Whenever the survey, examinations, drawings, and plans of an irrigation district, and the estimate of cost based thereon, shall provide that the works necessary for a completed project shall be constructed progressively over a period of years in accordance with a plan or schedule adopted by resolution of the board of directors of the district, it shall not be necessary for the secretary of state to certify at one time all of the bonds that have been voted for the said completed project; but such bonds may be certified from time to time, when approved by the director of ecology, as needed by the district. If the secretary of state shall certify all of the bonds necessary for the said completed project, even if said project is to be constructed progressively over a period of years in accordance with the aforesaid resolution of the board of directors, the bonds so voted and certified shall only be sold after prior written approval of said director.