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2021 Monsoon rains reactivate earth fissures in Cochise County, Arizona

/ 19 August 2021 / 0 Comments

Monsoon storms in July and August 2021 have dumped abundant rainfall across southern Arizona; with precipitation in the Willcox Basin area of Cochise County, including the towns of Dragoon, Elfrida, and Willcox upwards of ~8 inches, with the upper slopes of nearby mountain ranges receiving 14- to 16- inches. In early July a portion of U.S. 191 southwest of Willcox Playa was closed due to collapse along large surface fractures. These cracks underlie the highway and extend into the desert on both sides. Heavy rains flooded the open cracks, which were dramatically enlarged by rapid erosion.

Continued rains in July and August have resulted in repeated flooding and inundation of the landscape and roadways in Cochise County. Sometime around August 13th-14th the Cochise County Sheriff’s office reported damage to Dragoon Road west of U.S. 191, and to two areas of Parker Ranch Road east of Kansas Settlement Road. These roads are currently closed to traffic and marked with barriers. Much like the collapse along Highway 191, this damage was caused by erosion along open fractures driving partial collapse of the roadway.

Fissure Cochise County
Figure 1. View to the southwest from the damaged area of Dragoon Rd.

Damage to Dragoon Road occurred where a known earth fissure passes beneath the roadway (Figure 1). Erosion and collapse was observed on both the north and south sides of the road. Collapse on the south side was deeper with some portions of the fissure exceeding 3m (10 ft) wide and 4m (13 ft) deep (Figure 2). The southern portion (eastbound lane) of the roadway was completely undermined by the open fissure and the asphalt layer was sagging into the open void. Hairline cracks on both sides of the collapsed area extended across the entire roadway. On the north side the fissure extended into recently plowed land with a mix of long open reaches and zones of partial collapse and hairline cracks. The total length of the fissure is roughly 0.7 km (0.45 miles). This fissure was previously mapped by AZGS and is shown on the Dragoon Road Earth Fissure Map (Figure 3). The location of this earth fissure closely matches the trend of land subsidence depicted on the Arizona Department of Water Resources Kansas Settlement map. We now have active earth fissures on both sides of the subsidence bowl in this area. The earth fissure on the west side lies beneath the intersection of Cochise Stronghold and W Dragoon Roads and has repeatedly damaged the roadway, requiring costly repair.

Figure 2
Figure 2. View to the north along reactivated earth fissure passing beneath Dragoon Road. The Apache Generating Station is visible in the background
Figure 3
Figure 3. Excerpt from AZGS Dragoon Road fissure map (DM-EF-16). The reactivated fissure described here is annotated with a yellow arrow.

Four earth fissures pass beneath Parker Ranch Road east of Kansas Settlement Road. Several of these fissures were first observed in 2010 and are shown on the AZGS Three Sisters Buttes Earth Fissure Map. Two of these fissures re-opened during recent rains, resulting in major damage to the roadway (Figure 4). The western fissure crosses beneath Parker Ranch Road 1.7 km (1.1 miles) east of Kansas Settlement Road and is roughly 0.5 km (0.35 miles) long. On August 16, 2021, standing water was observed in the fissure on both sides of the road (figure 5). The southern portion (eastbound lane) of the road was completely undermined by collapse along the fissure, the asphalt layer was sagging into the underlying void, and hairline cracks in the asphalt extended across the road on both sides of the fissure (Figure 6). While on site, large chunks of the fissure wall periodically slumped into the water-filled fissure with loud splashes. At one point a large piece of asphalt in the road fell into the void below. Although the total depth of the open portion of this fissure was not observable due to the muddy water, widths of at least 3m (10 ft) and depths of 4m (13 ft) were observed in drier portions to the south.

Figure 4.
Figure 4. ) Excerpt from AZGS Three Sisters Buttes fissure map (DM-EF-22). The reactivated fissures described here are annotated with yellow arrows
Figure 5 and 6
Figure 5. A suspended fence extends across a water-filled earth fissure just south of Parker Ranch Road.   Figure 6. Collapse of Parker Ranch Road into a void caused by erosion along an earth fissure first observed in 2010. This is the second time this portion of the roadway has suffered major damage due to collapse at this location.

Collapse was most dramatic at the eastern fissure passing beneath Parker Ranch Road (approximately 3 km (1.85 miles) east of Kansas Settlement Road). Here the fissure was approximately 3m (10 ft) wide and up to 5m (16.5) deep with a total length of 0.7 km (0.45 miles). The roadway was completely undermined with only a thin bridge of asphalt and soil bridging the gap formed by collapse along the fissure (Figure 7). Broken and sagging asphalt was observed directly above the void with hairline cracks across the road on both sides of the collapsed zone. It was possible to see completely underneath the roadway from certain vantage points (Figure 8). Erosion and collapse along the fissure south of the road was significant. Large columns of soil and angular blocks of soil bearing vegetation were leaning or had already collapsed into the open fissure (Figure 9). Hairline cracks alongside and parallel to the fissure indicated unstable ground. Muddy pools and standing water were still present in some areas while high-water lines marked the extent of inundation in others. Some new collapse along gaps in between previously mapped fissures to the south was also observed.

Figure 7 and 8
Figure 7. Major undermining of Parker Ranch Road where it passes above an earth fissure first observed in 2010. This is the second time this portion of the roadway has suffered major damage due to collapse at this location.   Figure 8. Only a thin bridge of asphalt is intact above the void formed by erosion along the earth fissure.  

Reactivation and renewed collapse along existing earth fissures during and following heavy rains is a common process throughout Arizona, especially when rains are intense and repeated as sometimes observed during the monsoon season. Precipitation during monsoon-2021 has been especially heavy in Cochise County so new damage caused by erosion of existing fissures is not unexpected, although the extent and magnitude of collapse is impressive. Continued land subsidence in Cochise County due to groundwater withdrawal is likely to lead to the development of more earth fissures and flood-prone areas, so the potential for damage to infrastructure, loss of access to private property, and negative impacts on farmland is likely to increase as well.

Figure 9

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Posted by J.P. Cook (19 Aug. 2021)

 

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earth fissure
geologic hazard
Cochise County
Willcox Playa
monsoon

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