Centre manifold method is an accurate approach for analytically constructing an advection-diffusion equation (and even more accurate equations involving higher-order derivatives) for the depth-averaged concentration of substances in channels. This paper presents a direct numerical verification of converge to each other, with their velocities becoming practically equal. The obtained numerical results also demonstrate that the longitudinal diffusion can be neglected compared to the advection.

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We formulate and numerically analyse the averaged model of dispersion in turbulent canopy flows. The averaging is carried out across the flow, for example over the river depth. To perform the averaging, we use the general approach suggested by Roberts and co-authors in the late 1980s, which is based on centre manifold theory. We derive an evolution partial differential equation for the depth averaged concentration, involving first, second and higher order derivatives with respect to the downstream coordinate. The coefficients of the equation are expressed in terms of parameters characterising the turbulent flow. Preliminary numerical results are demonstrated. In particular, it is shown that the advection and diffusion coefficients coincide with their values obtained earlier for the flow over a smooth bottom in the limit of large depths.