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

Engineered log jams (ELJs) are commonly implemented in rivers to stabilize streambanks and diversify fish habitat, but few studies have verified the hydraulic benefits of ELJs by assessing their effects on the flow field. This field and modeling study assessed the effects of an ELJ on various characteristics of the flow field with increasing submergence. A 2D hydrodynamic model, iRIC SToRM, 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 the ELJ primarily affected the flow field by increasing the number of flow structures at higher discharges and submergence depths, with limited evidence of significant other impacts at high or low flows. Designs seeking to produce more profound effects for reducing erosive energy of flow and providing low energy refuge habitats at higher flows would benefit from structures that are located lower in the channel and that occupy a greater proportion of the channel width.