When it is determined to let the work by contract, contracts in amounts in excess of ten thousand dollars ($10,000.00) dollars [sic] shall be advertised after notice by publication as the board shall order, calling for bids, and the board may reject any or all bids, or may let said contract to the lowest or best bidder who shall give a good and approved bond, with ample security, conditioned on the carrying out of the contract. Such contract shall be in writing, and shall be accompanied by or shall refer to plans and specifications for the work to be done, prepared by the chief engineer. Said contract shall be let by the chief engineer, approved by the board, signed by the president and by the contractor, and shall be executed in duplicate; provided, that in case of sudden emergency when it is necessary in order to protect the district, the advertising of contracts may be waived upon the consent of the board, with the approval of the court; provided, further, that the provisions of this section shall not apply if it is determined by the board with the approval of the court that the work be done on force account.