A county may contract to maintain such flood control projects after the construction work is completed, which maintenance may be without expense to the United States of America, and may contract to and acquire easements and rights of way to relocate public roads or bridges when the replacement shall be rendered necessary by the construction of any flood control project and may give satisfactory assurance to the United States of America, or any agency thereof, that the location, relocation, building or rebuilding of such roads, rights of way, or bridges shall be done without expense to the United States of America or any agency thereof.