DISCRIMINATING MEANDERING AND BRAIDED CHANNEL PATTERNS ON THE BASIS OF DISCHARGE AND SLOPE USING A PHYSICAL LABORATORY MODEL: THE EMRIVERTM RIVER PROCESS SIMULATOR

River channel pattern is controlled primarily by sediment transport rate and stream power, which are largely a function of channel discharge (Q) and slope (S). Discriminators of the form S=aQ^-b have been shown to quantitatively demarcate the transition between meandering and braided rivers. We used a commercially available physical model, the Emriver^TMRiver Process Simulator (a stream table), to investigate whether this relationship could be produced under laboratory conditions.

The model is a metal box 2.13 m long, 0.91 m wide, and 0.15 m deep. The sediment is well sorted, ground plastic with a D₅₀ of 1.15 mm and a density of ~1.4 g/cm³. Prior to each experimental run, sediment was graded to a thickness of 2-4 cm, and an 8 cm-wide straight channel was excavated. Discharge was calibrated to a head gauge within the upstream inflow reservoir. Outflow was through a standpipe which could be adjusted vertically to effect channel slope. After each run, channel slope was measured with a laser level and point gauge. Thirty-three runs were conducted under various conditions of discharge and standpipe elevations. During each run, the channel was allowed to aggrade and evolve until it reached a state of dynamic equilibrium. Run times were typically 15-50 minutes.

Data were plotted on a log-log graph of dimensionless slope versus discharge. The data form two clearly defined fields of meandering and braided channels. The line demarcating the fields is defined by the equation S=2x10^-9Q^-1.70, a form comparable to data for natural rivers. However, coefficient a and exponent b are both markedly less than those for natural rivers, which are typically 10^-2-10^-4 and 0.25-0.44, respectively. These results indicate that the transition in channel pattern under our experimental conditions occurs at a lower slope for a given discharge and/or at a lower discharge for a given slope than for natural systems. This may be due to the lower density of the experimental sediment relative to natural sediment, or due to the effects of the relatively high experimental slopes on the force balance on grains (i.e., mobility). Despite the limited size and simplicity of the experiments, results are comparable to data for rivers, which speak to the “unreasonable effectiveness” of physical laboratory models and encourage their use to study processes of natural systems.