The powers conferred by the foregoing provisions of this Part of this Act shall not be exercised—
with respect to any stream, watercourse, ditch or culvert within the jurisdiction of a land drainage authority, except after consultation with that authority;
. . . Provided that nothing in this subsection shall apply in relation to the taking of proceedings in respect of a statutory nuisance.
Nothing in the foregoing provisions of this Part of this Act shall prejudice or affect the powers of any railway company or dock undertakers to culvert or cover in any stream or watercourse, or, without the consent of the railway company or dock undertakers concerned, extend to any culvert or covering of a stream or watercourse constructed by a railway company and used by them for the purposes of their railway, or constructed by dock undertakers and used by them for the purposes of their undertaking.