Miller and Ash canyons in the Huachuca Mountains have the potential for post-fire debris flows with volumes over 200,000 cubic yards, following the Monument wildfire. Other drainages have potential for debris flows ranging on up from 2,000 cubic yards.
These volumes and their probabilities of occurrence are part of the USGS assessment released publicly at the end of last week. AZGS geologists participated in the assessment. A table in the report lists each canyon, its potential volume and probabilities at multiple intervals over the next ten years:
This report presents a preliminary emergency assessment of the debris-flow hazards from drainage basins burned by the Monument wildfire in southeastern Arizona, in 2011. Empirical models derived from statistical evaluation of data collected from recently burned drainage basins throughout the intermountain Western United States were used to estimate the probability of debris-flow occurrence and volumes of debris flows for selected drainage basins. Input for the models include measures of burn severity, topographic characteristics, soil properties, and rainfall total and intensity for a (1) 2-year-recurrence, 30-minute-duration rainfall, (2) 5-year-recurrence, 30-minute-duration rainfall, and (3) 10-year-recurrence, 30-minute-duration rainfall.[right, location of drainages of interest and level of burn severity. Credit USGS]
Estimated debris-flow probabilities in the drainage basins of interest ranged from a low of 26 percent in response to the 2-year-recurrence, 30-minute-duration rainfall to 100 percent in response to the 10-year-recurrence, 30-minute-duration rainfall. The high probabilities in all modeled drainage basins are likely due to the abundance of steep hillslopes and the extensive areas burned at moderately to high severities. The estimated volumes ranged from a low of about 2,000 cubic meters to a high of greater than 200,000 cubic meters.
Ref: Ruddy, B.C. and Verdin, K.L., 2011, Probability and volume of potential postwildfire debris flows in the 2011 Monument burn area, southeastern Arizona: U.S. Geological Survey Open-File Report 2011–1181, 9 p.