An Efficient Method for Measuring the Dispersion Coefficient in Rivers
Carr, Meredith L.
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Permalink
https://hdl.handle.net/2142/83356
Description
Title
An Efficient Method for Measuring the Dispersion Coefficient in Rivers
Author(s)
Carr, Meredith L.
Issue Date
2007
Doctoral Committee Chair(s)
Chris Rehmann
Department of Study
Civil Engineering
Discipline
Civil Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Environmental
Language
eng
Abstract
The use of river velocity and bathymetry data measured with an acoustic Doppler current profiler (ADCP) to estimate the longitudinal dispersion coefficient K is evaluated. If shear dispersion controls mixing, the dispersion coefficient can be estimated from a theoretical formula involving velocity measurements in a cross section. K has typically been measured by costly and time-consuming tracer studies rather than by evaluating the theoretical formula because the velocity field could not be resolved sufficiently. However, ADCP transects, provide detailed velocity and bathymetry data quickly. The use of ADCP measurements was evaluated by investigating uncertainty caused by handling of unmeasured regions of flow, analysis of historical tracer and ADCP measurements at a number of rivers, and by conducting and analyzing simultaneous tracer and ADCP measurements at two rivers at two flowrates. ADCP measurements previously conducted at 20 sites with published tracer studies conducted in the previous 50 years were found to perform as well or better than empirical estimates. Half of the locations had ADCP estimates within 50% of the tracer reference value. Simultaneous measurements on two Florida rivers showed the ADCP estimated performed better at higher flows and on the river with less dead zones and secondary channels. Reach average ADCP estimates of K performed better than local estimates at individual sites and better than empirical methods for estimating K. Parameters representing the shape of the transverse profile and the importance of dead zones were used to characterize the rivers where the ADCP estimate may be more applicable. Using this ADCP measurement to evaluate K , an investigation of the varying relationship between flow and the dispersion coefficient at a specific site was performed, which was not feasible using conventional tracer studies. Estimation of the dispersion coefficient using ADCP measurements has shown the potential to be successful in many cases. This method will reduce the effort and expense of measuring the dispersion coefficient in many rivers, increase the understanding of river mixing, and improve the accuracy of predictions of contaminant transport in rivers.
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