Vertical distribution of velocity and suspended sediment, Middle Rio Grande, New Mexico

Abstract

Samples of suspended sediment and measurements of velocity at 3 to 5 points in a vertical are used to define vertical sediment concentration and velocity profiles for cross sections in reaches of the Rio Grande near Bernalillo and Socorro, and of the Rio Puerco near Bernardo, N. Mex. Velocity profiles show the velocity to vary logarithmically with depth. The coefficient of turbulent exchange, k, is found to decrease systematically for the transition zone and upper regime flow. For lower regime flow, k varies from about 0.4 to 1.2, indicating three-dimensional flow effects. Assuming a conventional logarithmic velocity equation to be applicable, it is found that the range of values of Ks, the height of the representative roughness element, is dependent upon the condition of the channel bed. The ratio of maximum to minimum Kf is about 20,000 for lower regime flow, 40 for transition and upper regime flow, and 4 for flow over a clay-armored bed. However, the relation of the average K, value to the average representative bed-material size for each of the reaches is about constant, and is approximately 400 times greater than d®, the size of bed material for which 65 percent by weight is finer. The Einstein-Barbarossa bar-resistance curve is shown to describe frictional losses for individual cross sections of the Bernalillo and Socorro reaches with reasonable accuracy. Twenty of the 23 computed values of mean velocity are within 25 percent of the measured values. When an average cross section is used for the Bernalillo reach, the computed mean velocities are within 15 percent of the measured values. Suspended sediment is more uniformly distributed through a vertical than predicted by conventional theory. The measured exponent of the sediment distribution, Z\, varies with about the 0.55 power of the fall velocity. The fall velocity is reduced appreciably by the presence of nigh concentrations of suspended fine material. The ratio of the sediment transfer coefficient to the momentum transfer coefficient, /3, appears to be a direct function of particle size and bed configuration. For a given particle size the highest values of /3 are associated with upper regime flow, while for lower regime flow, /3 values are close to unity.