To describe the mechanical process of accumulation and consequent pollution by a forest the differential equations of Navier–Stokes, incompressibility and diffusion are used with appropriate boundary conditions. For a solution of the problem the two-dimensional (in a vertical plane) formulation is used. The forest is simulated as a continuous medium, in which a resistance forces are disrtibuted. Two techniques of a solution are considered here: first method uses pressure and components of a velocity vector; second method applies stream and vorticities functions. In both cases the method of geometric decomposition is used, when the two-dimensional differential equations are substituted by a sequence of the one-dimensional tasks. The comparison of numerical solutions of the mentioned problem by two techniques has shown satisfactory coincidence.