This project will study river bedload transport dynamics prior to and following placement of large woody debris (LWD) in the Siuslaw National Forest in the Oregon Coast Range. The team will instrument bedload with passive integrated transponder tags and track their movements prior to emplacement of the LWD. Recent news that no other such projects are currently scheduled or proposed in this National Forest, in addition to the accessibility of the site, creates urgency and opportunity to capture the pre-emplacement sediment dynamics under a range of conditions. The study will (a) provide fundamental, and rare, information regarding bed material sediment transport in field conditions, information that will serve as a valuable reference point for evaluating sediment transport relations based on flume and engineered-stream studies; (b) address the sensitivities of mountain stream sediment transport to downstream debris characteristics and upstream supply limitations; and (c) determine whether grain-size dependence of transport is accentuated or dampened by debris and other channel irregularities.
This project will provide rarely obtained data that will inform not only theoretical understanding of sediment transport in mountain streams, but also stream restoration practitioners about the impacts of LWD placement. One application of LWD is for managing salmonid habitats. The team will interact with National Forest scientists and practitioners in the Pacific NW.
The project examined transport of gravel in a mountain stream by embedding 800 pieces of gravel with passive integrated transponder (PIT) tags, distributing them along the streambed, and tracking their movement. Tagged rocks were evenly divided into four size classes, 8 – 16 mm, 16 – 32 mm, 32 – 64 mm, and >64 mm, and each size class was uniformly distributed over some distance. Those distances were greater for smaller particles, which are expected to move more frequently and have greater transport distances. Results were obtained for an initial deployment of 400 pieces of tagged gravel. Transport was continuously monitored by an array of antennas fixed to the streambed and episodically with a mobile "wand" antenna. None of the tagged gravel passed the fixed antennas before they were turned off in summer 2013, but later mobile tracking did reveal that many particles moved during a storm in September 2013, when the antennas were turned off. We relocated 82% of the tagged rocks and determined their transport distances. For the two smaller size classes, the distribution of transport distances is approximately exponential, which indicates that distances are most commonly small, and bedload transport for these tracers is diffusive. For the two larger size classes, transport distances were predominantly smaller than the measurement uncertainty. The maximum magnitude of this uncertainty is reasonably represented by the smallest (most negative) distance measured, –4 m, so particles are classified as "mobile" if transport distance was greater than 4 m. Average transport distances were calculated from all relocated tracers, even those with apparently negative transport distances. Mobile fraction, average transport distance, and maximum transport distance for each size class are shown in the table. Note that, for twice the grain size, the mobile fraction is about one-half, whereas the average transport distance is about one-quarter. These results are consistent with previous studies that have found that, in conditions of partial bed mobility, an inverse dependence of mobile fraction on grain size compounds the inverse dependence expected between transport distance and size of mobile grains when distances are calculated from all grains and not only those that moved. Measurements of flow stage, discharge, and hydraulic geometry indicate that streamwise gradients in boundary shear stress vary with discharge not only in magnitude but also in sign. For the time series of downstream changes in shear stress shown in the figure, it appears that these changes are most often negative between antennas 4 and 5 and positive between 5 and 6, so we might expect deposition and scour, respectively, in these two stream segments. However, largely due to a fallen tree spanning the channel between antennas 5 and 6, this relationship changes at both the highest and lowest flow stages. For flows that interact strongly with this in-stream wood, we would expect the log to form a backwater and, therefore, decreasing shear stress between antennas 4 and 5. This backwater would lead to a steepened water surface between that backwater and the flow downstream of the log and, therefore, increasing shear stress between antennas 5 and 6. At moderate stages, the data reflect these expectations, but the changes in shear stress reverse sign at both the lowest and highest stages. The trunk of the tree is suspended above the channel bed, so the backwater effect is small when the flow passes beneath the obstruction. Regarding the gradient reversal at the highest stages, two mechanisms might contribute. First, if the flow depth is much greater than the log diameter, the effect of that wood may become small relative to trends in bed slope. Second, high flows passing over the log might be supercritical (shooting flow), and the transition back to critical or subcritical (tranquil) flow might create a standing wave or hydraulic jump downstream of the log and lead to a smaller water surface slope at antenna 6. The study site was part of a longer stream reach to which the Siuslaw National Forest added wood in an attempt to restore rearing habitat for coho salmon, which are listed under the Endangered Species Act as threatened in the Oregon coastal region, which includes the Siuslaw National Forest. The site has therefore served as a focal point for continued collaboration with the fish biologists who design and oversee implementation of this and other restoration projects. In collaboration with the Forest Service biologists, the investigators for this project designed two wood jams within the study reach, and these wood jams will serve, in part, as qualitative tests of the hypothesis that in stream wood structures may be designed to enhance both rearing and spawning habitats for coho salmon. The study site was also used for a field exercise addressing flow hydraulics in a class combining upper-level undergraduates and graduate students.
