(1)    If, after all contracts for construction of the improvement have been let, the aggregate of the contract prices, as certified by the flood control board created pursuant to s. 87.12, added to the department’s estimated cost of acquiring the necessary lands and of overhead expense and of the first 18 months’ operation and maintenance, exceeds the amount estimated by the department as the total cost of the improvement, but is less than the total amount found by the department to be collectible under s. 87.09, the work of constructing the improvement shall nevertheless proceed and the several public corporations shall provide, not later than the time fixed for the next ensuing general tax collection therein, the amounts required to make up the deficiency, in the same proportions as the original amounts were provided by such public corporations.

   (2)   But should the total cost, as ascertained and certified by the flood control board after the letting of the contracts, in the manner hereinabove set forth, exceed the total amount found by the department to be collectible under s. 87.09, all contracts for the construction of the work shall be null and void. At the expiration of one year after such certification, any moneys held by the secretary of administration on account of the project shall be refunded to the persons by whom they were paid to the secretary of administration; and funds in the hands of the flood control board shall be refunded to the public corporation by which they were paid to such board; any funds held by any town, village, or city, having been collected by special assessments against property benefited, shall be refunded to the owners of such property; any funds raised by any public corporation by the issuance of bonds on account of such proposed improvements shall constitute a fund for the retirement or payment of such bonds; and any fund held by any public corporation, having been raised otherwise than by special assessments or bond issues, shall be available for the general purposes of such public corporation. Provided, however, that if within one year after the last mentioned certification of the flood control board there shall be deposited with the treasurer of said board a sum equal to the difference between the aggregate cost of constructing the improvement as estimated by the department and the aggregate cost thereof as determined and certified by the flood control board after the letting of the contracts, said board shall proceed to relet the contracts for the construction of the improvement and to complete the same unless the aggregate of such new contract prices, together with the department’s estimate of the cost of acquiring lands and of overhead expenses and of the first 18 months’ operation and maintenance, shall again exceed the amount found by the department to be collectible under s. 87.09. The deposit herein referred to may be made by any person or any public or private corporation.

   (3)   If, after the letting of the contracts and during the progress of the work, it develops that the cost of completing the work exceeds the amount available therefor, the flood control board may borrow temporarily the amounts required for completing the work, upon promissory notes executed by the board, payable with interest on or before the 15th day of March next ensuing; and each town, village and city containing property found by the department to be benefited by the improvement shall upon certification of the flood control board as to the amount to be raised by such town, village or city to repay such temporary loan, include in its next general tax levy the amount so certified. Such certification by the flood control board shall require each such town, village and city to raise the same proportion of the sum required to pay the temporary loan as the total amount previously contributed by such town, village or city pursuant to s. 87.10 (1) (c) bears to the total amount contributed by all of such towns, villages and cities thereunder. If, by reason of the deferred collection of special assessments as authorized in s. 87.10 (1) (a), funds shall be required by the board before they are actually available, the board may borrow such funds upon its promissory notes payable with interest on or before March 15 of the year in which such deferred installments become due, and shall repay such loan out of the proceeds of such installments.

   (4)   If it shall develop that the total cost of constructing the improvement is less than the total amount available for the payment of such cost, the excess, except as provided in s. 87.13, shall constitute the commencement of the fund provided for in s. 87.14 for the maintenance and operation of the improvement.