If the proprietors of any such works shall construct therein next below the head gate or head thereof a waste gate, or other opening, so that the whole of the waters of such works may be discharged therefrom and returned to the source of supply thereof, and shall place therein a gate which may be set and locked in like manner as herein prescribed in respect of the head gate and shall provide and deliver the key thereof in like manner as hereinbefore provided in respect of a head gate, then and in such case, in lieu of closing the head gate of such works, such waste gate, sand-gate or other place of discharge may be opened and locked so as to discharge the whole or any necessary part of the waters diverted by such works and return the same to the source of supply thereof; and such waste gate shall be held to comply with all the requirements herein respecting the providing of a head gate for such works.