Pub. Contract Code, § 20991

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. 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 shall 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 shall be subject to the provisions of that chapter. The board shall have the right to reject any bid not suitable to the best interests of the district. In the event all proposals are rejected or no proposals are received pursuant to advertisement, or the estimated cost of the work does not exceed ten thousand dollars ($10,000), or the work consists of channel protection, dam protection, temporary work, maintenance work, or 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. 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).