The effect of bends on dispersion in streams

Abstract

Bends in streams induce secondary currents that alter the rates of both transverse mixing and longitudinal dispersion. Within a bend the transverse mixing coefficient depends on the square of the mean velocity, cube of the depth, and inversely on the shear velocity and square of the radius of curvature, as verified by a laboratory experiment. The longitudinal dispersion coefficient depends on the channel geometry, velocity distribution, rate of transverse mixing, and a dimensionless parameter that includes the mean velocity and length of an average bend. A numerical program for predicting dispersion coefficients in streams differs from previous theories by including the effect of the alternating direction of curvature in a series of bends; the difference is important for wide streams but not for narrow ones. Experimental findings in two streams, the Green‐Duwamish in Washington (width 20 meters), and the Missouri near Omaha, Nebraska (width 180 meters), verify the numerical program.