Closed storm sewers shall be constructed of precast or prefabricated pipe or built in place of closed box design to conform with city construction specifications. Storm sewers carrying runoff from streets may be designed to serve a 10-year frequency rain for the drainage area involved, provided that overflow from a 25-year frequency rain can reach a suitable outlet without inundating an elevation one foot (1') below any house pad elevation. In cases where the above cannot be accomplished, the storm sewer shall be designed to serve a 25-year frequency rain. Sizing shall be calculated by the Manning formula. (Prior code §7-7(b))