LONGITUDINAL DISPERSION IN MEANDERING CHANNELS

Abstract

Longitudinal dispersion is a bulk one-dimensional process by which a flowing stream spreads and dilutes a pollutant in the direction of flow. Therefore knowledge of longitudinal dispersion coefficient is essential for prediction of pollutant concentration in a stream. Considering that natural streams are rarely straight, the knowledge obtained from tests in straight channels is not always adequate to understand the basic mechanism of dispersion in natural streams. A large number of bends are always present in meandering rivers and these play an important role in the dispersion process. The dimensionless longitudinal dispersion coefficient in meandering channels is much greater than that in the corresponding straight channels with the same width to depth ratio and the same roughness distribution. Dimensional analysis indicates that the dimensionless longitudinal dispersion coefficient D,/RU in meandering channels is a function of, resistance to flow U/U^, width to depth ratio B/D, central radius to width ratio r /B, central angle of the bend & and vertical roughness density \ , expressed as bh/s(e+h), where b is the transverse length of the roughness element, h the height of the element, s the center to center longitudinal spacing of the elements and e the clear spacing between the elements. Accordingly, the objective of the present investigation is to study the effects of the above mentioned parameters on the longitudinal dispersion coefficient in meandering channels. In the dispersion experiments of the present study, the (ii) pollutant or the tracer used consisted of salt solution in water made neutrally buoyant with the help of denatured spirit. The injection of the pollutant was done instantaneously as a line source across the width of the flume. The concentration versus time records were obtained by using the conductivity probe - conductometer - amplifier - strip - chart recorder system. The longitudinal dispersion coefficient DL was first computed by the change of moments method, and then this value of DL was further improved by the Routing procedure, which employed the Gauss-Hermite quadrature technique. A systematic experimental programme was planned in three phases to study the effect of the above mentioned bend geometry and flow parameters on the dimensionless longitudinal dispersion coefficient. Experiments were carried out in the Hydraulic Laboratory, University of Roorkee, Roorkee. A sloping floor (slope = 0.024) covered with wooden planks was made for easy fabrication of various meandering channels, by bending and fixing of galvanized iron sheets as side walls. In all, seven meandering channels were fabricated and 109 dispersion runs were conducted in these meandering channels. The experimental data was subjected to analysis and by using the grid search method of optimization, an emperical predictive model was developed for predicting the dispersion coefficient in meandering or straight channels having smooth or rough bed conditions. The model was then validated by using the laboratory dispersion data of other investigators and the available field data. The performance of the model was found to be satisfactory.