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Sediment supply is widely acknowledged as a fundamental control on channel morphology, but the mechanisms by which sediment supply affect channel planform and dynamics are understood only qualitatively. This is problematic because understanding sediment supply effects is crucial to understanding past and future changes to river basins. Numerical models do not explicitly account for sediment supply effects on bed topography and planform morphology, and therefore cannot predict how changes in supply might affect channel morphology. This is due, in part, to a lack of observation of supply effects on channel morphology. We hypothesize that changes in topographic steering link sediment supply and bar morphology. This hypothesis posits that increased sediment supply would enhance topographic steering over the bar, causing the bar to advance laterally into the pool. Contrarily, decreased supply would diminish topographic steering, causing bars to shrink laterally and pools to expand. We are testing this hypothesis using flume experiments and will use those results to test and modify a quasi 3-D hydrodynamic model (IRIC) to include the effects of sediment supply.

We first examined this hypothesis in a 28-m long, 0.86-m wide flume with a constant discharge of 5.4 l/s and sand with a median grain size of 0.8 mm. The conditions were sufficient to support alternate bars downstream of the upper 8-10 m. Cutting off the feed initially resulted in an increase in flux at the bottom of the flume as fines were released from upstream. This resulted in shorter bar wavelengths, increased celerity, and no change in bar height. Once the initial pulse of fine sediment moved through the flume, the sediment flux began to gradually decrease. With the decreased sediment flux, bar migration decreased and bar wavelength increased until migration stopped. As the sediment flux decreased the bar height decreased as the pools filled in. These shallower pools then eroded the edges of the bar resulting in a wider pool and narrow bar, as our theory would predict. Overall, bar wavelength was the most responsive variable to the change in sediment supply, with shorter wavelengths associated with higher flux, but the bar height was relatively constant until the bars stopped migrating and became emergent.

Previously, we were able to create a meandering river in the laboratory using a combination of alfalfa sprouts to provide strength, and fine sediment to fill in chutes between the bar and the floodplain. The sinuosity in the previous experiments was low relative to meandering channels in the field. To increase sinuosity, we increased the bank strength by tripling the alfalfa density. This slowed the bank erosion rate and limited chute developments which are a locus for cutoffs. The resulting channel was more sinuous than previous experiments (up to a sinuosity of 1.5), but the channel eventually avulsed due to a feeder malfunction. At the conclusion of these experiments, we fixed the walls in place using roughened sheet metal to test the effect of supply changes on a non-migrating sinuous channel. We ran the flume with a steady feed rate until the sediment flux, topography, and bed material size equilibrated. We then doubled the feed rate and measured the morphological response.

The morphologic changes included a transient response while the water surface slope and sediment flux reached a new equilibrium. During the transient phase, which lasted approximately 55 hours, the bed fined and some of the bars responded by expanding upstream and filling the pools with coarse sediment, while the topography of bars in tight bends with little room upstream to expand remained virtually the same. The equilibrium water surface slope for the high feed was 30% higher than the equilibrium supply. Once the water surface slope and sediment flux out equilibrated, the channel morphology began to evolve to a topography very similar to that prior to the feed increase, only at a higher slope. This change did not happen immediately, however, as the channel topography finally stabilized 100 hours after the feed change. We are still analyzing data from this experiment and have just begun analyzing changes to the stress field using IRIC.

To supplement our RFS experiments, we assisted in an experiment at the St. Anthony Falls Outdoor Stream Lab (OSL) at the University of Minnesota in October 2009. These experiments were sponsored by National Center for Earth-Surface Dynamics and conducted in conjunction with researchers from Utah State University, the University of Minnesota, and Johns Hopkins University. The OSL, at 3 m wide and 0.5 m deep, is a sinuous near-field-scale flume that does not migrate. Because it is so large, we can measure velocity profiles, which cannot be measured in our smaller flume at RFS, where we must rely on numerical models to quantify the stress field. At the OSL, we ran the flume at an equilibrium supply, took detailed measurements of the bed topography, water surface elevation, and velocity, and then increased the feed by approximately five times. During the equilibrium run, the topography at the central bar was steady. The feed increase caused the bar grew upstream and laterally into the pool, and the pool filled in somewhat. The feed was then returned to equilibrium and the resulting bar morphology to a morphology was very similar to that prior to the feed increase.

To date we have conducted several experiments on supply effects on channel morphology. Over the next year, we will drop the feed in the sinuous fixed wall flume at the RFS, repeat the sinuous fixed wall experiment under freely migrating conditions (using alfalfa to provide bank strength), and continue working with numerical models to predict the magnitude and duration of supply effects on bar morphology.

This project has supported the Ph.D. of Christian Braudrick (expected completion 2011), enabling him to expand on previous work on meandering rivers to include the specific effects of sediment supply on channel form. Two undergraduates (Cory Hiraga and Sumner Collins) and one Master's student (Russell McArthur) have had their first exposure to research as laboratory assistants.