Pub. Contract Code, § 21461

All contracts for any improvement or unit of work, when the cost according to the estimate of the engineer will exceed five thousand dollars ($5,000), shall be let to the lowest responsible bidder or bidders as provided in this article. The board shall first determine whether the contract shall be let as a single unit, or divided into severable parts. The board shall advertise for bids by three insertions in a daily newspaper of general circulation or by two insertions in a weekly newspaper of general circulation printed and published in the agency, inviting sealed proposals for the construction or performance of the improvement or work. The call for bids shall state whether the work shall be performed in one unit or divided into parts. The work may be let under a single contract or several contracts, as stated in such call. The board shall require the successful 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 payment of their claims for labor and material. The bonds shall comply with Title 3 (commencing with Section 9000) of Part 6 of Division 4 of the Civil Code. The board may reject any and all bids. In the event all proposals are rejected or no proposals are received, or the estimated cost of the work does not exceed five thousand dollars ($5,000), or the work consists of channel protection, maintenance work, or emergency work, the board of supervisors may have the work done by force account 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). The district may purchase in the open market without advertising for bids, materials and supplies for use in any work, either under contract or by force account.