  The correct description of the surface water dynamics in the model of shallow
water requires accounting for friction. To simulate a channel flow in the Chezy
model the constant Manning roughness coefficient is frequently used. The
Manning coefficient n M is an integral parameter which accounts for a large
number of physical factors determining the flow braking. We used computational
simulations in a shallow water model to determine the relationship between the
Manning coefficient and the parameters of small-scale perturbations of a bottom
in a long channel. Comparing the transverse water velocity profiles in the
channel obtained in the models with a perturbed bottom without bottom friction
and with bottom friction on a smooth bottom, we constructed the dependence of n
M on the amplitude and spatial scale of perturbation of the bottom relief.
