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In this paper, the authors investigate analytically the transformation from the initial guided mode distribution to the stationary state and the effects of the bidimensional roughness profile, in multimode polymeric buried waveguides. In these structures, due to the geometrical dimensions and the operating wavelength, about a thousands of guided modes can propagate, even for weak core/cladding dielectric contrast. The coupling coefficients are computed by exploiting the geometrical features of the optical channels, such as the waveguide dimensions and the roughness surface statistics. The analysis gives insight on the guided/guided and guided/radiated mode interaction, and higher order solution is proposed, in the case of a great number of modes interacting over distances that are extremely long as compared to the signal wavelength and the roughness correlation length. Experimental results are valuated by means of semicontact atomic force microscopy as well as compared with existing numerical models.