Experimental and Numerical Study of Advection- Dispersion of Pollutant in a Gravel Bed Rivers

Water quality assessment of rivers, in order to insure human health and environmental sustainability, is one of the most important purposes of the physical and numerical models of solute transport in rivers. In this research, some tracer experiments were done and also the numerical model of OTIS was used to simulate solute transport. This model predicts the solute BT curve for a given hydraulic and geometrical parameters at downstream sections of river with respect to concentration of pollutant resource at upstream boundary condition. The experiments were conducted in a flume with length, width and depth of 12, 1.2 and 0.8m. Two longitudinal slopes of 0.004 and 0.007 and three discharges of 7.5, 11.5 and 15.5 (l/s) were selected for the experiments. A given mass of NACL solution was instantaneously injected into upstream of the flume and then the breakthrough curves were plotted based on the measured electric conductivity values along the centerline of the flume for the different sections at downstream of the injection point. To assessment the goodness of the simulated and observed BT curves, the statistical indices including the root mean square error (RMSE), Nash and Sutcliffe (NS) and mean absolute error (MAE) were extracted. With comparison of the observed BT curves with the numerical results (obtained with OTIS), the longitudinal dispersion coefficient values in different reach of flume were calculated. Analysis of the results showed that for a constant longitudinal slope and discharge, by increasing the distance from the injection location, the coefficient of dispersion was increased. Also, for constant slope, with enhancing the discharge rate, the dispersion coefficient was increased but for a given discharges, the dispersion coefficient was decreased with increasing the longitudinal -slope.