Impact of Lateral Gap on Flow Distribution, Backwater Rise, and Turbulence Generated by a Logjam

Abstract Logjams may form at natural obstructions and are also used as nature‐based solutions for river restoration and natural flood management. Previous research has described backwater rise due to logjams that span the full channel cross‐section and logjams with a gap between the lower edge of the logjam and the bed. Logjams that fill the channel depth, but not its width, leaving a lateral gap between the logjam and the channel bank, are also common natural formations and the focus of this study. The flow distribution between the logjam and the lateral gap, backwater rise, and wake turbulence are key factors in determining the ecologic and flood risk impact of a logjam. Specifically, relative to a channel‐spanning logjam, the introduction of a lateral gap can reduce backwater rise and increase the potential for trapping particles, such as nutrients or microplastics, within the wake region, but may also promote erosion in the gap. The choice of logjam and gap widths can be used to maximize flow and habitat diversity in rivers, while reducing erosion risk. We present experimental results demonstrating that the flow distribution between the logjam and the lateral gap can be predicted by assuming equal resistance through the logjam and gap sections. Further, we show that backwater rise can be determined from the predicted discharge through the logjam using a momentum balance developed for channel‐spanning logjams. Finally, turbulence generated within the jam was observed directly downstream of the logjam, and, for the densities considered, increased with jam density.