Wildfire, Rain and Floods: A case study of the June 2010 Schultz Wildfire, Flagstaff, Arizona
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.
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