Stress partitioning in streams by large woody debris

Using simple theoretical models and field measurements from a spring-domina ted stream, we quantify how large woody debris affect channel hydraulics an d morphology at both the local and reach-averaged scales. Because spring-do minated streams have nearly constant discharge, they provide a unique natur al opportunity to study flow and transport processes near the channel-formi ng flow. We first show that the drag on a floating log is identical to the theoretical value for widely separated cylinders at similar Reynolds number s. We then use simple theoretical models to estimate the partitioning of fl ow shear stress between woody debris and streambeds. The inferred stress pa rtitioning is consistent with an estimate based on a comparison of local an d reach-averaged measurements of the water surface slope. Our measurements show that even though large woody debris cover less than 2% of the streambe d, they provide roughly half of the total flow resistance. As large woody d ebris 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 a nalysis shows that simple theoretical models of stress partitioning may pro vide a convenient mathematical framework for assessing how changes in debri s loading affect streams.