The elevation or grade of the top of the curb lines on either side of the above named streets and avenues shall conform to the elevation of grade of the centerline of the street or avenue as the same may be established, the elevation of the top of either curb at any given point being the same as that of the top of the center of the street at the point opposite. The elevation of the curb lines upon streets and avenues within the business portion of the city, may be altered from the above by resolution of the City Council to conform to the public convenience and necessity.
(`90 Code, § 19-9)