Sediment transport in the Rio Grande, New Mexico

This report describes hydraulic data, observed and computed sediment concentrations, and size distributions of bed-material samples for 293 observations and presents the results of a series of investigations at six sediment stations on the Rio Grande in New Mexico. The Rio Grande at Otowi Bridge, near San Ildefonso, N. Mex., the farthest upstream station, has a typical pool-and-riffle channel configuration, and the bed material consists of both sand and gravel. The Rio Grande at Cochiti and at San Felipe has a sand-gravel channel, but the marked controlling influence of riffles is lacking. The three downstream stations, near Bernalillo, at Albuquerque, and near Belen, have sand-bed channels. Slopes through the 110-mile reach range from about 12 feet per mile at Otowi to 4 feet per mile near Belen. Transport rates of bed material computed by the modified Einstein method are related to the simple hydraulic variables, discharge, unit discharge, and velocity, for each of the stations. The sediment transport relations are found to vary systematically in a downstream direction, that is, with bed-material size, and to fall into two distinct groupings, one for the confined or partially confined sections and the other for the laterally unrestricted sections. Sediment transport rates are greater at the wide sections for higher discharges and greater at the narrow sections for the lower flows, probably because the wider sections have a tendency to aggrade and channelize at the lower flows. Flow characteristics differ markedly for the pool-and-riffle channel at Otowi and the sand-bed channel near Bernalillo. At Otowi, the depth, slope, bed shear stress, resistance to flow, and the bed-material size all increase with increasing discharge. Near Bernalillo, slope and bed-material characteristics are approximately constant, flow resistance, which is dependent upon bed configuration, decreases with increasing discharge, and the range in bed shear stress, compared to Otowi, is very limited. The mean velocities and sediment discharges for the two stations are comparable, in spite of the wide differences in flow characteristics. Sediment transport rates are found to relate reasonably well to "effective shear," a measure of the shear stress resisted by the grain roughness only. However, the variable size distribution of the bed material introduces considerable scatter for the Otowi relation. The scatter is reduced by converting the shear-transport relations to dimensionless form, similar to the parameters used by Einstein (1950, Bagnold (1956). and A. A. Bishop ("Sediment transport in alluvial channels: a critical examination of Einstein's theory: Colorado State Univ. Ph. D. thesis, 1961). Consideration of the curves developed by Bishop to predict total bed-material discharge and of the dimensionless relations for the Rio Grande indicate that to be generally applicable, the shear stress-transport functions probably should include additional parameters to explain the influence of temperature and the effects of flow depth. Systematic changes in bed-material characteristics for Otowi introduce changes in the sediment transport, independent of the hydraulics of the flow; a single simple parameter such as median diameter or a representative grain size is, therefore, not sufficient for characterization of the bimodal distribution of the bed material in the shear stress-transport functions. The influence of temperature on sediment transport for the Rio Grande data is apparent, but a precise quantitative evaluation is impossible because the effects of temperature changes are not independent of the effects of interrelated changes in other