STUDIES CONCERNING LONGITUDINAL DISPERSION. IN OPEN CHANNELS

Abstract

Whenever a pollutant is discharged into a river it becomes mandatory that its concentration at all points downstream, where an intake may have to be placed, be known. If the pollutant is conservative, after a certain distance, decrease in its concentration due to its mixing with ever-increasing volume of water is a result of longitudinal dispersion. The most popular model for the longitudinal dispersion is Taylor's model. This model is based on the assumption of a constant dispersion coefficient, DL -thx_aughaut_ the dispersion reach. Recent studies have indicated that DL increases with the distance from the point of injection. Many investigators have studied the variation ofDL with thannel characteristics and flow parameters but none of them have considered the longitudinal distance from the point of injection as a parameter. Moreover none of the relations developed for DL give satisfactory result under all conditions. Hence a study of the available data was undertaken to study this variation. As a result of this study a predictor for DL was derived which incorporates this distance as a parameter and shows high correlation. Some of the recently developed finite difference schemes were studied with & view to incorporating a variable dispersion coefficient with longitudinal distance. But the methods studied herein were not found to give satisfactory ii results. Finally a computer program has been developed which uses the derived predictor equation to calculate DL in various sub- reaches and applies Fischer's solution of longitudinal dispersion equation to calculate concentration-time profile at the end of each sub-reach.