Maxium Sediment Capacity of Saltating Sediment Size Mixtures
Watershed Physical Processes Research Unit
2013 Annual Report
1a.Objectives (from AD-416):
Improve predictive capability of the maximum transport capacity of sediment size mixtures in shallow flow
1b.Approach (from AD-416):
Experimental studies will be conducted in laboratory channels to determine the mode and rate of non-cohesive granular material in relation to differences in material characteristics and hydraulic flow regimes. Improved predictions of sediment transport capacities will be determined from experimental data sets, taking into account the effect of the boundary layer thickness in relation to particle size.
This work consisted of experiments in which two sand sizes with d50 of 1.29 mm (coarse sand) and 0.78 mm (medium sand), respectively, were added into shallow water streams of 21.6 l/m in different weight proportions using two independently controlled feeder arrangements. The addition rates of the mixtures were also varied from a low value, where the transported rates were smaller than the addition rates, to a high value, where a sediment wave structure developed. Two photonic probes arranged inline with flow captured signals from the passing particle clusters. Particle cluster velocities were evaluated by cross-correlations of captured signals. Results with single size material showed that the cluster velocities with coarse sand were larger than those of the medium sand for the same solid concentration. For mixtures in which the percentage by weight of medium sand increased from 10% to 90% (total weight of mixture remained constant) the measured cluster velocities decreased. This reduction was 50% when 90% of mixture contained medium sand. This observation could be attributed to inter-particle interactions as well as increased cluster fluxes from medium sand. Post-saltation measurements revealed that even a small percentage (10% by weight) of medium sand in the mixture can cause significant changes in bed morphology. The shallow bed quickly transits into a meandering mode with smaller transport rates. The small scale wave structures were less pronounced when the particle composition of mixtures changed towards the medium size material.