Within sixty days after the notice is posted, the claimant must commence the excavation or construction of the works in which he intends to divert the water, or the survey, road or trail building, necessarily incident thereto, and must prosecute the work diligently and uninterruptedly to completion, unless temporarily interrupted by snows or rain; provided, that if the erection of a dam has been recommended by the California dGbris commission at or near the place where it is intended to divert the water, the claimant shall have sixty days after the completion of such dam in which to commence the excavation or construction of the works in which he intends to divert the water; provided, that whenever any city and county, or any incorporated city or town within this state makes, or has made, or acquires, or has acquired any appropriation of any of the waters of this state in accordance with the provisions of section 1415 of this code, it shall not be necessary for such city and county, city or town to commence the work for development of more of the water so claimed than is actually necessary for the immediate needs of such city and county, city or town and it shall be held to be a sufficient compliance with the requirements of this chapter, to the full amount of water stated in the notice posted and recorded, for such city and county, city or town to within sixty days make the necessary surveys, or within six months to authorize the issuance of municipal bonds, for the construction of the necessary works designed to supply such city and county, city or town with the water required for immediate use. Any appropriation heretofore made by any such city and county, city or town in connection with which surveys were at any time made, or an issue of bonds authorized for the construction of any portion of the works necessary for a diversion of any part of the water appropriated, is hereby confirmed to the full amount of water stated in the original notice or notices.