21Restriction on works over navigable waters
Nothing in section 17 (Power to construct works), 18 (Power to deviate), 19 (Power to improve bridge) or 20 (Supplementary works powers) of this Act shall authorise the carrying out of any works either—
(a)in navigable waters; or
(b)over navigable waters at a level below the level of the soffit of the bridge constructed across the river under the powers of the [1846 c. cccxxxv.] Cornwall Railway Act 1846.