18. Before commencing a tidal work or a part of a tidal work the undertaker shall submit plans thereof to the Port Authority for their approval (which approval shall not be unreasonably withheld but may be given subject to reasonable conditions) and shall not commence the same until such plans have been approved by the Port Authority: but if the Port Authority do not within 42 days after the receipt of any such plans notify to the undertaker their disapproval thereof and the grounds for their disapproval they shall be deemed to have approved them.