  Starting from the rigorous excitation equation, the propagation of waves
through a 2D waveguide with the periodically corrugated finite-length insert is
examined in detail. The corrugation profile is chosen to obey the property that
its amplitude is small as compared to the waveguide width, whereas the
sharpness of the asperities is arbitrarily large. With the aid of the method of
mode separation, which was developed earlier for inhomogeneous-in-bulk
waveguide systems [Waves Random Media \textbf{10}, 395 (2000)], the corrugated
segment of the waveguide is shown to serve as the effective scattering barrier
whose width is coincident with the length of the insert and the average height
is controlled by the sharpness of boundary asperities. Due to this barrier, the
mode spectrum of the waveguide can be substantially rarefied and adjusted so as
to reduce the number of extended modes to the value arbitrarily less than that
in the absence of corrugation (up to zero), without changing considerably the
waveguide average width.
