6. If any pile, stump or other obstruction to navigation becomes exposed in the course of constructing any tidal work (other than a pile, stump or other obstruction on the site of a structure comprised in any permanent work), the undertaker, as soon as reasonably practicable after the receipt of notice in writing from the Statutory Conservancy and Navigation Authority requiring such action, must remove it from the river or, if it is not reasonably practicable to remove it—
(a)cut the obstruction off at such level below the bed of the river as the Statutory Conservancy and Navigation Authority may reasonably direct; or
(b)take such other steps to make the obstruction safe as the Statutory Conservancy and Navigation Authority may reasonably require.