  We study theoretically and experimentally how to reduce the backscattering of
water waves in a channel with multiple turns. We show that it is not only
possible to cancel backscattering but also to achieve a remarkable transmission
in such geometries. In order to avoid the reflection that naturally arises at
each turn of the waveguide, an anisotropic metamaterial made of closely-spaced
thin vertical plates is used. The efficiency of the metamaterial arrangement
depends only slightly on the frequency of the incident wave, as long as its
wavelength is much larger than the periodicity of the array. This phenomenon is
not restricted only to water wave channels but also applies to any type of
waves with Neumann boundary conditions.
