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ROCK-FALL-HAZARD MAP FOR THE ST. GEORGE–HURRICANE METROPOLITAN AREA

Basemap consists of National Agricultural Imagery Programnatural color aerial photography.

Universal Transverse Mercator Projection, zone 12.North American Datum 1983.

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DISCUSSION

USING THIS MAP

MAP LIMITATIONS

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High rock-fall-hazard areas include steep slopes below resistant cliff-forming units where the acceleration and runout zones are litteredwith abundant rock-fall boulders greater than 1.5 feet in diameter. Such large boulders can do significant damage to property andthreaten lives. Rock units in high rock-fall-hazard areas chiefly include the Shinarump Member of the Chinle Formation, upper KayentaFormation, Springdale Sandstone Member of the Kayenta Formation, Navajo Sandstone, Virgin Limestone Member of the MoenkopiFormation, Kaibab Formation, Toroweap Formation, and Tertiary–Quaternary basalt flows. Where jointed or fractured, outcrops ofthese rock units can produce large, angular boulders.

Moderate rock-fall-hazard areas are present where slopes provide sufficient relief to create an acceleration zone, but where onlysparse rock-fall debris is present on slopes or in the runout zone at the base of the slope. Typically rock units in moderate-hazardareas crop out in the slope instead of forming a capping unit, or where a capping unit is present, the resulting rock-fall debris istypically less then 1.5 feet in diameter. Rock units in moderate rock-fall-hazard areas include the Kayenta Formation, MoenaveFormation, and portions of the Kaibab Formation and Navajo Sandstone.

Low rock-fall-hazard areas are present where fine-grained, comparatively soft rock units such as mudstone and shale crop out onsteep slopes, or where rock units typical of moderate- or high-hazard categories crop out in areas of low to moderate relief. Low rock-fall-hazard areas typically contain sparse rock sources of limited extent. Low rock-fall hazard is most common adjacent to low reliefoutcrops of Navajo Sandstone and basalt.

SYMBOLS

Municipality boundary

Interstate Highway

State Highway

Major local surface street

Other road

The Rock-Fall-Hazard Map is based on limited geologic and geotechnical data; site-specific studies are required to produce more detailedgeotechnical information. The map also depends on the quality of those data, which varies throughout the study area. The mappedboundaries between rock-fall-hazard categories are approximate and subject to change with additional information. The rock-fall hazard at anyparticular site may be different than shown because of geological variations within a map unit, gradational and approximate map-unitboundaries, and the regional map scale. Small, localized areas of higher or lower rock-fall hazard may exist within any given map area, buttheir identification is precluded because of limitations of map scale. This map is not intended for use at scales larger than the published scale,and is designed for use in general planning to indicate the need for site-specific studies.

planes, and potential clast size. Slopes below rock sources should be evaluated for slope angle,aspect, substrate, surface roughness, vegetation, and distribution, size range, amount of embedding,and weathering of rock-fall boulders. In addition, evaluation of the runout zone below a source can beestimated using a simple 2-dimensional model such as the Colorado Rock-Fall Simulation Program(CRSP). This map does not consider rock-fall hazards caused by cuts, fills, or other alterations to thenatural terrain.

2008

William R. Lund, Tyler R. Knudsen, Garrett S. Vice, and Lucas M. Shaw

PLATE 5

Rock fall is a natural mass-wasting process that involves the dislodging and downslope movement of individual rocks and small rock masses.The combination of steep slopes capped by well-jointed, resistant bedrock formations such as the Shinarump Member of the Chinle Formationand numerous Quaternary basalt flows, makes rock fall the most common slope-failure type in the St. George – Hurricane metropolitan area.Rock falls pose a hazard because a rolling boulder can cause significant damage to property, roadways, and vehicles and thus pose a serioussafety threat. Rock-fall hazards are found where a source of rock exists above slopes steep enough to allow rapid downslope movement ofdislodged rocks by falling, rolling, and bouncing. Rock fall is also the most common type of slope failure caused by earthquakes. Earthquakesas small as magnitudes 4.0 have triggered rock falls. Additionally, slope modification such as cuts for roads and building pads or clearing ofslope vegetation for development can increase or create a local rock-fall hazard.

For additional information about the rock-fall hazard in the St. George – Hurricane metropolitan area, refer to the Rock-Fall-Hazard textdocument in this report.

HAZARD REDUCTIONEarly recognition and avoiding areas subject to rock fall is the most effective means of reducing rock-fall hazards. However, avoidance maynot always be a viable or cost-effective hazard-reduction option, especially for existing developments, and other techniques are available toreduce potential rock-fall damage. These may include but are not limited to rock stabilization, engineered structures, or modification of at-riskstructures or facilities. Rock-stabilization methods are physical means of reducing the hazard at its source using drilled bolts, steel mesh, orgrout on susceptible outcrops. Engineered catchment or deflection structures such as berms or benches can be placed below source areas, orat-risk structures themselves could be designed to stop, deflect, retard, or retain falling rocks.

The UGS recommends retaining a geotechnical firm familiar with rock-fall hazards early in the project design phase to conduct a site-specificinvestigation of the proposed site. If a rock-fall hazard is present, the geotechnical consultant should provide design or site preparationrecommendations as necessary to reduce the hazard. In areas where a site-specific evaluation indicates that rock falls are possible, but therock-fall hazard is low, disclosure of the hazard to landowners and residents may be an acceptable alternative to avoidance or costly hazard-reduction efforts. Disclosure ensures that buyers are informed of the hazard and are willing to accept the associated risks.

The Rock-Fall-Hazard Map shows areas of relative rock-fall hazard in the St. George – Hurricanemetropolitan area where site-specific hazard studies are recommended prior to construction (see tablebelow). These studies can resolve uncertainties inherent in generalized hazard mapping and helpensure safety by identifying the need for rock-fall-resistant design or mitigation. For most areas, site-specific assessment may only require a field geologic evaluation to determine if a rock-fall source ispresent. However, if a source is identified, additional work to adequately assess the hazard is needed.Rock-fall sources should be evaluated for the following parameters: rock type, joints, fractures, bedding

Although this product represents the work of professional scientists, the Utah Department of Natural Resources,Utah Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use.The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under anycircumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims byusers of this product.

For use at 1:24,000 scale only. The Utah Geological Survey does not guarantee accuracy or completeness ofdata.

Recommended Requirements for Site-Specific Investigations Related to Rock-Fall Hazards to Protect Life and Safety

High, Moderate

One- and Two-Family

Dwellings andTownhouses

Buildings and OtherStructures thatRepresent a

Substantial Hazardto Human Life in

the Event of Failure

All OtherBuildings and

Structures ExceptThose Listed in

Groups II, III, andIV

Buildings andOther

Structures Designated as

EssentialFacilities

Buildings and OtherStructures that

Represent a LowHazard to Human Lifein the Event of Failure

HazardPotential

LowNone

I II III IV

YesYes YesYesYes

YesYesYes

No NoNoNoNo

No2No2

2 Property damage possible, but little threat to life safety.

Classification of Buildings and Other Structures for Importance Factors1

1Occupancy category from the International Code Council (2006).

UTAH GEOLOGICAL SURVEYa division of

Utah Department of Natural Resources

Plate 5

Utah Geological Survey Special Study 127

Geologic Hazards and Adverse Construction Conditions,

St. George−Hurricane Metropolitan Area, Washington County, Utah