The flow line of any com­bi­na­tion curb and gut­ter sec­tion, as well as the flow line of any storm sew­er, shall have a fall of at least 0.5 of a foot per one hun­dred (100) lin­eal feet, except where ver­ti­cal curves in the grade line of the street make this pro­vi­sion inap­plic­a­ble with respect to the curb and gutter.