Pub. Contract Code, § 21081

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 in the manner provided in this article. The board shall first determine whether the contract shall be let as a single unit for the whole of the work, or shall be divided into severable parts, or both, according to the best interests of the district. The board shall call for bids and advertise the call 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 district inviting sealed proposals for the construction or performance of the improvement or work before any contract is made. The call for bids shall state whether the work is to be performed as a unit or divided into severable specific parts, or both, as stated in the call. The board may let the work by single contract for the whole or it may divide such work into severable parts by separate contracts, as stated in such call, according to the best interests of the district. 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 their claims for labor and material, the bonds to contain the terms and conditions set forth in Title 3 (commencing with Section 9000) of Part 6 of Division 4 of the Civil Code and to be subject to the provisions of that title. The board shall also have the right to reject any bid. In the event all proposals are rejected or no proposals are received pursuant to advertisement, or the estimated cost of such 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, without advertising for bids, have the work done by force account. 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.