5U.K.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 any permanent work), the nominated undertaker shall, as soon as reasonably practicable after the receipt of notice in writing from the port authority requiring such action, remove it from the river or, if it is not reasonably practicable to remove it, cut it off at such level below the bed of the river (not being more than two metres below bed level in the main navigation channel or one metre below bed level elsewhere in the river) as the port authority may reasonably direct and in the absence of such action within a reasonable period of time the port authority may carry out the removal and recover its reasonable expenses in so doing from the nominated undertaker.