Why did the fissure cross the road? New and old earth fissure activity in Cochise County, Arizona
Joseph P. Cook, ARIZONA GEOLOGICAL SURVEY, Tucson
Tools for Monitoring and Predicting Earth Fissures
Once an earth fissure has breached the surface there is little than can be done to mitigate it, other than preventing water from entering and further eroding the sidewalls. Filling fissures with loose unconsolidated fill usually fails as surface flow eventually washes the fill laterally or down into the fissure. Authorities in Pinal County have filled the “Y-Crack” fissure near Chandler Heights several times, just to see the fissure reopen as monsoon rains and sheet flow erode and remobilize the fill.
Roads cut by fissures may require repeated repairs and continuous monitoring especially following intense rains. Because earth fissures form in response to overdrafting groundwater, the only guaranteed means of preventing their occurrence is to cease groundwater pumping in excess of recharge. But even if groundwater overdrafts were to cease today the compaction of valley filling sediments would likely continue for some time, perhaps years, as the compaction process lags behind decades of falling groundwater levels.
Realistically, southern Arizona is not likely to cease overpumping of groundwater any time soon. In Cochise County, numerous pecan and pistachio orchards exist with new plantings occurring near the Dragoon Road and Bowie-San Simon earth fissure study areas. With continued groundwater withdrawal we can expect continued land subsidence and formation of new earth fissures. If overdrafts of groundwater continue, how can we plan around the eventuality of new earth fissures? To prepare for fissures, we need to know where they are likely to form. It is reasonable to expect earth fissures around valley margins that have undergone significant water level decline (100 feet or more) since the early 1900s. However, it is also possible for fissures to form further from the mountain front, similar to those cropping out north of the Sulphur Hills. Because earth fissures form where differential compaction is most severe, it is beneficial to know where and at what rate subsidence is taking place.
Interferometric Synthetic Aperture Radar (InSAR) is a highly accurate method of detecting centimeter-scale changes in ground elevation (i.e., subsidence). Repeat InSAR data can be used to create an interferogram which depicts net subsidence (or uplift) over a window of time. The hydrology and geophysics/surveying unit at the Arizona Department of Water Resources (ADWR) collects InSAR data throughout Arizona and processes it to monitor ground subsidence. ADWR hydrologists have noted a highly pronounced visible correlation between the surface expression of earth fissures and areas of greatest differential subsidence as shown on InSAR interferograms (pers. comm. Brian Conway December 2010).
InSAR interferograms are proving a useful predictive tool for pinpointing new fissure formation. As Figure 5 shows, the new fissures in the Three Sisters Buttes study area correspond closely with high differential subsidence areas portrayed on InSAR interferograms. We have had some success, too, using InSAR images to guide us to unmapped earth fissures. The ability to accurately quantify subsidence trends and outline fissure-prone areas is a valuable tool for civil authorities, developers, landowners, and the general public.
Figure 5. A) Giant desiccation crack field in the Three Sisters Buttes study area. These GDCs have disrupted the surface of Fern Ct (N-S dirt road), which required repair. The distance across the white line in the GDC polygon is 250 feet. B) Polygonal crack network near known earth fissures. These polygonal cracks were previously identified as combination earth fissure-giant desiccation cracks. In the Three Sisters map they are shown as earth fissures. The polygon here measures 425 feet across. Continuous fissures are indicated in black and discontinuous fissures are indicated in red.
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