Effects of an engineered log jam on spatial variability of the flow field across submergence depths

Abstract

Engineered log jams (ELJs) are commonly implemented in rivers to diversify fish habitat, but few studies have verified the hydraulic benefits of ELJs by assessing their effects on the spatial variability and turbulence structure of the flow field. This field and modelling study assessed the effects of an ELJ on various characteristics of the flow field with increasing submergence. A 2‐D hydrodynamic model was applied to simulate flow conditions of the Calapooia River, Oregon, with an ELJ installed at the upstream end of a meander. Output was analyzed using coherent flow structure identification, maps of turbulence measures, and wavelet analysis along linear transects. At the highest discharge (62% ELJ submergence), more and smaller coherent flow structures were identified than in the no ELJ model run, but the magnitudes of turbulence metrics were unaffected. At low flow (1% jam submergence), the presence of the ELJ did not affect the number of coherent flow structures identified and produced only small and localized effects on turbulence measures. Taken together, results indicate that the ELJ primarily affected the flow field by increasing the number of flow structures at higher discharges and submergence depths. However, these changes were small and local to the ELJ. The limited effects of this wood jam over limited spatial scales raise questions about the conditions under which such structures increase hydraulic diversity and highlight the need for more detailed understanding on spatial variability around habitat structures with complex shapes.

Scale dependent spatial structuring of mountain river large bed elements maximizes flow resistance

“…However, use of a constant roughness value is common in 2D modeling and has been shown to both perform well (MacWilliams et al, 2006;L'Hommedieu et al, 2020;Reid et al, 2020; and produce results similar to models with spatially varied roughness . Further, 2D model hydraulic predictions are equally if not more sensitive to topographic inaccuracies than to typical model calibration parameters such as roughness Pasternack, 2011;McKean et al, 2014).…”

Flow and wake characteristics associated with large wood to inform river restoration

“…Design guidelines on ELJs have been summarized in several publications (Brooks 24 , Abbe and Brooks 26 , USBR & ERDC 8 ), with special focus on Australia and the US Pacific Northwest. The majority of previous studies focused on emergent ELJs with different designs 27 , 28 , discharge conditions 29 , and porosity 30 . For ELJs with increasing porosity, the flow passing through the ELJ into the downstream wake (bleed flow) reduced the downstream turbulence levels and scour 29 , 30 .…”

“…The majority of previous studies focused on emergent ELJs with different designs 27 , 28 , discharge conditions 29 , and porosity 30 . For ELJs with increasing porosity, the flow passing through the ELJ into the downstream wake (bleed flow) reduced the downstream turbulence levels and scour 29 , 30 . Emergent ELJs located at the channel wall created reduced velocity just downstream of the structure, a lateral mixing layer extending about one-half of the structure’s width into the flow, and enhanced velocity at the center of the channel, extending to the opposite bank 27 .…”

Logjams With a Lower Gap: Backwater Rise and Flow Distribution Beneath and Through Logjam Predicted by Two‐Box Momentum Balance

“…Instream large wood (LW, defined as logs with diameter m and length m, Keller & Swanson, 1979; Wohl & Jaeger, 2009) increases spatial heterogeneity of flow and sediment transport, providing improved habitat complexity with recognized benefits to fish and invertebrate populations (Bouwes et al., 2016; Faustini & Jones, 2003; L'Hommedieu et al., 2020; Schalko et al., 2018, 2021; Wohl et al., 2016). Wood presence increases the average roughness of stream reaches (Follett et al., 2020; Hankin et al., 2020; Shields & Gippel, 1995), enhancing channel‐floodplain connectivity.…”