Pub. Contract Code, § 21061

(a) All contracts for any improvement or unit of work, except as provided in this article, estimated to cost in excess of twenty-five thousand dollars ($25,000), shall be let to the lowest responsible bidder in the manner provided in this article.

(b) The board of supervisors of the district shall advertise by five or more insertions in a daily newspaper of general circulation, or by two or more insertions in a weekly newspaper of general circulation, printed and published in the district, inviting sealed proposals for the construction of the improvement or work. The board shall require the successful bidder or bidders to file with the board good and sufficient bonds, to be approved by the board, conditioned upon the faithful performance of the contract and upon the payment of all claims for labor and material, the bonds to contain the terms and conditions set forth in Chapter 5 (commencing with Section 9550) of Title 3 of Part 6 of Division 4 of the Civil Code and to be subject to that chapter.

(c) The board may also reject any bid not suitable to the best interests of the district. If all proposals are rejected or no proposals are received pursuant to advertisement therefor, or the work consists of channel protection, dam protection, temporary work, maintenance work, or of emergency work, the board of supervisors may, without advertising for bids, have the work done by force account. Emergency work may also be done by negotiated contract without advertising for bids or requiring bonds. In case of an emergency, if notice for bids to let contracts will not be given, the board shall comply with Chapter 2.5 (commencing with Section 22050).

(d) The board of supervisors, acting as the board of the district, may, by ordinance, resolution, or board order, authorize the flood control engineer or other county officer to order changes or additions in work being performed under a construction contract. When so authorized, any change or addition in the work shall be ordered in writing by the flood control engineer, or other designated officer, and the extra cost for any change or addition to the work so ordered shall not exceed five thousand dollars ($5,000) when the total amount of the original contract does not exceed fifty thousand dollars ($50,000), nor 10 percent of the amount of any original contract that exceeds fifty thousand dollars ($50,000), but does not exceed two hundred fifty thousand dollars ($250,000). For contracts whose original cost exceeds two hundred fifty thousand dollars ($250,000), the extra cost for any change or addition to the work so ordered shall not exceed twenty-five thousand dollars ($25,000), plus 5 percent of the amount of the original contract costs in excess of two hundred fifty thousand dollars ($250,000). In no event shall any such change or alteration exceed one hundred fifty thousand dollars ($150,000).