Wildfire, Rain and Floods: A case study of the June 2010 Schultz Wildfire, Flagstaff, Arizona
Ann Youberg, ARIZONA GEOLOGICAL SURVEY, Tucson
Karen Koestner and Dan Neary - Rocky Mountain Research Station, Flagstaff
The summer of 2010 brought wildfires and near record monsoon rains to the San Francisco Peaks near Flagstaff, Arizona. The human-caused Schultz Fire on the Coconino National Forest northeast of Flagstaff (Figure 1) was the largest wildfire in Arizona during 2010 (www.inciweb.org). Ignited by an abandoned campfire on June 20th at Schultz Tank and Elden Trail, the Schultz Fire burned hot and fast (http://inciweb.org/incident/1996/). High winds quickly drove the blaze across the steep eastern slopes of the San Francisco Peaks: approximately 60% of the total 15,075 acres (23.5 sq mi) burned that first day (Figures 2 and 3; U.S. Forest Service, 2010). Over a thousand residents from nearby housing developments were evacuated, but no structures were directly impacted. By the time the fire was 100% contained on June 30th, the assessment of damages and preparation for monsoon rains was well underway. Following the fire, heavy rains from the 4th wettest monsoonon record in Flagstaff resulted in numerous debris flows, significant erosion, and substantial flooding of the residential areas below. While debris flows were confined to the forest upslope of residential neighborhoods, multiple sediment and ash-laden floods caused extensive damage to homes, property and infrastructure up to 4 miles from the burn. There were no fatalities from the fire itself, but a 12-year old girl was tragically killed in a flash flood in her neighborhood on the afternoon July 20th.
Figure 1. Location map of Schultz Fire (orange outline)
northeast of Flagstaff, Arizona.
Figure 2. Schultz Fire Progression Map.
Data from Coconino National Forest.
Figure 3. View of the Schultz Fire on June 24. The white almost horizontal line on the steep
is FR146 (waterline). Timberline Estates in the foreground. Photo: D. Fleishman, USFS.
In early August of 2010, AZGS geoscientists joined researchers from the U.S. Forest Service Rocky Mountain Research Station’s (RMRS) Southwest Watershed Team to document and quantify the effects of fire on geomorphic and watershed processes following the Schultz Fire. Our goals are to provide land managers and government agencies with the tools necessary for assessing and effectively mitigating post-fire geologic hazards, potential impacts to long-term water quality and supply, threats to infrastructure, human life and property, and to understand local landscape evolution processes.
Watersheds subject to moderate to high severity burns are prone to much greater runoff – both in volume and velocity - due to decreased interception and infiltration (Neary and Ffolliott, 2005). Excess runoff increases hillslope and channel erosion resulting in sediment-laden flood flows or debris flows. Newly formed rills and gullies on the upper hillslopes of the Schultz Fire burn area fed into existing drainages or swales, further concentrating runoff and scouring channels to bedrock. Precipitation that, prior to the fire, would not have produced much runoff, produced very large post-fire flood flows and debris flows.
The Schultz Fire burned several peaks on the east side of the San Francisco Peaks composite volcano. Drainages descend from the steep mountain slopes into confined channels within Pleistocene alluvial fan deposits. Near the forest boundary the channels emerge onto unincised, heavily modified Holocene alluvial fans where housing developments have sprung up over the past quarter century. The Schultz Fire was a high-impact fire with the majority of the fire area burned at moderate (27%) or high (40%) severity on moderate to very steep slopes that were covered by Ponderosa Pine and mixed conifer forests (Figure 4, U.S. Forest Service, 2010). Several watersheds burned almost completely (Figure 4). Removal of the forest floor litter, alteration of soil properties, development of fire-induced water repellency, and loss of tree canopy in the moderate and high severity burn areas dramatically impacted the hydrologic behavior of this landscape.
Figure 4. Schultz Fire BAER basins with burn severity and ALERT stations. BAER basins identified as Basins of Concern are in blue. Data from Coconino National Forest. Basin outlines generated from a 10m DEM.
Burned Area Emergency Response (BAER)
Several days before containment, a U.S. Forest Service Burned Area Emergency Response (BAER) team began to asses the fire impacts on the forest to identify potential resources at risk and to determine appropriate mitigation measures. Their mitigation goals were to reduce flooding potential and retain on-site soils (Robichaud and others, 2010). Of particular concern were the City of Flagstaff’s waterline road (FR146), which provided approximately 20% of Flagstaff’s summer water from the Inner Basin of the San Francisco Peaks, cultural resources, soil erosion, and flooding impacts to downstream developments (U.S. Forest Service 2010). The BAER team assessed 11 basins within the burned area (Figure 4) and identified five as basins of concern (Figure 4) due to the burn severity, total area burned, and the steepness of the slopes (US Forest Service, 2010).
Initial BAER Treatments
The Coconino National Forest, Flagstaff Ranger District (formerly Peaks Ranger District), initiated treatments recommended by the BAER team immediately after completion of the report and concurrence from regional and national BAER Program coordinators. BAER team recommendations included:
- Aerial application of certified weed-free straw mulch on lower slopes
(5-60%) of high severity burn
- Manufactured wood straw on high severity burn slopes (40-100%).
- Removal of 30 culverts along Schultz Pass Road (FR420) to facilitate flood passage.
- Placement of large rock rip-rap on targeted fill slopes along the FR146 (waterline).
Seeding was initially considered but not recommended. Channel treatments were not recommended due to the steep terrain of the high-severity burn. Only straw mulch was applied to maximize the area of coverage with available funds; application was complete on July 22nd. Culvert removal along FR420 and rip-rap application along FR146 was complete by July 19th.
Post-Fire Storms, Floods and Debris Flows
The first rain fell on July 16th with minimal impacts. Flood waters resembled typical black, post-fire, ash-laden flows (Figure 5). The next storm, on July 20th, produced 1.78 in of rain in 45 minutes, with a very high peak 10-minute intensity of 0.98 in (Figure 6). This short duration, high-intensity precipitation event produced debris flows and flooding that eroded channels and broke the waterline in two places. Downstream flooding was surprisingly widespread, extending into the community of Timberline west of US89, through the Fernwood development east of the highway, and ultimately inundating low-lying areas in the Doney Park development 4 miles from the burn. Another high intensity storm on the 16th of August delivered 1.06 in of rain in 46 minutes with a peak 10-minute intensity of 0.59 in and produced a second round of debris flows. Several other storms caused repeated flooding between and after these events, but the storms of July 20th and August 16th were the only ones known to generate multiple debris flows in seven basins.
Figure 5. Flood flow in drainage crossing FR420 on July 16. Flow from upper right to lower left. Recorded precipitation amounts at the ALERT rain gauges varied from 0.01-0.55 in. Photo: A. Stevenson, USFS.
Figure 6. Flood flow in drainage crossing FR420 early in the storm on July 20th. Flow from upper right to lower left. Recorded precipitation amounts at the ALERT rain gauges varied from 1.46-1.78 in. Photo: A. Stevenson, USFS. Video: B. Prusse, USFS.
Additional BAER Treatments
A second BAER rapid assessment of July 29th, 2010, evaluated the impacts of the July 20th flood event. Four additional treatments were recommended and implemented in August 2010. These treatments included aerial seeding with a blend of native grasses and non-persistent annuals, applying straw mulch in select areas, removing log-debris jams in two drainages, and constructing waterbars and drains on 23 miles of Forest roads.
A third round of BAER funding was made available in October 2010. By October 29th select areas were reseeded with mix of native grasses and barley (a non-native annual), intended to "jumpstart" native regeneration and reduce erosion in 2011. Another application of mulch with weed-free straw was complete November 7th. Areas with slopes greater than 40% required a second application of seed and mulch to replace materials lost to wind or water.
The series of BAER treatments qualify as emergency stabilization. Recent research by Robichaud and others (2010) has shown that straw mulch applied on slopes <60% is effective at stabilizing slopes and inhibiting rill and gully development. However, treatments on the low to moderate slopes have had very little impact on flooding and sediment movement in the Shultz fire area because of the concentration of high-severity burn on steep mountain slopes. On these slopes, the efficacy of straw mulch and seed applications for mitigating runoff and erosion is questionable because these treatments are susceptible to removal by surface runoff or wind (Robichaud and others, 2010).
Within the Schultz Fire burn area, debris flows and high energy flood flows originate high in the watersheds on the extremely steep and severely burned slopes of the San Francisco Peaks. In the upper-basins around FR146 straw mulch washed down the hillslopes resulting in the need for re-applications of mulch. Based on our observations, over half of the straw mulch applied to the steep slopes of the Schulz Fire has been removed by surface runoff and wind. Nonetheless, multiple straw mulch and seed applications were made to aid in the reduction of runoff and soil erosion during the summer and fall of 2010. By using the winter snowpack to stabilize the straw mulch and seeds, runoff and erosion should be reduced further in 2011.
Figure 8. Same drainage as Figure 7. This photo was taken on August 9th after the July 20th debris flows and several other rain storms. Photo: D. Fleishman, USFS.
Figure 7. Drainage swale along FR146 (waterline) taken on June 29 after the fire but before rain. Note the swale and smooth hillslopes. Photo: City of Flagstaff Water Department.
Post-fire flooding and debris flows accomplished a significant amount of geologic work in a very short period of time. The first major flood event of July 20th exposed bedrock in channels that had previously contained substantial amounts of sediment. The rip-rap barriers installed to protect FR146 and the waterline beneath at drainage crossings were completely destroyed and the waterline was broken at two drainage crossings. The following photos from two drainages (Figures 7, 8 and 9) and along FR146 (waterline, Figure 10) illustrate the impact of this fire and subsequent rains on Coconino National Forest lands. There are several key points to note in these photographs. Photos taken after the fire but before monsoon rains show drainage swales with no defined channel bottom or banks (Figure 7), while post-storm channels are either filled with fresh debris (Figure 8) or scoured to bedrock with steep banks up to 3 m high (Figure 9). Notice the size of material, both rock and woody debris, and depths of scour in these channels. Over time, the channel banks will collapse and more sediment will feed into the channel from the adjacent hillslopes, refilling the channel. Before the rains, hillslopes were smooth; now they have numerous rills and gullies (Figure 10). Also note that vegetation was already growing on steep hillslopes by mid-August (Figure 11). Sediment scoured from hillslopes and channels on the steep slopes of the burned area was deposited on fans and in channels at the base of those slopes.
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.