Large woody debris and river morphology in scour pool formation, dam removal, and delta formation

Abstract

The interactions between large woody debris, fluid flow, and sediment transport are a first-order control on river processes, affecting channel morphology at multiple scales. This thesis investigates how wood geometry and orientation affect scour pool formation around individual pieces of woody debris. We find that the presence or absence of roots on woody debris is the main determinant of scour pool size. The size of scour pools and flow disturbances around woody debris also increases as root cross-sectional area relative to flow increases. The Elwha River Restoration project, which includes two large dam removals, provides an opportunity to investigate how woody debris and logjam dynamics change in response to large-scale wood influxes and changes in channel connectivity. Tracking of individually tagged pieces of woody debris shows wood traveled from the upstream reservoir location to the channel reach downstream of both dams after dam removal. The two former reservoirs followed very different logjam development trajectories, with very little logjam development on the upstream reservoir and unexpectedly rapid log jam development on the downstream reservoir. These differences reflect a greater influx of wood into the downstream reservoir and sediment erosion patterns that exposed 1 - 3 m diameter stumps rooted in the pre-dam valley floor that became anchors for logjam formation. One of the questions for Elwha River Restoration project planning was how woody debris buried in reservoir sediments would affect patterns of reservoir delta erosion during and after dam removal. I use flume experiments to investigate the effects of woody debris on dam removal and delta morphodynamics. Experimental runs with woody debris as compared to experiments without woody debris show increases in channel roughness, channel mobility, and the number of active channels on the delta. Several types of physical aquatic habitats increase with woody debris, including hydraulic variability, number of pools, and lengths of channel edges and delta shorelines. Together these results advance our understanding of how woody debris interacts with channel and delta morphodynamics, and suggest that wood interactions at multiple scales should be considered in dam removals, river restoration projects, and delta restoration projects