PCC §21631: (a) Any improvement or unit of work when the cost, according to the estimate of the engineer, will exceed six thousand five hundred dollars ($6,500), shall be done by contract and shall be let to the

(a) Any improvement or unit of work when the cost, according to the estimate of the engineer, will exceed six thousand five hundred dollars ($6,500), shall be done by contract and 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, 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 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 shall be divided into severable parts, or both, as stated in the call. The board may let the work by single contract or it may divide the work into severable parts by separate contracts, as stated in the call, according to the best interests of the district. The board shall require the successful bidder or bidders to file with the board a good and sufficient bond, to be approved by the board, conditioned upon the payment of their claims for labor and material. The bond shall 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 be subject to the provisions of that title. The board may reject any and all bids and readvertise for new bids, or by a two-thirds vote may elect to undertake the work by force account. (b) If no proposals are received pursuant to advertisement or the estimated cost of the work does not exceed six thousand five hundred dollars ($6,500), 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 employees of the district, by day labor, under the direction of the board, by contract, or by a combination of those methods. 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). (c) The district may acquire in the open market, without advertising for bids, materials, equipment, and supplies for use in any work or for any other purpose; provided, however, that materials and supplies for use in any new construction work or improvement, except that work referred to in subdivision (b), may not be purchased if the cost exceeds six thousand five hundred dollars ($6,500), without advertising for bids and awarding the contract to the lowest responsible bidder.