On and after May 23, 1955, the funds appropriated for flood prevention purposes may be expended in watersheds heretofore authorized by section 13 of the Flood Control Act of December 22, 1944, as amended, for necessary measures for the prevention of erosion, floodwater, and sediment damages, including gully control, floodwater detention, and floodway structures, in areas other than those over which the Department of the Army has jurisdiction and responsibility, and where the Army does have jurisdiction and responsibility, may enter into agreements with the Army to carry out jointly the measures heretofore set out and in areas where the Secretary is authorized to purchase land rights for structural measures, the Secretary in lieu of such acquisition, may reimburse local organizations for such proportionate share of the cost of land rights furnished by local organizations as the Secretary deems equitable in consideration of the national interest.