ROCKVILLE – Residents living within high rock-fall-hazard zones in Rockville face the possible consequences of a large rock fall similar to the fatal event that occurred last December. That is the principle finding of a geologic investigation into the rock fall that killed two people on December 12, 2013.
That afternoon, a huge, joint-controlled rock mass, with an estimated volume of almost 1,400 cubic yards and weighing about 2,700 tons, detached from the cliff face at the top of the Rockville Bench, near Zion National Park. The rock mass fell onto the steep slope below the cliff, and shattered into numerous fragments. The rock fall debris then moved rapidly downslope before striking and destroying a house, detached garage, and a car. The largest boulder to strike the house weighed an estimated 520 tons.
Following that event, the Utah Geological Survey completed a geologic investigation to determine what caused the incident and what risk still remained for the area. The purpose of the recently-released 20-page report was to document the characteristics of the fatal rock fall; evaluate future rock fall hazard at and near the site; and provide recommendations for homeowners, the Town of Rockville, and other officials to consider in managing future rock fall risk.
Results of the investigation show that a second, large, joint-controlled rock mass is partially detached from the cliff face above the site, and could fall at any time. Additionally, the slope below the cliff is littered with boulders related to both the December 12th and earlier rock falls all of unknown stability.
Elsewhere along the bench top, cliff-face-parallel joints are present which can serve as detachment surfaces for subsequent rock falls. The UGS concluded that the rock-fall hazard at and adjacent to this site remains very high.
“The December 12th rock fall, and the frequency and distribution of past rock falls in Rockville amply demonstrate that residents living anywhere within high rock-fall-hazard zones in Rockville face the possible consequences of a large rock fall similar to the one that occurred in December,” said Bill Lund, senior scientist for the UGS geologic hazards program.
Although this was the town’s first fatal event, this rock fall is at least the sixth large rock fall within Rockville in the past 13 years. Three of those struck and damaged buildings at the base of the Rockville Bench. In 2011, the UGS prepared a rock fall-hazard map of Rockville and identified numerous houses and other structures at the base of the Rockville Bench that are in a high rock fall-hazard zone, as was the house where the fatalities occurred. Statewide, at least 20 deaths are known to have resulted from rock falls since 1850.
An evaluation of the December 2013, rock-fall site by Utah Department of Transportation geotechnical engineers (included as an appendix in RI 273), concluded that rock-fall mitigation technologies such as rock fall fences or catchment ditches would be ineffective due to the large size and high energy of the typical boulders involved, and that the cost of prevention strategies such as rock bolting the cliff areas would greatly exceed the value of the endangered properties and the effectiveness would be questionable.
Results of the UDOT analysis leave residents living within high rock-fall-hazard areas of western Rockville with two options: (1) accept the hazard and continue to live in the high-hazard area knowing that the consequences of that decision could result in significant property damage and may prove fatal, or (2) relocate from high-hazard areas.
It is the responsibility of the Town of Rockville to ensure that current and prospective future residents and land owners within high rock-fall-hazard areas are made fully aware of the hazard, so they can make an informed decision regarding their future course of action regarding rock-fall hazard.
The UGS recommends that Rockville investigate the possibility of acquiring the properties in high rock fall-hazard areas, so occupied structures can either be retired (torn down), or move the houses to safe locations outside of those areas.
Submitted by Utah Geological Survey
The UGS has been gradually mapping out hazardous areas in Washington County for years. It released its first study of this type in 2008, focusing on the more populous areas surrounding St. George and Hurricane. A 2013 study on a previously unmapped area that the UGS labeled “the state Route 9 Corridor” was released in March, as reported by St. George News. The state Route 9 corridor surveyed includes recently mapped areas of Springdale, Rockville, Virgin and LaVerkin. Of nine geological hazard categories included in that study, flooding was the most prevalent and destructive.
To see particulars of hazards affecting these areas, and hazards mapped for the St. George – Hurricane area, see St. George News March 20 report here where folio links are included for those specific areas.
Resources and related posts
- “Investigation of the December 12, 2013, Fatal Rock Fall at 368 West Main Street, Rockville, Utah” (UGS Report of Investigation 273) is available on CD (20 p.) for $14.95 and is also available as print on demand from the Natural Resources Map & Bookstore. The report can be viewed on the UGS website athttp://geology.utah.gov/online/ri/ri-273.pdf.
- Floods, rockfalls, landslides; geologists map hazard areas along SR-9 Corridor – by St. George News March 20, 2014
- “Geologic Hazards of the state Route 9 corridor, LaVerkin City to Town of Springdale, Washington County, Utah” (UGS Special Study 148)
- Rockslide in Rockville, one home destroyed, 2 fatalities; STGnews photo gallery
- Santa Clara Heights landslide: 30 years of problems, no solution in sight
- Rockslide isolates homes, communication towers
- Highway through Zion Park closed due to flooding
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Copyright St. George News, StGeorgeUtah.com Inc., 2014, all rights reserved as to the section included, “Beyond Rockville.”