On and after July 31, 1947, no appropriation under the Corps of Engineers shall be available for any expenses incident to operating any power-driven boat or vessel on other than Government business, and that Government business shall be construed to include transportation, lodging, and subsistence on inspection trips of Federal and State officials, having a public interest in authorized or proposed improvements for river and harbor and flood control, and any expenses incurred therefor shall be chargeable to river and harbor and flood control appropriations heretofore or hereafter made under rules and regulations to be prescribed by the Chief of Engineers: Provided, That such expenditures shall be certified by the Division Engineer as necessary and proper expenditures.