Monday, August 01, 2011

Debris flow predictions in Monument wildfire area

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.

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.

[right, location of drainages of interest and level of burn severity. Credit USGS]

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.


  1. Debris flow as a result from wildfires is interesting - I've never heard of this before. Can this be expected to occur during rainfall deluge, or through the effects of normal and average rainfall in the area, or is it due to steep terrain? Thanks, Phill

  2. Phill from Art Gallery Earth posed the question above about when to expect post-fire debris flows. Following a wildfire the hydrology of a watershed can be significantly altered due to the consumption of the vegetation and ground litter, and the effects of the heat on the soil. Depending on the extent and severity of the burned area, post-fire runoff can be orders of magnitude larger than pre-fire conditions. Thus, post-fire debris flows can be generated from frequent, low-magnitude rainstorms with recurrence intervals of <2 yrs (Cannon and others, 2008). Research has shown that the key factors for generating post-fire debris flows are duration and intensity of rainfall, basin ruggedness (how steep are the slopes), and percent of the basin burned at high and moderate severity on slopes greater than 30% (Cannon and others, 2009). In Arizona, post-fire debris flows are typically generated in steep, burned basins during the first few rains after a fire from short-duration (< 1hr), high-intensity convective storms. While frequent (<2 yr) storms can produced post-fire debris flows in Arizona, larger storms (> 10 yrs) are quite likely to generate post-fire debris flows (depending on watershed conditions). Following the Monument Fire, post-fire debris flows were generated in Miller Canyon from a 10 yr storm with a total rainfall of 1.64” and a peak rainfall intensity of 0.72” in 10 minutes.

    Cannon, S.H., Gartner, J.E., Rupert, M.G., Michael, J.A., Rea, A.H., and Parrett, C., 2009, Predicting the probability and volume of postwildfire debris flows in the intermountain western United States: Geological Society of America Bulletin, p. B26459.26451.

    Cannon, S.H., Gartner, J.E., Wilson, R.C., Bowers, J.C., and Laber, J.L., 2008, Storm rainfall conditions for floods and debris flows from recently burned areas in southwestern Colorado and southern California: Geomorphology, v. 96, no. 3-4, p. 250-269.