Whenever the city council shall provide for the construction of any trunk sewer, or sewerage and drainage, or trunk water main, it may divide the territory to be served thereby into subdistricts. The construction of such improvement may be made under separate contracts for such subdistrict thereof, and the city council may levy assessments in each subdistrict and issue bonds to be paid by the collection of assessments against property in each subdistrict independent of any subdistrict; provided however, that such subdistrict shall be set forth in said ordinance providing for such improvement, and when it is proposed to pay any portion of the cost of such improvement from the general fund such ordinance shall specify approximately the amount to be apportioned to such subdistrict. (Prior code § 13.48.230.)