The sum of $15,000,000 is authorized to be appropriated as an emergency fund to be allocated by the Secretary of the Army on the recommendation of the Chief of Engineers in rescue work or in the repair or maintenance of any flood-control work on any tributary of the Mississippi River threatened or destroyed by flood heretofore or hereafter occurring: Provided, That the unexpended and unallotted balance of said sum, or so much thereof as may be necessary, may be allotted by the Secretary of the Army, on the recommendation of the Chief of Engineers, in the reimbursement of levee districts or others for expenditures incurred or made prior to June 15, 1936 for the construction, repair, or maintenance of any flood-control work on any tributaries or outlets of the Mississippi River that may be threatened, impaired, or destroyed by the flood of 1927 or subsequent flood; and also in the construction, repair, or maintenance, and in the reimbursement of levee districts or others for the construction, repair, or maintenance of any flood-control work on any of the tributaries or outlets of the Mississippi River that may have been impaired, damaged, or destroyed by caving banks or that may be threatened or impaired by caving banks, of such tributaries, whether or not such caving has taken place during a flood stage: Provided further, That if the Chief of Engineers finds that it has been or will be necessary or advisable to change the location of any such flood-control work in order to provide the protection contemplated by this section, such change may be approved and authorized.