Abstract:
Using simple theoretical models and field measurements from a
spring-dominated stream, we quantified how large woody debris affects
channel hydraulics and morphology at both the local and reach-averaged
scale. 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 quantified the reach-averaged
flow resistance due to woody debris by comparing local and reach-averaged
measurements of the water surface slope. Our measurements show that, even
though large woody debris covers only about 2% of the streambed, it
provides roughly half of the total flow resistance. Our measurements also
imply that the drag coefficient of large woody debris is identical to the
theoretical coefficient for widely separated cylinders, implying that
simple theoretical models can be used to estimate the partitioning of flow
shear stress between woody debris and streambeds. As large woody debris is
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.