How Engineered Log Jams (elj) of Different Designs Affect Channel Morphology and Hydraulics In A High Energy Gravel and Sand Channel

The addition of large wood is a common feature of river restoration projects in Western Washington State where wood jams comprised of logs over a meter in width were once a distinguishing characteristic of many rivers. Wood is typically added in a defined jam structure, referred to as an Engineered Log Jam (ELJ). ELJ are incorporated into restoration projects with the goal of influencing local and reach scale channel bathymetry and velocity patterns and, through these effects, improving aquatic habitats. The impact of LWD on channel morphodynamics is largely a function of the residence time of the logs in the channel. Stable wood jams may extend into the channel creating shade and over water cover, scour a downstream plunge pool, and enhance groundwater to surface water exchange. Channel morphodynamics adjust to the presence of the wood and the channel planform is stabilized by the wood as the wood decays in place. ELJ design and construction methods have adjusted over time to increase the likelihood that ELJ will remain in place through large floods. Log jam design includes as variables the number of logs, the number with rootwads, along channel roughness length of ELJ, structure height, structure spacing, and the orientation of the logs. This study focused on the impact of ELJs on channel bed topography, flow complexity, and overall reach complexity in the South Fork Nooksack River, a high energy, gravel and sand bed channel in the Cascades Range in Washington State. Two adjacent reaches were studied over a flood season. The upstream reach has been the focus of multiple restoration efforts since 2006 and numerous ELJ of different sizes and shapes have been constructed. Notably, the ELJ were designed to increase complexity in the channel morphology by maintaining gravel bars and islands, creating self-sustaining side channels with lower flow velocities in winter and spring, and forming deep pools of cooler water at the jams. The downstream reach has remained a single thread channel with leveed banks and few restoration projects. The sizes and designs of the ELJ in the upstream reach vary, enabling a comparison between jam types, ages, and river location. The influence of the ELJ on channel bathymetry and hydraulics was measured over a flood season to quantify the impact of ELJs on channel bed topography, flow complexity, and overall geomorphic unit complexity in a high energy, gravel and sand channel. Accurate predictions of ELJ local and reach scale impacts are needed for restoration projects to achieve maximum results. ELJ design and size were not as important as ELJ spacing and location in the channel cross section where the goal is influencing channel flows and morphology. Bedform sequence and reach scale morphologies will adjust over time to the presence of the ELJ depending on how multiple ELJ create feedback between bed sediment and flow velocity patterns.