Drainage boards and officials operating under chapters 20 and 21 may employ the procedures of this chapter as an alternate method instead of the procedures in chapters 20 and 21 for the apportionment of costs and the preparation and collection of a special assessment roll.

     When 2 or more public corporations other than the state or a county or counties are to be assessed, the drainage board, or if appropriate in chapter 21 proceedings, the drain commissioner, may determine that costs to be assessed to the public corporations, in excess of those apportioned for drainage to state or county highways, shall not be apportioned by the establishment of a fixed percentage of costs to be borne by each public corporation, but that the costs shall be assessed at large to all of the public corporations in the aggregate and apportioned annually between the public corporations on the basis of the relative valuations, as equalized, of each within the area served by the drain, if the drainage board or drain commisssioner shall determine that this method of apportionment will more fairly result in the costs of the drain being apportioned in accordance with the benefits to be derived therefrom. Notice of tentative apportionment in the foregoing manner shall include a calculation of the apportionment of costs between public corporations assessed in the aggregate based on the latest available valuations.

     When employing this chapter, the chairman of the drainage board in chapter 20 proceedings or the secretary of the drainage board in chapter 21 proceedings shall prepare a special assessment roll which shall contain the name of each public corporation assessed, the total estimated or actual cost of the project, the fixed percentage of the cost, if any, apportioned to public corporations and to the state or counties for drainage of highways, the aggregate amount assessed at large against all other public corporations, and if the assessment is divided into annual installments, the amount of each installment, listed separately for the state and counties and for public corporations assessed a fixed percentage if applicable, and in the aggregate for all other public corporations. After approval of the roll by the drainage board, the chairman shall then send a certified copy of the roll to each public corporation assessed.

     On or before June 1 of each year, or, if necessary in connection with notification of the due date of the initial installment only at any appropriate time, the chairman of the drainage board shall notify each public corporation to which costs are apportioned as provided in this chapter of the installment of assessment and interest thereon coming due before the next June 1. On or before the due date of an installment each public corporation shall pay to the county treasurer the full amount, together with interest accruing to the due date. Failure to notify a public corporation shall not excuse it from making payment of the installment and interest. In determining the amounts due from public corporations assessed in the aggregate, the chairman shall calculate the total state equalized valuation of each within the area served by the drain and shall assign to each public corporation as its annual installment the same percentage of the total aggregate installment next falling due as the state equalized valuation of the public corporation within the area served by the drain bears to the total state equalized valuation within the area served by the drain. In making those calculations, the chairman shall use the state equalized valuation for the latest year in which the valuations are finalized for the public corporations so assessed, even though the valuations may no longer be current. If noncurrent valuations are used, the drainage board may determine to adjust later payments when valuations are finalized if the board in its sole discretion shall determine the adjustment to be necessary to properly apportion costs between public corporations so assessed.

     In calculating amounts of assessments chargeable against debt limitations of public corporations assessed under this chapter, each public corporation shall be charged with the same percentage of the total aggregate at large assessment as the state equalized valuation of the public corporation within the area served by the drain bears to the total state equalized valuation within the area served by the drain. The state equalized valuations used in those calculations shall be those used in the latest notification of installment sent by the chairman pursuant to section 628, and prior thereto shall be the latest valuations available.

     It is the purpose and intention of this chapter to provide an additional and alternate method of apportioning drainage costs under chapters 20 and 21 to public corporations other than the state and counties. Therefore, this chapter when employed shall be construed to be integrated with chapters 20 and 21 and all provisions of those chapters inconsistent with this chapter shall not be applicable, but all other provisions of those chapters shall remain in full force and effect.