The resettlement franchise shall be granted upon the express condition that the city or city and county may, at a valuation for the purpose of public acquisition, fixed and determined as provided in Section 6462, assume ownership by purchase, or take over and possess the property used and useful, or, in the discretion of the city or city and county, prospectively useful property of the franchise grantee, its successors or assigns, upon giving the grantee written notice of its intention to purchase and take over the property, which notice shall be given only when authorized by ordinance of the legislative or governing body of the city or city and county.