In the case of an emergency or disaster not in contemplation at the time of making the annual estimate of costs, declared to be such by resolution of such board, the diking commissioners 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 diking districts and the servicing thereof, and all such warrants so issued shall be valid and legal obligations of such district and its taxable lands and improvements as shown upon the then current roll of said district filed with the county auditor.