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

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.