural-colored barrier was accompanied by an increase in time spent underneath the barrier, which was also found by (Müller et al, 2021). Possibly the orange LB1 provides a clearer visual cue for fish passage or may not have been associated with shelter due to its unnatural color.…”

“…manuscript submitted to Water Resources Research though fish movement was not impeded, porosity rather than discharge influenced spatial usage and fish movement, with more fish passing upstream in the presence of a nonporous barrier (Müller et al, 2021).…”

“…Key parameters, namely total barrier height, and the gaps underneath and within the barriers, have been considered (Dodd et al., 2016), but without considering porosity, longitudinal barrier length, and flow alterations in relation to fish behavior. Our previous work on the fish response to leaky barriers revealed that although fish movement was not impeded, porosity rather than discharge influenced spatial usage and fish movement, with more fish passing upstream in the presence of a nonporous barrier (Müller et al., 2021).…”

“…These include large recirculation zones immediately downstream of the structure (Ead & Rajaratnam, 2002) and flow exiting the barrier top (Müller et al, 2021). Most leaky barriers, however, feature high porosity allowing flow through the structure (Figure 1 -3-…”