(a)   Upon the declaration of a water supply emergency as provided in § 15-603, the governing body of the city shall have the power to adopt emergency water rates by ordinance designed to conserve water supplies.
   (b)   Such emergency rates may provide for, but are not limited to:
      (1)   Higher charges for increasing usage per unit of the use (increasing block rates);
      (2)   Uniform charges for water usage per unit of use (uniform unit rate); or
      (3)   Extra charges in excess of a specified level of water use (excess demand surcharge).