All contracts shall be let to the lowest responsible bidder. The board shall advertise for two or more days in a newspaper of general circulation published in the county, inviting bids for furnishing labor, material or supplies before any contract is made therefor. The board may require such bonds as it may deem adequate from the successful bidder to secure the faithful performance of the contract and the payment of all claims for labor and materials, and may reject any and all bids. In an emergency, work found by a four-fifths vote of the board to be necessary to protect life or property from impending flood damage may be done by negotiated contract without advertising for bids therefor.