If any pile, stump or other obstruction to navigation becomes exposed in the course of constructing any specified 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 the bed of the river) as the Port Authority may reasonably direct.