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1813141516171818 1314151613 17141516 17

ROCK-FALL HAZARDZION NATIONAL PARK GEOLOGIC-HAZARD STUDY AREA

EXPLANATION

T 38 ST 39 S

37 30'

113 7' 30"

2010

DISCUSSION

USING THIS MAP

MAP LIMITATIONS

SYMBOLSHigh rock-fall hazard: Areas where steep slopes below resistant cliff-forming bedrock units provideacceleration and runout zones littered with abundant rock-fall boulders >1.5 feet in diameter. Suchlarge boulders can damage property and threaten lives. Rock units in high-hazard areas include theShinarump Member of the Chinle Formation, Springdale Sandstone Member of the KayentaFormation, Navajo Sandstone, Lamb Point Tongue Member of the Navajo Sandstone, andQuaternary basalt flows. Where jointed or fractured, these rock units can produce large (>30 feet inlong dimension) angular boulders.Moderate rock-fall hazard: Areas where (1) slopes provide sufficient relief to create an accelerationzone, but where only sparse rock-fall debris is present on slopes or in the runout zone at the base ofthe slope; typically bedrock units in these areas crop out in the slope instead of forming a cappingunit; (2) talus/cliff-retreat deposits form steep slopes due to erosion and previous rock-fall boulders onthose slopes may remobilize; or (3) resistant, cliff-forming bedrock units extend to the canyon floorwith no underlying acceleration zone; rock falls in these areas tend to fall straight down and stopimmediately at the base of the cliff.Low rock-fall hazard: Areas where fine-grained, comparatively soft bedrock units such as mudstoneand shale crop out on steep slopes, or where rock units typical of moderate- or high-hazardcategories crop out in areas of low to moderate relief. Low rock-fall hazard areas typically containsparse rock sources of limited extent.Low probability, high hazard: Canyon bottoms subject to very large low-probability, high-hazard rockfalls. These areas are typically bordered by towering, jointed Navajo Sandstone cliffs. The largest ofthese events (fin collapse/rock avalanche) may involve thousands of cubic yards of material, aretypically sourced high on canyon walls, and have recurrence intervals likely measured in thousandsof years. Runout zones can extend much farther than for the smaller, more frequent rock-fall events.

ByWilliam R. Lund, Tyler R. Knudsen, and David L. Sharrow

HAZARD REDUCTION

Plate 2Utah Geological Survey Special Study 133

Zion National Park Geologic-Hazard Study Area, Washington and Kane Counties, Utah

Camp

Creek

Creek

Horse RanchMountain

Taylor Fork

North

Fork

Middle

Fork

South

LeePass

NaguntMesa

BuckPastureMountain

Creek

Timber

Kolob

CanyonsRoad

ShuntaviButte

GregoryButte

KolobArch Langston

Mountain

Creek

La Verkin

WillisCreek

BEAR CANYONTRAP

KO

LO

B

CA

NY

ON

S

BurntMountain

HO P

VA L L E Y

FirepitKnoll

PineValleyPeak

NorthgatePeaks

NorthGuardian

Angel

SouthGuardianAngel

Kolob

Terrac

eRo

ad

Kolob

Terrace

Road

North

Creek

Right

Fork

Left

Fork

Blue

Creek

Goose

Creek

LavaPoint

Kolob

Creek

DeepCr

eek

Coalp

itsWa

sh

Wash

Scoggins

Altar ofSacrifice

The WestTemple

MeridianTower The

Sentinel

MountKinesava

Wash

Hube

r

Creek

Oak

The Watchman

JohnsonMountain FORK

EAST

RIVER

VIRGIN

P A R U N U W E A P

C A N Y O N

BridgeMountain

The EastTemple

MountSpry

CraterHill

TwinBrothers

Mountainof the Sun

The GreatArch

The GreatWhite Throne

LadyMountain

CastleDome

Mount

AngelsLanding

ObservationPoint

Mountainof Mystery

WynopitsMountain

IvinsMountain

Timber

HU

RR

I CA

NE

CL I

FF

S

Death Point

Tucupit Point

Paria Point

Beatty Point

Top

Mountain

Herbs

Po

intLA

NGST

ON

CANYO

N

Currant

Creek

Cane

Creek

POLE

CANYON

L OW

E R

K OL O

B

P L A T E A U

UP P E R

K OL O

B

P L A T E A U

HO

RS

EP A

ST

UR

E

PL A

TE

AU

ZI O

N

CA

NY

ON

O R D E R V I L L E C A N Y O N

TheSubway

GREAT

WEST

CANYON

COUG

A R MO U N TA I N

CourtPatriarchsTheof

CreekBirch

Creek

Pine

Creek

Pine

Creek

Clear

Chec

kerbo

ard

Mes

a

Creek

Clear Creek

Co-op

GIFFORDCANYON

FORK

NORTH

RIVE

R

VIRGIN

ZION-MOUNT CARMEL TU NN EL

TUNNEL

Wash

Terry Wash

Jennin

gs

Wash

Coalpits

TheBishopric

HEAPS

CANYON

ECHO

CANYON

CableMountain

Temple ofSinawava

TELEPHONE

CANYON

IMLAY

CANYON

BULLOCH

GULC

HT H E

N A R R OWS

FORK

NORT

H

RIVER

VIRGIN

WILDCAT

CANYON

RUSSELLGULCH

North

Creek

North

Creek

Zion

Cany

on

Scen

icDr

ive

9

9

The Grotto

Zion CanyonVisitor Center

ZionLodge

Little

Creek

Grapevine

Wash

TOW

E RS

V I R G I NT H E

O F

Abraham Isaac

Jacob

ChurchMesa

PHANTOM VALLEY

CORRAL

HOLLOW

KolobCreek

WeepingRock

Cathedral Mountain

Majestic

Kolob CanyonsVisitor Center

WASH

INGT

ON C

OKA

NE C

O

IRON COWASHINGTON CO

Neag

le R

idge

BullpenMountain

Pocket Mesa

OAK SPRING VALLEY

PINE

VALLEYLEE

VALLEY

CaveKnoll

CAVE

VALLEY

TabernacleDome

Wash

SpringPine

TRAIL

CANYONGrea

theart

Mesa

BEHUNIN

CANYON

Creek

Shunes

Shunesburg Mountain

Wash

Hepworth

Wash

DennettCanyon

Bee HivePeak

HIDDENCANYON

CANYON

MYSTERY

R 12 W R 11 W

T 38 ST 39 S

R 11 W R 10 W

T 40 ST 41 S

T 41 ST 42 S

T 39 ST 40 S

T 41 ST 42 S

T 39 ST 40 S

T 40 ST 40.5 S

T 41 ST 42 S

R 11 W R 10 W

R 10 W

R 12 W R 11 W

R 10 W R 9.5 W R 9.5 W R 9 W

37 27' 30"

37 25'

37 22' 30"

37 20'

37 17' 30"

37 15'

37 12' 30"

37 10' 37 10'

37 12' 30"

37 15'

37 17' 30"

37 20'

37 22' 30"37 22' 30"

37 25'

113 5'

113 2' 30"

113

37 22' 30"

112 57' 30" 112 55'

112 52' 30"

112 52' 30"

112 55'112 57' 30"

113

113 2' 30"113 5'

113 7' 30"

113 10'

113 12' 30"

113 12' 30"

Zion National Park Geologic-Hazard Study Area boundaryZion National Park boundaryCounty boundaryState highwayMinor roadFoot trailExisting park structureArea not studied

This map shows areas of relative rock-fall hazard in the Zion National Park Geologic-Hazard Study Area. Site-specific, rock-fall-hazard investigations should be performed for future development in the study area asrecommended in the table below. Existing park facilities, campgrounds, and high-use trails should be evaluatedas time and funding allow, also as recommended in the table below. A geotechnical consultant should providedesign or site-preparation recommendations as necessary to reduce the rock-fall hazard. These investigationscan resolve uncertainties inherent in generalized hazard mapping and help ensure safety by identifying the needfor rock-fall-resistant design or mitigation.For some areas, site-specific assessment may only require a field geologic evaluation to determine if a rock-fallsource is present. 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 and other fractures, beddingplanes, and potential clast size. Slopes below rock sources should be evaluated for slope angle, aspect,substrate, surface roughness, and vegetation. Previous rock-fall deposits should be evaluated for distribution,clast-size range, amount of embedding, and weathering of rock-fall boulders. In addition, evaluation of the runoutzone below a source can be estimated using a simple two-dimensional model, such as the Colorado Rock FallSimulation Program (Jones and others, 2000).The hazard presented by large rock falls in areas designated "low probability, high hazard" is high, but thelikelihood of such an event at any particular location is low. These areas are considered subject only to veryinfrequent events. Site-specific investigations are not recommended for future development in these areas.

The map boundaries between rock-fall-hazard categories are approximate and subject to change as newinformation becomes available. The rock-fall hazard at any particular site may be different than shown becauseof geological variations within a map unit, gradational and approximate map-unit boundaries, and map scale.Small, localized areas of higher or lower rock-fall hazard may exist within any given map area, but theiridentification is precluded because of limitations of the map scale. This map is not intended for use at scaleslarger than the published scale, and is designed for use in general planning and design to indicate the need forsite-specific investigations.

Early recognition and avoiding areas subject to rock fall are the most effective means of reducing rock-fallhazard. However, avoidance may not always be a viable or cost-effective option, especially for existing facilities,and other techniques are available to reduce potential rock-fall damage. These may include, but are not limitedto, rock stabilization, engineered structures, and modification of at-risk structures or facilities. Rock-stabilizationmethods are physical means of reducing the hazard at its source using rock bolts and anchors, steel mesh, orshotcrete on susceptible outcrops. Engineered catchment or deflection structures such as berms or benches canbe placed below source areas, or at-risk structures themselves could be designed to stop, deflect, retard, orretain falling rocks. Conversely, after careful consideration of the hazard, it may be possible to conclude that thelevel of risk is acceptable and that no hazard-reduction measures are required.

37 27' 30"

UTAH GEOLOGICAL SURVEYa division of Utah Department of Natural Resourcesin cooperation withNational Park Service

Approximate meandeclination, 2009

12 9'o

TRUE

NOR

THMA

GNET

IC N

ORTH

Map Location

This geologic-hazard map was funded by the Utah Geological Survey and theU.S. Department of the Interior, National Park Service. The views andconclusions contained in this document are those of the authors and should notbe interpreted as necessarily representing the official policies, either expressedor implied, of the U.S. Government.Although this product represents the work of professional scientists, the UtahDepartment of Natural Resources, Utah Geological Survey, makes no warranty,expressed or implied, regarding its suitability for a particular use. The UtahDepartment of Natural Resources, Utah Geological Survey, shall not be liableunder any circumstances for any direct, indirect, special, incidental, orconsequential damages with respect to claims by users of this product.For use at 1:24,000 scale only. The Utah Geological Survey does notguarantee accuracy or completeness of data.

www.geology.utah.gov

Base map consists of U.S. Department of Agriculture 2006 National AgriculturalImagery Program natural color aerial photography and shaded relief generatedfrom digital elevation data acquired from the Utah Automated GeographicReference Center.

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

REFERENCESBiek, R.F., Willis, G.C., Hylland, M.D., and Doelling, H.H., 2003, Geology of Zion National Park, in Sprinkel, D.A.,

Castleton, J.J., 2009, Rock-fall hazards in Utah: Utah Geological Survey Public Information Series 94, 3 p.Cruden, D.M., and Varnes, D.J., 1996, Landslide types and processes, in Turner A.K., and Schuster, R.L., editors,

Elliott, A.H., and Giraud, R.E., 2009, 2nd damaging Y Mountain rock fall in four years: Utah Geological Survey,

Hylland, M.D., 1995, Fatal Big Cottonwood Canyon rock fall prompts UGS emergency response: Utah Geological

International Code Council, 2009, International building code: Country Club Hills, Illinois, 678 p.Jones, C.L., Higgins, J.D., and Andrew, R.D., 2000, Colorado Rock Fall Simulation Program, version 4.0: Report

Keefer, D.K., 1984, Landslides caused by earthquakes: Geological Society of America Bulletin, v. 95, p. 402-421.Keller, E.A., and Blodgett, R.H., 2006, Natural hazards – Earth’s processes as hazards, disasters, and

Lund, W.R., Knudsen, T.R., and Brown, K.E., 2009, Large rock fall closes highway near Cedar City, Utah: Utah

Varnes, D.J., 1978, Slope movement types and processes, in Schuster, R.L., and Krizek, R.J., editors, Landslides

Chidsey, T.C., Jr., and Anderson, P.B., editors, Geology of Utah’s parks and monuments (second edition):Utah Geological Association Publication 28, p. 106-137.

Landslides – investigation and mitigation: Washington, D.C., National Academy of Sciences, NationalResearch Council, Transportation Research Board Special Report 247, p. 36-75.

Survey Notes, v. 41, no. 3, p. 6-7.

Survey, Survey Notes, v. 27, no. 3, p. 13.

prepared for the Colorado Department of Transportation, 127 p.

catastrophes: Upper Saddle River, New Jersey, Pearson Prentice Hall, 395 p.

Geological Survey, Survey Notes, v. 41, no. 3, p. 8-9.

– analysis and control: Washington, D.C., National Academy of Sciences, National Research Council,Transportation Research Board Special Report 176, p. 12-33.

1 0 10.5 MILE1000 0 1000 2000 3000 4000 5000 6000 7000 FEET

1 0 10.5 KILOMETER

SCALE 1:24,000

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

High, Moderate

FamilyDwellings andCampgrounds

Buildings and OtherStructures That

Represent aLow Hazard

to Human Life inthe Event of Failure

All OtherBuildings and

Structures ExceptThose Listed in

Groups I, III, andIV

Buildings andOther

Structures Designated as

EssentialFacilities

Buildings and OtherStructures That

Represent a SubstantialHazard to Human Lifein the Event of Failure

HazardPotential

LowNone

I II III IV

No2

No2 YesYesYesYesYesYes

No NoNoNoNo

Occupancy Category1

1Modified from International Code Council (2009). 2Property damage possible, but little threat to life safety.

YesYes

Rock fall is a natural mass-wasting process that involves the dislodging and downslope movement of individualrocks and small rock masses (Varnes, 1978; Cruden and Varnes, 1996). Rock falls pose a safety threat becausea falling or rolling boulder can cause significant damage to property, roadways, and vehicles as well as injury oreven loss of life (see, for example, Hylland, 1995; Keller and Blodgett, 2006; Elliot and Giraud, 2009; Lund andothers, 2009). Rock-fall hazards exist where a source of rock is present above slopes steep enough to allowrapid downslope movement of dislodged rocks by falling, rolling, and bouncing.Rock-fall hazard is based on a number of factors including geology, topography, and climate. Rock-fall sourcesinclude bedrock outcrops or boulders on steep mountainsides or near the edges of escarpments such as cliffs,bluffs, and terraces. Talus cones and scree-covered slopes are indicators of a high rock-fall hazard, but otherless obvious areas may also be vulnerable.Rock falls are initiated by freeze/thaw action, rainfall, weathering and erosion of the rock and/or surroundingmaterial, and root growth. Rock fall is also the most common type of mass movement caused by earthquakes.Keefer (1984) indicates that earthquakes as small as magnitude (M) 4.0 can trigger rock falls. All nine of Utah'shistorical earthquakes of M 5 or greater have caused rock falls. Sources of earthquake ground shaking thatmight produce rock falls in the Zion National Park Geologic-Hazard Study Area include a large earthquake on theHurricane fault west of the study area, or a moderate earthquake (< M 6.5) within the study area itself (IvanWong, URS Corporation, written communication, 2008).Slope modification, such as cuts for roads and building pads or clearing of slope vegetation for development, canincrease or create a local rock-fall hazard. However, in many cases a specific triggering event is not apparent.Although not well documented, rock falls in Utah appear to occur more frequently during spring and summermonths. This is likely due to spring snowmelt, summer cloudburst storms, and large daily temperature variations(Castleton, 2009).Rock fall is the most common mass-movement type in the Zion National Park Geologic-Hazard Study Area. Thecombination of steep slopes capped by well-jointed, resistant bedrock formations provides ample opportunity togenerate rock falls. Bedrock units particularly susceptible to rock fall in the study area include the ShinarumpMember of the Chinle Formation; Springdale Member of the Kayenta Formation; Lamb Point Tongue Member ofthe Navajo Sandstone; other ledge- and cliff-forming strata in the Moenkopi, Moenave, and Kayenta Formations;and the massive, pervasively jointed, cliff-forming Navajo Sandstone. Rock falls are particularly prevalent andhazardous where softer, more easily eroded bedrock units crop out on slopes below stronger, more resistantbedrock formations. Erosion of the underlying soft units and subsequent undercutting of the more resistantbedrock formations triggers many rock falls.Talus deposits blanket steep to moderate slopes throughout the study area. These deposits are derived fromupslope ledges and cliffs and consist chiefly of accumulations of poorly sorted, coarse, angular blocks of varioussizes. The boulders in talus deposits may exceed 30 feet in long dimension (Biek and others, 2003). Thewidespread distribution of talus and the direct relation of talus deposits to the rock-fall process attest to thewidespread extent of the rock-fall hazard in the study area.For additional information about the rock-fall hazard in the Zion National Park Geologic-Hazard Study Area, referto the Rock-Fall Hazard chapter in this report.

_