Utah Geological Survey investigation into fatal Rockville rock fall; high risk remains

A rockslide in Rockville that destroyed a home and killed two people, Rockville, Utah, Dec. 12, 2013 | Photo by Dan Mabbutt, St, George News

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.

Utah Geological Survey rock-fall hazard zones and significant historical rock falls (stars) in the Town of Rockville | Map courtesy of Utah Geological Survey, St. George News
Utah Geological Survey rock-fall hazard zones and significant historical rock falls (stars) in the Town of Rockville | Map courtesy of Utah Geological Survey, St. George News

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

Beyond Rockville

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.

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Copyright St. George News, StGeorgeUtah.com Inc., 2014, all rights reserved as to the section included, “Beyond Rockville.”

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4 Comments

  • Bub April 3, 2014 at 10:01 pm

    I could have saved them the trouble of writing this report… BIG LOOSE ROCKS + GRAVITY + HOUSE BUILT BELOW BIG LOOSE ROCKS = EXHIBIT A. Just sayin’ 😀

    • Brian Daniels April 4, 2014 at 12:17 am

      I agree… I’m sorry these people died… but you know they never thought it would happen to them. My life is worth way more than a view.

  • chupacabra April 4, 2014 at 10:07 am

    I mean c’mon, only 20 people have perished in Utah since 1850 due to falling rocks. I guess I should move to a nuclear bunker for fear of the sky falling. We all live with risk each day. The likelihood of being killed in traffic is exponentially greater for every household in ROCKville. People know what town they live in and accept the added risk. Placing a burden on the town of Rockville and its residents is nothing more than a fear-mongering tactic led by yet another beaureaucracy.

    • Bender April 4, 2014 at 12:34 pm

      You miss the point of the article, oh mythical beast. You and I and almost all of the rest of the state are at an almost zero risk of dying of rockfall in our homes. The people living under these geologic time bombs are at a hugely elevated risk of becoming the meat in a bed sandwich. The risk of dying in an auto accident may still be higher, but certainly not exponentially higher. The important point is that these people can completely remove this risk by not living under these crumbling cliffs.
      .
      I would choose to use my nine lives doing something more exciting than living at the base of a cliff. That’s just me though.

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