Aspen Burn Increased Flood Potential Bonito Wash Return Intervals
Prepared by William B. Reed, Colorado Basin River Forecast Center

Within Arizona post burn runoff has been observed to be 5 times greater than runoff during pre-burn conditions . In areas of sparse desert vegetation, shallow soils, and bare rock--the increase may not be as great--since runoff is already relatively high due to low initial abstractions. However, a value of 2.5 was used here, in an attempt to be conservative, and to reflect that only half of the watershed was burned.

The pre-burn 10-year return interval flow should be approximately the bankfull flow. The post burn 25-year return interval flow should be approximately the calculated event flow. This assumes the event was caused by a rainfall of 1.5 inches in 25 minutes, which has a return interval of approximately 25-years.

The above indicates that the pre-burn 100-year flood now has a return interval less than 25 years. These enhanced flood prone conditions will likely exists until such time as the watershed has recovered--perhaps 3 to 5 years. There is an 18 % chance or likelihood of one or more floods of equal or greater magnitude occurring in the next 5 years.

Please note that the site has not been visited by the author, and is an ungaged stream; therefore, this report should be considered preliminary and subject to revision (revised 8/25/2003).