Where any county in connection with any such flood control project has given or may give assurances of local cooperation required by the federal authorities, as authorized by law, the board of supervisors of such county shall have the added power and authority, if necessary or desirable for the fulfillment of such assurances, to acquire all lands and easements and rights-of-way, and the fee title to such lands where advisable, either by purchase or by condemnation and, if by condemnation, according to the existing statutes applicable to the acquisition by counties of property for public use.

Where any county of the state which operates any such project has been required to give its assurances by the federal authorities or other agency of the government of the United States of local cooperation and participation in any such project by agreeing to pay any part of the construction costs of such project or projects, then the board of supervisors of such county shall have the added power and authority, if necessary and desirable for the fulfillment of such assurances, to sign agreements with such federal authorities or other agency of the government of the United States whereby such participating county agrees to pay its part of the cost of such construction or any fractional part thereof, including interest of not more than three per cent (3%) per annum, and provided further that said assurances shall be due and payable within the primary term of forty years from the time such assurances are given.