Reports: AC847935-AC8: The Role of Sediment Supply in Morphodynamics: Bars, Bends, and Planform

William E. Dietrich , University of California (Berkeley)

Although sediment supply is thought to be a fundamental control on channel morphology, we have little understanding of how changes to sediment supply affect bar-pool morphology in straight and meandering channels. Changes to sediment supply are common due to climatic and land use changes, and interpreting the role of sediment supply on morphology is critical to interpreting stratigraphic data. 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 river morphology, particularly for meandering rivers. This is due, in part, to a lack of observation of supply effects on channel morphology. Here we focused on the response of bar pool morphology to sediment supply changes by compiling a database on the controls on channel morphology from the literature, and conducting experiments focusing on bar response to sediment supply changes.

We compiled a database of gravel bed meandering channels to explore the controls on meandering and guide our experiments. By accounting for lateral mobility, we found that gravel bed meanders with cutoffs had very low bankfull Shields stresses relative to gravel braided streams, meandering channels without cutoffs, and sinuous channels without bars (see figure). The Shields stress for gravel bed meanders in the field ranged from 0.016 to 0.046 with a mean value of 0.032. These very low Shield stresses reflect that gravel transport rates are very low, with strongly selective transport, and gravel transport may be limited to areas of high shear stress (pools and the upstream end of point bars). Calculation of the Rouse Number for coarse sand (a measure of whether a given sediment travels as bedload or suspended load) indicates that coarse sand should be transported as bedload in these streams. Indeed, field measurements of bedload in gravel bed meanders find that 30-60% of the bedload is sand, even if the bed is entirely gravel.

We examined the response to increased sediment supply at the Richmond Field Station (RFS_ and the University of Minnesota’s Outdoor Stream Lab (OSL). The RFS experiments tested supply response in a scaled-down gravel bed channel while the OSL experiments tested the supply response in a near field scale sand bed meander. At RFS, we fixed the channel boundary following experiments with alfalfa to provide bank strength. The channel walls were roughened sheet metal to test the effect of supply changes on a 45-cm wide, 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 to assess the effects of sediment supply changes in non-migrating sinuous channels.  The channel response could be divided into two phases: a transient response that included cross-stream morphologic adjustments, and a long-term response that included a new water surface slope. During the transient phase, which lasted approximately 55 hours, the bed fined and cross-stream topographic relief decreased due to pool filling. Once the water surface slope and sediment flux out equilibrated, the channel morphology began to evolve to a topographic form very similar to that prior to the feed increase, only at a 30% higher slope.

The OSL experiments were conducted in conjunction with researchers from Utah State University, the University of Minnesota, and Johns Hopkins University. At the OSL, we ran the flume at an equilibrium supply, increased the feed by approximately five times, and reduced the feed back to the equilibrium value. 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 was very similar to that prior to the feed increase. The sand bedded channel showed a much stronger topographic response to changing sediment supply than the scaled down gravel channels at the Richmond Field Station.

We examined the channel response to decreased supply in both a straight flume with alternate bars and the sinuous flume with fixed walls at RFS. In both cases, the primary response was a decrease in the water surface slope through incision into pools. In the straight fume the decreased sediment flux caused bar migration to decrease and increased bar wavelength until downstream bar migration stopped. During this period bar relief decreased as pools filled in and bars were laterally eroded leaving behind narrow emergent bars and a relatively flat channel bed. The topographic response in the sinuous flume was similar except bar wavelength did not adjust. The sinuous channel incised into the pools in bends leaving the point bars behind as terraces. This had the effect of narrowing the channel width and while increasing overall topographic relief, decreased the topographic complexity in the low flow channel.

Finally, we tried testing the effects of sediment supply change in alluvial laboratory channels. These channels differ from the fixed wall channels in that the channel high rates of sediment supply may be partially accommodated by alterations in migration rate, but also in that aggradation leads to channel infilling and potentially avulsions and cutoffs. We have found that for channels with high bank strength (generated by alfalfa sprouts), channel migration is very localized, and even slight overfeeding of sediment leads to channel infilling. We will continue to conduct experiments examining the dynamics of sediment supply in alluvial channels for the remainder of 2011.  

This project has supported the Ph.D. of Christian Braudrick (expected completion 2012), 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.

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