Theoretical methods have been developed to estimate the longitudinal dispersion coefficients (k) in natural rivers. The triple integral expression for longitudinal dispersion caused by the transverse velocity gradient and the solution for the depth-averaged streamwise velocity distribution in the transverse direction in a rectangular flume are used in this study. The longitudinal dispersion coefficient was calculated after changing the nonintegral formula for the velocity distribution into a trigonometric function series by Fourier transformation and then by substituting this series into the triple integral expression. A dimensionless formula for longitudinal dispersion in the flume was then obtained by regression analysis and was consistent with the experimental results obtained in previous studies and this study. By analyzing the measured dispersion coefficients of the natural rivers and the corresponding values obtained from the formula for a flume with the same hydraulic parameters, a formula was derived for the longitudinal dispersion coefficient of natural rivers. The formula provides a suitable method for estimating pollutant transport in natural rivers.
