Wildfire, Rain and Floods: A case study of the June 2010 Schultz Wildfire, Flagstaff, Arizona
Ongoing Post-Fire Problems and Recovery
Photographic evidence shows that the first small post-fire rainstorm on July 16th began eroding hillslopes and scouring channels. While each post-fire precipitation event contributed to erosion, sediment transport, and flood flows in adjacent developments, the majority of the geologic work occurred on July 20th, and to a lesser degree on August 16th. As rills and gullies formed on hillslopes and channels scoured to bedrock, the response to rain changed. Runoff was more efficiently routed to bedrock channels. Flood hydrographs became flashier with higher flood flows reaching the developed areas more quickly. This is typical of post-fire erosion and flooding. As vegetation recovers and channels fill in with sediment, runoff and channel flows will decrease to near pre-fire levels, typically in three to five years (Neary and others, 2006). Research in Arizona on the Rattle Fire (1977), Coon Creek Fire (2000), and the Rodeo-Chediski Fire (2002) demonstrated that maximum peak flows most likely occur during the first monsoon immediately following wildfire, with flows in subsequent years substantially diminished. This does not preclude substantial future flooding, however, if high intesity rainfall occurs.
Figure 11. Native vegetation regrowth on steep hillslopes above FR146 (waterline) Photo: A. Youberg, AZGS, August 17th.
While vegetation has begun to recover even on very steep, high-severity burn slopes (Figure 11), it will take a few years to significantly reduce runoff. Sediment-laden floods repeatedly inundated residential areas this past summer. Coconino County Emergency Management and Coconino County Flood Control have worked around the clock to help residents prepare and deal with the flooding. The County continues to work on solutions for routing flood waters to minimize widespread flooding and to protect communities. There has been some discussion regarding the construction of retention basins to accommodate debris and flood flows from the burn area. But, construction of retention basins is expensive, requires constant sediment-removal, will not retain flood waters, and probably could not be built before vegetation recovers sufficiently to reduce runoff and flood flows.
More large floods may occur within the next few years. Future flooding depends on future rainfall amount and intensity and the recovery of vegetation on the mountain slopes, and is impossible to predict in the long-term. Until robust vegetative cover returns to hillslopes, floods over the next few years will likely carry a fair amount of sediment. With the natural re-growth of vegetation, and with the mitigation efforts of the Coconino National Forest and Coconino County, flooding should diminish over the next several years to near pre-fire levels.
Residents from the areas affected by post-Schultz Fire flooding can attest to the havoc of flooding on the environment, their lives, houses and infrastructure. A recent study in the Proceedings of the National Academy of Science shows Arizona and New Mexico have lost up to 18% of their high-altitude forest over the past 24 years due to drought, wildfires and pest infestation. This same study suggests Arizona could lose more than half of its high-altitude forests by mid-century (Williams, 2010).
We can expect more wildfires in the future, with increasing deleterious impact on human society. Many communities throughout Arizona face similar hazards as the developments downstream of the Schultz Fire dealt with this past summer. Over the past two decades, several communities throughout Arizona have dealt with post-wildfire flooding, rapid erosion, and sediment remobilization. The recent flooding and impacts to residential areas from the Schultz Fire remind us once again of the need to consider potential post-fire impacts to existing communities and future developments now, while there is time to plan and implement mitigation measures. Arizona communities in the urban-wildland interface urgently need building and flood ordinances that anticipate wildfires and potential post-fire events.
Neary, D.G.; Ffolliott, P.F. 2005. Chapter 5: Hydrologic cycle and water resources. Pp. 107-118. In: Neary, D.G.; Ryan, K.C.; DeBano, L.F. 2005. Fire effects on soil and water. USDA Forest Service, Rocky Mountain Research Station, General Technical Report RMRS-GTR-42, Volume 4: Fort Collins, CO.
Neary, D.G.; Gottfried, G.J. Beyers, J.L.; Ffolliott, P.F. 2006. Floods and sediment yields from recent wildfires in Arizona. Proceedings of the 8th Federal Interagency Sediment Conference, Las Vegas, NV April 3-6, 2006.
Robichaud, Peter R.; Ashmun, Louise E.; Sims, Bruce D. 2010. Post-fire treatment effectiveness for hillslope stabilization. Gen. Tech. Rep. RMRS-GTR-240. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 62 p.
U.S. Forest Service. 2010. Burned Area Emergency Response Report, July 8, 2010. Coconino National Forest, Flagstaff, Arizona. 167 p.
Williams, A.P., Allen, C.D., Millar, C.I., Swetnam, T.W., Michaelsen, J., Still, C.J., and Leavitt, S.W., 2010, Forest responses to increasing aridity and warmth in the southwestern United States: Proceedings of the National Academy of Sciences, v. 107, p. 21289-21294.
Other Useful Resources
RMRS Flagstaff Photo Gallery Schultz Photos
NOAA/National Weather Service's Schultz Burn Area Flood page
Coconino National Forest Schultz Fire BAER page
Coconino County Emergency Management page
Beware: Floods can follow fires. Tom Beal, Arizona Daily Star, Tucson, 10 January 2011.
Flagstaff wildfire aftermath study done. Associated Press, 9 January 2011.
Bad, but not hopeless: Revegetation hasn't worked, but the burn area may soon heal itself. Cyndy Cole, Arizona Daily Sun, Flagstaff, 6 January 2011.
Fire & Flood: Lessons from Flagstaff's Shultz Fire apply to Verde Valley. Steve Ayers, Camp Verde Bugle, Camp Verde, AZ, 6 January 2011.
Arizona: Schultz Wildfire Case Study. Hobart King, Geology.com, http://geology.com/news/2011 5 January 2011.
Arizona Fires, Floods, Earthquakes, and a Grand Canyon Time line. Jonathan DuHamel ~WryHeat, Tucson Citizen, Tucson, Arizona, 4 January 2011.
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