In the case of an emergency or disaster not in contemplation at the time of making the annual estimate of costs and declared to be such by resolution of the board, the board may incur additional obligations and issue valid warrants therefor in excess of such estimate in the manner provided by law for issuance of warrants by drainage districts and the servicing thereof, and all such warrants so issued shall be valid as shown upon the then current roll of said district filed with the county auditor.