Measuring the Dispersion Coefficient with Acoustic Doppler Current Profilers

A method is evaluated for estimating the longitudinal dispersion coefficient K from velocities and bathymetry measured with an acoustic Doppler current profiler (ADCP). If shear dispersion controls the mixing, the dispersion coefficient can be estimated from measurements of velocity and depth in a cross section. The dispersion coefficient has typically been measured by costly and time-consuming tracer studies because the velocity field could not be resolved sufficiently before the flow changed. However, ADCP transects, which now are routinely used to measure discharge, provide detailed velocity and bathymetry data quickly. The dispersion coefficient is estimated from ADCP measurements from the United States Geological Survey and compared with estimates from dye studies. Half of the estimates of K fall within 50% of the values from tracer studies, and 85% are within a factor of 3. The ADCP method is at least as accurate as the best empirical formula considered. Both the comparison of field data and an analysis with theoretical velocity profiles suggest that the error in K will be largest when the velocity profile is nearly uniform.