Post-Monument Fire Floods and Debris Flows in the Huachuca Mountains, southern Arizona

Environmental Geology

Sep 12, 2011

Post-Monument Fire Floods and Debris Flows in the Huachuca Mountains, southern Arizona

Article Author(s):

Ann Youberg
Phil Pearthree

The summer of 2011 saw huge wildfires burning across Arizona. The Wallow Fire (538,049 acres) set a new state record, burning a larger area than the 2002 Rodeo-Chediski Fire (468,638 acres). The Horseshoe 2 Fire (222,954 acres) burned approximately 70% of the Chiricahua Mountains and became the 4^th largest fire in Arizona history. In addition to the Horseshoe 2 Fire, two other large wildfires (Murphy Complex and the Monument Fire) and numerous smaller fires burned a total of 366,679 acres in the Coronado National Forest. The wildfires of this summer continued a pattern of large fire occurrence that began in the 1990s; during the past 20 years, fires larger than any that occurred earlier in the 20^th century have been fairly common.

Indeed, as of August 31st, 2011, the total acreage burned in Arizona was 1,008,753 acres, exceeding the previous record of 975,000 acres set in 2005 (Table 1; Southwest Coordination Center). Lightning-caused wildfires continue to burn in the state so the final number of acres burned will be higher by the close of 2011. In Arizona, monsoon rains immediately follow wildfire season and are often how fires are ultimately extinguished. But the quick, intense burst of rainfall from relatively common (< 2-5 yr frequency) monsoon storms can generate large floods and debris flows in watersheds disturbed by wildfires.

Table 1. Ten largest fire years for Arizona, by cause and acreage, from 1990 to August 31, 2011. (http://gacc.nifc.gov/swcc/predictive/intelligence/ytd_historical/histori...)

	Number of Fires		Acres
	Year	Human Caused	Lightning Caused	Human Caused	Lightning Caused	Total
2011 (as of August 31, 2011)	1,013	693	927,408	81,345	1,008,753
2005	2,731	1,276	197,752	777,426	975,178
2002	1,833	1,385	599,383	30,493	629,876
1995	3,318	1,526	119,366	125,397	244,763
2004	1,479	1,552	49,074	185,011	234,085
2009	1,452	875	88,772	133,857	223,036
1994	2,469	2,110	40,793	182,106	222,899
2003	1,346	1,701	118,283	86,536	204,819
1993	3,719	1,016	117,049	87,725	204,774
1996	1,747	2,033	89,916	98,271	188,187

The Monument Fire near Sierra Vista was not a particularly large fire in this context, but because of its proximity to developed areas and extreme fire behavior it resulted in the greatest damage to homes and other buildings. The fire started on June 12^th and burned 30,526 acres by the time of containment on July 12^th (http://www.inciweb.org/incident/2324/), covering the southern 30% of the Huachuca Mountains and extending out into adjacent grasslands. A total of 62 residences, 18 outbuildings and 4 businesses were lost to the fire. Within the fire perimeter approximately 7% of the area was burned at high severity, mostly on higher steep mountain slopes, 39% at moderate severity, 40% at low severity, and approximately 14% remained unburned (Figure 1).

Debris flows and flooding occurred in canyons of the Huachuca Mountains after fires in 1977, 1988, and 2006, so we anticipated that similar activity would occur this summer. A U.S. Forest Service Burned Area Emergency Response (BAER) team conducted a rapid assessment of damages from the fire and recommended watershed treatments to attempt to mitigate the increased risks from flooding, erosion and debris flows. As an extension of the BAER work, the USGS modeled the probability and volumes of potential post-fire debris flows for the Monument Fire (http://pubs.usgs.gov/of/2011/1181/). Cochise County and the Arizona Department of Water Resources (ADWR) installed two ALERT rain gauges (Figure 2), one in Ash Canyon and one on the ridge between Miller and Carr Canyons, to provide real-time rainfall information for issuing flood warnings in cooperation with the National Weather Service.

Figure 1. Map of the Monument Fire burn severity (Coronado National Forest).

The first significant rainfall on the Monument Fire burn area occurred on the afternoon of July 10^th. The storm was centered over Miller and Carr Canyons, causing floods and debris flows in both canyons. Beatty’s Guest Ranch, a private inholding in Miller Canyon, was severely damaged by debris flows generated from this storm (Figure 2). Tom and Edith Beatty have lived on their property in Miller Canyon for over 40 years, and thus experienced a wildfire in 1977 that burned through Miller Canyon and their property. That fire was also followed by debris flows that caused significant damage to the Beatty’s property, so they were well aware of the increased risks of floods and debris flows from the Monument Fire.

We visited the Beatty’s property on July 8^th, before any significant rainfall, to see how the fire had affected their property and to photo document channel and hillslope conditions. On the afternoon of July 10^th, during the first significant rainfall, the ALERT gauge installed on the ridge between Miller and Carr canyons (Figure 2) recorded 1.64” of rainfall between 13:31:40 and 14:35:40 (MST). The ALERT data indicates a peak 10-minute rainfall intensity of 0.72”, more than enough to generate post-fire debris flows, and indeed Tom Beatty reported significant and damaging flows through his property. On July 11^th, we returned to the Beatty’s to assess damage from debris flows and floods. Photos from July 8^th and 11^th document the dramatic changes caused by the post-fire floods and debris flows (yellow arrows indicate the same object between photos, like trees or rocks, and blue arrows indicate direction of stream flow).

Debris flows and floods emanated from several drainages above the Beatty’s property. The largest drainage is Miller Creek, which crosses the south end of the property (Figure 2). The next two tributary drainages north are informally called Marshall Canyon and Beatty’s Gulch, respectively (Figure 2). On July 10^th Miller Creek had a debris flow that terminated just upstream of the confluence with Marshall Canyon. The snout of this flow was deposited in the main channel and was approximately 3 feet thick with sediment ranging from silt and sand to small boulders. The debris flow generated in Marshall Canyon was quite large and probably consisted of several pulses or multiple debris flows. It damaged a cabin and buried an apple orchard. At the confluence with Miller Creek, the Marshall Canyon debris fan is approximately 7 feet thick with sediment sizes ranging from silt and sand to large boulders. Part of the debris flow continued down Miller Creek depositing sediment in the former channel and scouring a new channel to the south. Sediment deposited by the Marshall Canyon debris flows extends down Miller Canyon to the Miller Canyon Trailhead parking area. Beatty’s Gulch produced sediment-laden flood flows that damaged another cabin and filled a pond with sediment. The largest material from this drainage was large cobbles to small boulders, but most of the material was composed of sand and pebbles.

Figure 2. Map showing location of Beatty’s property (white box), the ALERT rain gauge (white dot), and drainages with July 10th debris flows (green asterisk). High burn severity is shown in red, moderate in yellow and low in blue (see Figure 1 for burn severity of the entire fire).

Although these flows moved a substantial amount of sediment, the upper watersheds still have plenty of material available for transport, so the risk of post-fire floods and debris flows is ongoing. The now-disturbed channels that cross the Beatty’s property will continue to evolve with subsequent flows. Channels will likely fill and scour as pulses of sediment move through the system. With this in mind, we visited the Beatty’s again on August 3^rd and on September 1^st to assess if subsequent rains had produced debris or flood flows. Floods continue to shift sediment, but we did not find evidence of any new debris flows on the Beatty’s property.

During the August and September field surveys, we also visited other canyons in the burned area to look for evidence of floods and debris flows. Carr Canyon had a post-fire debris flow on July 10^th, as reported by a BAER implementation team member. Ash, Montezuma and Copper Canyons did not have evidence of post-fire debris flows on August 3^rd. During the September 1^st field visit evidence of post-fire debris flows were observed in Copper and Hunter Canyons. These were likely generated by an August 11^th storm that caused flooding at the Coronado National Memorial Visitor’s Center (link to video by USGS: http://gallery.usgs.gov/videos/437). Ash and Lutz Canyons may have also had post-fire debris flows generated from the August 11^th storm, but this is still being assessed. The end of the monsoon season is approaching, and we anticipate that the potential for flooding and debris flows will diminish over time as vegetation recovers. Runoff will decrease, reducing the magnitude of flood flows and lessening the potential for debris flows, resulting in more stable hillslopes and channels. This decrease in runoff to something close to pre-fire conditions typically takes 3 to 5 years following a wildfire, but may take longer depending on watershed conditions and storm patterns.