The consideration of electromagnetic wave propagation in irregular waveguiding structures usually assumes either smooth variation of medium parameters [1] or their rigorously periodical perturbation [2, 3]. Combination of smooth and oscillated parameter perturbations leads to additional peculiarities of Bragg scattering [4]. Anisotropy of parameters causes coupling of waves with different polarization [3]. In the present paper the mutual transformation of waves in open guided structures has been theoretically considered using a complex form [4] of the asymptotic method of Krylov, Bogoliubov and Mitropolsky (KBM) [5]. The influence of surface waves on scattering TM and TE wave beams by a surface with spatially oscillated anisotropic surface impedance has been analyzed. For some types of perturbations analytical solutions to coupled-wave equations with the KBM method have been obtained and expressions for perturbed wavenumbers of eigenwaves have been derived. Comparisons with some known in literature results have been done. In general case the differential equations with slowly varying coefficients are rather easily integrated numerically. Longitudinal distributions of wave complex amplitudes describe a combination of different physical phenomena in the considered structure.