Stress partitioning in streams by large woody debris

Using simple theoretical models and field measurements from a springdominated stream, we quantify how large woody debris affect channel hydraulics and morphology at both the local and reach-averaged scales. Because spring-dominated streams have nearly constant discharge, they provide a unique natural opportunity to study flow and transport processes near the channel-forming flow. We first show that the drag on a floating log is identical to the theoretical value for widely separated cylinders at similar Reynolds numbers. We then use simple theoretical models to estimate the partitioning of flow shear stress between woody debris and streambeds. The inferred stress partitioning is consistent with an estimate based on a comparison of local and reachaveraged measurements of the water surface slope. Our measurements show that even though large woody debris cover less than 2% of the streambed, they provide roughly half of the total flow resistance. As large woody debris are added to a stream, the total shear stress increases (because the water depth increases), but the shear stress borne by the bed decreases, as a growing fraction of the total shear stress is borne by the debris. Our analysis shows that simple theoretical models of stress partitioning may provide a convenient mathematical framework for assessing how changes in debris loading affect streams.