Postwildfire Debris-Flow Hazards

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Wildfire is a natural process in forest ecosystems, and occurs with varying frequencies and severities depending on landscape characteristics, climatic conditions, and the historical fire regime. Although attention often is focused on the potential damages from wildfire in the wildland-urban interface, wildfire also presents a threat to critical infrastructure including flood water conveyances and water conveyances critical to municipal water supplies. Further, burned landscapes are at risk of damage from postwildfire erosion, such as that caused by debris flows and flash floods, which can be the most catastrophic of the postwildfire threats to an area. 

Detail from figure 8 in SIR 2014-5161 showing integrated debris-flow hazard index values for basins outside of Albuquerque, NM

Detail from figure 8 in SIR 2014-5161 showing integrated debris-flow hazard index values for basins outside of Albuquerque, NM

(Public domain.)

Debris flows are high-density slurries of water, rock fragments, soil, and mud that can have enormous destructive power particularly when they are fast moving. Wildfire can drastically increase the probability of debris flows in landscapes that have otherwise been stable throughout recent history. A primary watershed effect of wildfire is rapid and dramatic decrease in infiltration because of widespread removal of vegetation and development of hydrophobic soils (Cannon and Gartner, 2005). 

Debris flows have been documented after many fires in the western United States (Cannon and others, 2001a and b; Cannon and others, 2010; DeGraff and others, 2011; Kean and others, 2011). In addition, debris flows following wildfire can be generated in response to low-recurrence interval/high intensity rainfall. Recently burned landscapes may be at risk of such postwildfire hydrologic hazards for severalto many years following the fire (Cannon and Gartner, 2005). The U.S. Geological Survey (USGS) has developed a model (Cannon and others, 2010) to estimate postwildfire debris-flow probability and volume. This information can be used to determine watersheds of concern or areas most at risk for loss of life and property. 

The USGS postwildfire debris-flow hazard models have been applied after four fires in New Mexico since the model was developed: the 2011 Track and Las Conchas Fires and the 2012 Little Bear and Whitewater-Baldy Fires. These reports were used by land managers to help select and prioritize post-fire flood and debris flow mitigation treatments. 

In 2013, the USGS developed a new method for estimating post-fire erosion hazards before a wildfire actually burns with a study in the Sandia and Manzano Mountains. This prewildfire assessment approach is valuable to resource managers because the analysis of the debris-flow threat is made before a wildfire occurs, which facilitates prewildfire management, planning, and mitigation. The prewildfire study to determine the potential for postwildfire debris flows in the Sandia and Manzano Mountain areas in Central New Mexico was initiated by the USGS in cooperation with the Bernalillo County Natural Resources Services as a part of the Rio Grande Water Fund. 

The U.S. Department of Agriculture Forest Service and The Nature Conservancy provided support for this effort principally through fire simulation modeling. The study was conducted to provide information on which subwatersheds might constitute the most serious, potential, debris-flow hazards in the event of a large-scale wildfire and subsequent rainfall in the Sandia and Manzano Mountain areas. The USGS will follow the Sandia and Manzano report with a similar prewildfire assessment of the postwildfire debris-flow hazards in the Jemez and Sangre de Cristo Mountains (due in 2015 and 2016 respectively). 

The maps in these reports may be used to prioritize areas where forest thinning or other protective measures may be needed prior to wildfires within these drainage basins, their outlets, or areas downstream from these drainage basins to help reduce potential burn severities. These assessments evaluate only postwildfire debris flows and will not consider hazards associated with flash floods; such hazards may remain for many years after a fire.

 

Video Interview with local reporter

 

Debris blocking a gage, tributary to Whitewater Canyon, Gila National Forest, Glenwood, NM, NMWSC

Debris blocking a gage, tributary to Whitewater Canyon, Gila National Forest, Glenwood, NM, NMWSC

(Public domain.)

Products

The report “Postwildfire debris flows hazard assessment for the area burned by the 2011 Track Fire, northeastern New Mexico and southeastern Colorado” by Anne C. Tillery, Michael J. Darr, Susan H. Cannon, and John A. Michael, is published as a U.S. Geological Survey Open-File Report: 2011-1257 and is available on the Web at (https://pubs.usgs.gov/of/2011/1257/

The report "Postwildfire preliminary debris flow hazard assessment for the area burned by the 2011 Las Conchas Fire in north-central New Mexico" by Anne C. Tillery, Michael J. Darr, Susan H. Cannon, and John A. Michael, is published as a U.S. Geological Survey Open-File Report: 2011-1308 and is available on the Web at (https://pubs.usgs.gov/of/2011/1308/

The report "Estimated probability of postwildfire debris flows in the 2012 Whitewater-Baldy Fire burn area, southwestern New Mexico" by Anne C. Tillery; Anne Marie Matherne, and Kristine L. Verdin, is published as a U.S. Geological Survey Open-File Report: 2012-1188 and is available on the Web at (https://pubs.usgs.gov/of/2012/1188/

The report "Postwildfire debris-flow hazard assessment of the area burned by the 2012 Little Bear Fire, south-central New Mexico", by Anne C. Tillery and Anne Marie Matherne is published as a U.S. Geological Survey Open-File Report: 2013- 1108 and is available on the Web at (https://pubs.usgs.gov/of/2013/1108/

The report "Potential postwildfire debris-flow hazards: a prewildfire evaluation for the Sandia and Manzano Mountains and surrounding areas, central New Mexico" by Anne C. Tillery, Jessica R. Haas, Lara W. Miller, Joe H. Scott, and Matthew P. Thompson, is published as a U.S. Geological Survey Scientific Investigations Report: 2014-5161 and is available on the Web at (https://pubs.usgs.gov/sir/2014/5161/

 

Large debris flow, tributary to Whitewater Canyon, Gila National Forest, Glenwood, NM

Large debris flow, tributary to Whitewater Canyon, Gila National Forest, Glenwood, NM

(Public domain.)

Apex of large debris flow, tributary to Whitewater Canyon, Gila National Forest, Glenwood, NM

Apex of large debris flow, tributary to Whitewater Canyon, Gila National Forest, Glenwood, NM

(Public domain.)

 

Cited References

Cannon, S.H., and Gartner, J.E., 2005, Wildfire-related debris flow from a hazards perspective, chap. 15 of Jacob, M., and Hungr, O., eds., Debris-flow hazards and Related Phenomena: Berlin, Praxis, Springer-Verlag, p. 363-385. 

Cannon, S.H., Bigio, E.R., and Mine, E., 2001a, A process for fire-related debris flow initiation, Cerro Grande fire, New Mexico: Hydrological Processes, v. 15, no. 15, p. 3011-3023. 

Cannon, S.H., Kirkham, R.H., and Parise, M., 2001b, Wildfire-related debris-flow initiation processes, Storm King Mountain, Colorado: Geomorphology, v. 39, no. 3-4, p.171-188.

Cannon, S.H., Gartner, J.E., Rupert, M.G., Michael, J.A., Rea, A.H., and Parrett, C., 2010, Predicting the probability and volume of postwildfire debris flows in the intermountain western United States: Geological Society of America Bulletin, v. 122, p. 127-144. 

DeGraff, J.V., Wagner, D., Gallegos, A.J., DeRose, M., Shannon, C., and Ellsworth, T., 2011, The remarkable occurrence of large rainfall-induced debris flows at two different locations on July 12, 2008, Sierra Nevada, California: Landslides, v. 8, no. 2, p. 343-353. 

Kean, J.W., Staley, D.M., and Cannon, S.H., 2011, In situ measurements of post-fire debris flows in southern California-Comparisons of the timing and magnitude of 24 debris-flow events with rainfall and soil moisture conditions: Journal of Geophysical Research, v. 116, 21 p. [Also available at doi:10.1029/2011JF002005.]