No license shall be granted under which a new wharf is to be built, or an old wharf extended, in counties coextensive with cities of the first class, unless there shall be, after the building or extension of such wharf, a dock or water surface between it and the adjoining wharves on each side of at least one hundred and fifty feet unless said board of wardens, by a vote of a majority of the whole board, shall decide that the public convenience demands a variance from this rule in any particular case.
53 P.S. § 14217