Effects of sediment supply and slope on channel topographic roughness and sediment transport
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We investigate evolution of mountain channel morphology and riverbed surface roughness by conducting laboratory experiments. The experimental flume is 4m long by 0.1m wide with a working length of 2.5m. We control initial sediment size distribution, flume slope, water discharge, and sediment feed rate. Measurements include topographic profiles, flow depth, surficial grain-size distribution, sediment transport rate, and sediment size distribution. Experiments begin with a gravel bed of a broad sediment size distribution, at two initial flume slopes: 8.2% and 12.4%. Discharge is held constant until transport rates and topographic changes indicate the system is at near steady state. Coarse sand is then fed into the channel at 1,000 g/min as a means to perturb the system. Sediment feed is held constant until the perturbed bed reach steady-state conditions. The feed is subsequently ceased and measurements continue until sediment transport rates and topography stabilize. These laboratory experiments provide first-hand observations of channel systems evolving after perturbations. Transport rates decay exponentially following perturbations and remain very low when the channel bed is stabilized. The introduction of coarse sand acts to smooth the channel bed by filling in topographic lows in the 8.2% sloped channel. At a 12.4% slope, increased mobility of sand allows steady state conditions to be met with little smoothing of the bed. The sand also increases the mobility of coarser sediment that was previously stable, likely due to local surface smoothing at grain scale. The increased fraction of surface sand cover maintains increased scouring and mobilization of coarser grains. These post-perturbation mechanisms are interpreted to be responsible for topographic adjustments as the system readjusts towards new steady-state conditions. Surface sorting and transporting distributions reflect high sand fractions well after perturbations have ceased. This suggests that brief pulses of fine sediment can increase coarse sediment mobility for prolonged periods.