Whenever the drainage district shall need the sum of five thousand dollars or more, either for the purpose of paying outstanding warrants issued under section 31-416, or to refund bonds issued for the purpose of paying or providing funds for the payment of work done under the provisions of sections 31-401 to 31-450, the board of directors may issue negotiable bonds for such sums as may be needed, not, however, exceeding the amount that the engineer of the district shall certify as being required in the case of an original issue of bonds, nor exceeding the amount of outstanding bonds in the case of refunding bonds. Such bonds shall be signed by the president, secretary and treasurer, under the corporate seal, with coupons attached, and shall be payable in not to exceed twenty annual installments. They shall be sold at not less than par.