As soon as the board of supervisors has adjudicated, fixed, and established the classification and benefits as provided by sections 31-327 to 31-329, it may at once levy a tax on the lands and other property in the district to which benefits have been assessed, equal in amount to the cost of such drainage works and improvements as estimated by the engineer and as modified and confirmed by the board, plus the actual expenses of organizing the district, the probable working and administrative expenses, and damages (as estimated by the board of supervisors) in the completion of the works and improvements, and the carrying out of the objects of the district. In case bonds are issued, as provided in section 31-336, then the amount of the interest (as estimated by the board of supervisors) which shall accrue on such bonds shall be included and added to the tax. The tax shall be apportioned to and levied on each tract of land or property in the district in proportion to the benefits assessed, and not in excess thereof. The board shall determine whether the tax shall be collected and paid in a single assessment, or by dividing into not to exceed twenty annual installments. If any assessment of benefits heretofore or hereafter made shall be found or declared to be invalid for any reason, either as to the whole assessment or the assessment as to any particular tract, interest, county, town or city, or other corporation, either to the whole of such assessment or to any part thereof, it shall be lawful and valid, and it shall be the duty of the board of the district to reassess such property or interest against which the former assessment has been found and declared to be invalid, after giving notice and taking the same steps and following the same procedure as required in making the original assessment of benefits. Such reassessment of benefits shall be made so as to do justice to all property, parties and interests, and shall take into consideration and give credit for all payments made under the assessment which has been found and declared to be valid. A new report of benefits, as to the part declared to be invalid, shall be made against the property or interest that was released by the finding of invalidity of the former assessment. In case any such drainage district shall have accumulated funds over and above the necessary cost of construction and upkeep, the board of supervisors of such district in its discretion may cause so much of such accumulated funds as are not required to be apportioned to each tract of land or property in the district prorated accordingly and in proportion to the benefits assessed and collected, and such board shall draw warrants on the treasurer of the drainage district for the payment of such accumulated funds to the proper owners.