Peat Landslide Hazard and Risk Assessment - Scottish Executive
S ZION NATIONAL PARK GEOLOGIC-HAZARD STUDY AREA LANDSLIDE HAZARD - Utah · 2015-12-15 · For...
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The term includes both deep-seated and shallow mass movements (Cruden and Varnes, 1996). The moisture content of the affected materials at the time of landsliding may range from dry to saturated. Landslides can be both damaging and deadly. The U.S. Geological Survey (USGS) estimated that in the United States, landslides on average cause $1-2 billion in damage and more than 25 deaths annually (USGS, 2008). Giraud and Shaw (2007) compiled mapping of approximately 14,000 landslides statewide in Utah. Anderson and others (1984) estimated that the total direct costs of landslides in Utah in the abnormally wet spring of 1983 exceeded $250 million. The 1983 Thistle landslide, Utah’s single most destructive landslide, is recognized, both in terms of direct and indirect costs, as the most expensive individual landslide in North American history (University of Utah, 1984; Schuster, 1996; USGS, 2008). Rock and soil units susceptible to landsliding underlie parts of the Zion National Park Geologic-Hazard Study Area. Historical landslides have disrupted transportation routes, houses, commercial sites, and public utilities within and adjacent to the study area (Black and others, 1995; Lund and Sharrow, 2005; Lund and others, 2007). For additional information about the landslide hazard in the Zion National Park Geologic-Hazard Study Area, refer to the Landslide Hazard chapter in this report. By William R. Lund, Tyler R. Knudsen, and David L. Sharrow Plate 3 Utah Geological Survey Special Study 133 Zion National Park Geologic-Hazard Study Area, Washington and Kane Counties, Utah Camp Creek Creek Horse Ranch Mountain Taylor Fork North Fork Middle Fork South Lee Pass Nagunt Mesa Buck Pasture Mountain Creek Timber Kolob Canyons Road Shuntavi Butte Gregory Butte Kolob Arch Langston Mountain Creek La Verkin Willis Creek BEAR CANYON TRAP KOLOB CANYONS Burnt Mountain HOP VALLEY Firepit Knoll Pine Valley Peak Northgate Peaks North Guardian Angel South Guardian Angel Kolob Terrace Road Kolob Terrace Road North Creek Right Fork Left Fork Blue Creek Goose Creek Lava Point Kolob Creek Deep Creek Coalpits Wash Wash Scoggins Altar of Sacrifice The West Temple Meridian Tower The Sentinel Mount Kinesava Wash Huber Creek Oak The Watchman Johnson Mountain FORK EAST RIVER VIRGIN PARUNUWEAP CANYON Bridge Mountain The East Temple Mount Spry Crater Hill Twin Brothers Mountain of the Sun The Great Arch The Great White Throne Lady Mountain Castle Dome Mount Angels Landing Observation Point Mountain of Mystery Wynopits Mountain Ivins Mountain Timber HURRICANE CLIFFS Death Point Tucupit Point Paria Point Beatty Point Top Mountain Herbs Point LANGSTON CANYON Currant Creek Cane Creek POLE CANYON LOWER KOLOB PLATEAU UPPER KOLOB PLATEAU HORSE PASTURE PLATEAU ZION CANYON OR D E R V I LLE CANY O N The Subway GREAT WEST CANYON COUGAR MOUNTAIN Court Patriarchs The of Creek Birch Creek Pine Creek Pine Creek Clear Checkerboard Mesa Creek Clear Creek Co-op GIFFORD CANYON FORK NORTH RIVER VIRGIN ZION-MOUNT CARMEL T U N N E L TUNNEL Wash Terry Wash Jennings Wash Coalpits The Bishopric HEAPS CANYON ECHO CANYON Cable Mountain Temple of Sinawava TELEPHONE CANYON IMLAY CANYON BULLOCH GULCH THE NARROWS FORK NORTH RIVER VIRGIN WILDCAT CANYON RUSSELL GULCH North Creek North Creek Zion Canyon Scenic Drive 9 9 The Grotto Zion Canyon Visitor Center Zion Lodge Little Creek Grapevine Wash TOWERS VIRGIN THE OF Abraham Isaac Jacob Church Mesa PHANTOM VALLEY CORRAL HOLLOW Kolob Creek Weeping Rock Cathedral Mountain Majestic Kolob Canyons Visitor Center WASHINGTON CO KANE CO IRON CO WASHINGTON CO Neagle Ridge Bullpen Mountain Pocket Mesa OAK SPRING VALLEY PINE VALLEY LEE VALLEY Cave Knoll CAVE VALLEY Tabernacle Dome Wash Spring Pine TRAIL CANYON Greatheart Mesa BEHUNIN CANYON Creek Shunes Shunesburg Mountain Wash Hepworth Wash Dennett Canyon Bee Hive Peak HIDDEN CANYON CANYON MYSTERY R 12 W R 11 W T 38 S T 39 S R 11 W R 10 W T 40 S T 41 S T 41 S T 42 S T 39 S T 40 S T 41 S T 42 S T 39 S T 40 S T 40 S T 40.5 S T 41 S T 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 boundary Zion National Park boundary County boundary State highway Minor road Foot trail Existing park structure Area not studied This map shows areas of relative landslide hazard, and provides a basis for requiring site-specific hazard investigations. Site-specific investigations can resolve uncertainties inherent in generalized geologic-hazard mapping and help ensure safety by identifying the need for hazard mitigation. This map identifies areas, based on previous landslide history, material characteristics, and slope, where site- specific slope-stability conditions (such as material strength, orientation of bedding or fractures, ground-water conditions, erosion or undercutting) should be evaluated prior to development. The level of investigation needed at a given site depends on the relative hazard and the nature of the proposed development (structure type, size, use, and placement; required cuts and fills; and changes in ground-water conditions). A valid landslide-hazard investigation must address all pertinent conditions that could affect, or be affected by, the proposed development, including earthquake ground shaking. This can only be accomplished through the proper identification and interpretation of site-specific geologic conditions and processes (Blake and others, 2002). Nearby conditions that may affect the site must also be considered. The analysis of natural and modified slopes for static and/or seismic stability is a challenging geotechnical problem. Blake and others (2002, p. 3) consider the following steps required for a proper static and seismic slope-stability analysis. The stability calculations are then carried out using an appropriate analysis method for the potential failure surface being analyzed. A seismic slope-stability analysis requires consideration of each of the above factors for static stability, as well as characterization of: Blake and others (2002) consider all of the above factors vital for a proper slope stability analysis, but note that some factors are more easily characterized than others. They identify two factors—subsurface stratigraphy/geologic structure and soil shear strength—as particularly challenging to accurately characterize. Accordingly, landslide-hazard investigations must be interdisciplinary in nature and performed by qualified, experienced geotechnical engineers and engineering geologists working as a team. Utah Geological Survey Circular 92 Guidelines for Evaluating Landslide Hazards in Utah (Hylland, 1996) presents minimum standards for performing landslide-hazard evaluations. Turner and Schuster (1996), and Blake and others (2002) provide additional guidance for evaluating landslide hazards. Local jurisdictions may adopt more stringent requirements for slope-stability investigations, as they deem necessary, to meet local needs and conditions. The UGS recommends that the following site-specific investigations be conducted for each of the landslide-hazard categories. 37 27' 30" 1. Surface topography, 2. Subsurface stratigraphy, 3. Subsurface water levels and possible subsurface flow patterns, 4. Shear strength of materials through which the failure surface may pass, 5. Unit weight of the materials overlying potential failure planes. "Accurate characterization of: Design-basis earthquake ground motions at the site, and Earthquake shaking effects on the strength and stress-deformation behavior of the soil, including pore pressure generation and rate effects.” 1. 2. MAP LIMITATIONS This map is based on 1:24,000-scale UGS geologic mapping, and the inventory of landslides obtained from that mapping and shown on this map reflects that level of mapping detail. Some smaller landslides may not have been detected during the mapping or are too small to show at that scale. Therefore, site-specific geotechnical and geologic-hazard investigations should be preceded by a careful field evaluation of the site to identify any landslides present. The mapped boundaries of the landslide-hazard categories are approximate and subject to change as new information becomes available. The landslide hazard at any particular site may be different than shown because of variations in the physical properties of geologic units, ground-water conditions within a map unit, gradational and approximate map-unit boundaries, and the generalized map scale. Small, localized areas of higher or lower landslide hazard may exist within any given map area, but their identification is precluded by limitations of map scale. This map is not intended for use at scales other than the published scale, and is intended for use in general planning and design to indicate the need for site-specific investigations. HAZARD REDUCTION As with most geologic hazards, early recognition and avoidance are the most effective ways to mitigate landslide hazards. However, avoidance may not always be a viable or cost-effective option, especially for existing developments, and engineering techniques are available to reduce potential landslide hazards. Techniques for mitigating landslide hazards include, but are not limited to, care in site grading; proper engineering, construction, and compaction of cut-and-fill slopes; careful attention to site drainage and dewatering of shallow or perched ground water; construction of retaining structures within the toe of slopes; and use of mechanical stabilization including tiebacks or other means that penetrate the landslide mass to anchor it to underlying stable material. Other techniques used to reduce landslide hazards include benching, bridging, weighting, or buttressing slopes with compacted earth fills, and installation of landslide warning systems (Keller and Blodgett, 2006). However, some geologic units, for example the Petrified Forest Member of the Chinle Formation, may be too weak to buttress, and may continue to move upslope of the buttress (Francis Ashland, UGS, written communication, 2007). Where development is proposed in areas identified on this map as having a potential for landsliding, we recommend that a phased site-specific investigation be performed early in the project design phase. A site- specific investigation can establish whether the necessary conditions for landsliding are present at a site; if they are, appropriate design and construction recommendations should be provided. UTAH GEOLOGICAL SURVEY a division of Utah Department of Natural Resources in cooperation with National Park Service Approximate mean declination, 2009 12 9' o TRUE NORTH MAGNETIC NORTH This geologic-hazard map was funded by the Utah Geological Survey and the U.S. Department of the Interior, National Park Service. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. 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 any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims by users of this product. For use at 1:24,000 scale only. The Utah Geological Survey does not guarantee accuracy or completeness of data. www.geology.utah.gov Base map consists of U.S. Department of Agriculture 2006 National Agricultural Imagery Program natural color aerial photography and shaded relief generated from digital elevation data acquired from the Utah Automated Geographic Reference Center. Universal Transverse Mercator Projection, zone 12 North American Datum of 1983 REFERENCES Anderson, L.R., Keaton, J.R., Saarinen, T. F., and Wells, W.G., II, 1984, The Utah landslides, debris flows, and Ashland, F.X., 2003, Characteristics, causes, and implications of the 1998 Wasatch Front landslides: Utah Black, B.D., Mulvey, W.E., Lowe, M., and Solomon, B.J., 1995, Geologic effects, in Christenson, G.E., editor, The Blake, T.F., Hollingsworth, R.A., and Stewart, J.P., editors, 2002, Recommended procedures for implementation of Cruden, D.M., and Varnes, D.J., 1996, Landslide types and processes, in Turner, A.K., and Schuster, R.L., editors, Giraud, R.E., and Shaw, L.M., 2007, Landslide susceptibility map of Utah: Utah Geological Survey Map 228DM, 11 Hylland, M.D., editor, 1996, Guidelines for evaluating landslide hazards in Utah: Utah Geological Survey Circular Keller, E.A., and Blodgett, R.H., 2006, Natural hazards – Earth’s processes as hazards, disasters, and Lund, W.R., and Sharrow, D.L., 2005, Field Trip 7 – Engineering geology and geologic hazards Las Vegas, Nevada Lund, W.R., Sharrow, D.L., and Knudsen, T.R., 2007, Engineering geology in the Rocky Mountain West – effective Neuendorf, K.K.E., Mehl, J.P., Jr., and Jackson, J.A., editors, 2005, Glossary of geology (fifth edition): Alexandria, Schuster, R.L., 1996, Socioeconomic significance of landslides, in Turner, A.K., and Schuster, R.L., editors, Turner, K.A., and Schuster, R.L., editors, 1996, Landslides – investigation and mitigation: Washington, D.C., University of Utah, 1984, Flooding and landslides in Utah ─ an economic impact analysis: Salt Lake City, University floods of May and June 1983: Washington, D.C., National Academy Press, 96 p. Geological Survey Special Study 105, 49 p. September 2, 1992 M L 5.8 St. George earthquake, Washington County, Utah: Utah Geological Survey Circular 88, p. 2-11. DMG Special Publication 117 – Guidelines for analyzing and mitigating landslide hazards in California: Los Angeles, Southern California Earthquake Center, 125 p. Landslides – investigation and mitigation: Washington, D.C., National Academy of Sciences, National Research Council, Transportation Research Board Special Report 247, p. 36-75. p. pamphlet, scale 1:500,000, digital video disk. 92, 16 p. catastrophes: Upper Saddle River, New Jersey, Pearson Prentice Hall, 395 p. to Zion National Park, Utah: Association of Engineering Geologists 48 th Annual Meeting, Las Vegas, Nevada, 33 p., compact disk. geologic practice, in, Lund, W.R., editor, Field guide to geologic excursions in southern Utah: Utah Geological Association Publication 35, 39 p., compact disk. Virginia, American Geological Institute, 779 p. Landslides – investigation and mitigation: Washington, D.C., National Academy of Sciences, National Research Council, Transportation Research Board Special Report 247, p. 12-35. National Academy of Sciences, National Research Council, Transportation Research Board Special Report 247, 673 p. of Utah Bureau of Economic and Business Research, Utah Department of Community and Economic Development, and Utah Office of Planning and Budget, 123 p. EXPLANATION Very High: Existing landslides (susceptibility category A). High: Areas where the Petrified Forest Member of the Chinle Formation and the overlying Moenave Formation (susceptibility category B) crop out on slopes greater than 15 percent (8 o ). Moderate C: Areas where susceptibility-category C geologic units crop out on slopes greater than 20 percent (12 o ). Moderate B: Areas where the Petrified Forest Member of the Chinle Formation crop out on slopes less than 15 percent (8 o ). Low: Areas where susceptibility-category D geologic units crop out on slopes greater than 30 percent (17 o ). Landslide Susceptibility of Geologic Units A Susceptibility Category Geologic Units 1 Comments B C D Existing landslides Petrified Forest Mbr., Chinle Fm.; Moenave Fm. where above slopes of Petrified Forest Mbr. Existing landslides are considered the most likely units in which new landslides may initiate (Ashland, 2003). 1 See chapter 1, figure 1.4, in accompanying text for complete geologic unit names. Harrisburg Mbr., Kaibab Fm.; Shnabkaib Mbr. and red members, Moenkopi Fm.; Moenave Fm. not above slopes of Petrified Forest Mbr.; Kayenta Fm. above Springdale Sandstone Mbr.; Temple Cap Fm.; Carmel Fm.; Cedar Mountain Fm. The Petrified Forest Member consists chiefly of bentonitic clay, which is expansive and has low shear strength. This unit includes the greatest number of landslides in the study area. Numerous landslides have also formed in the overlying Moenave Formation where the Petrified Forest crops out on lower slopes. These bedrock units contain varying amounts of gypsum, shale, claystone, mudstone, siltstone, or a combination of these rock types that impart weak shear strength to the units, at least locally, and make them susceptible to landsliding. These units contain the second greatest number of landslides in the study area, which often occur as landslide complexes. These geologic units either contain a higher percentage of stronger rock types, crop out on slopes too gentle to generate landslides, or generate failures that are too small to map at 1:24,000-scale. As a result, they exhibit few or no mapped landslides. Landslides identified within these units typically result from mass movement or erosion in underlying, weaker geologic units. Remaining bedrock and unconsolidated geologic units exclusive of the Navajo Sandstone. Slope failures in the Navajo are limited to rock falls and fin-collapse mass wasting which are discussed in the Rock-Fall Hazard section of this study. Map Location 1 0 1 0.5 MILE 1000 0 1000 2000 3000 4000 5000 6000 7000 FEET 1 0 1 0.5 KILOMETER SCALE 1:24,000 Landslide-Hazard Category Recommended Site-Specific Study Very High High Moderate Low Detailed engineering geologic and geotechnical-engineering investigation necessary. Predevelopment stabilization recommended for historical and geologically young (late Pleistocene or Holocene) landslides. Detailed engineering geologic and geotechnical-engineering investigation necessary. Geologic evaluation and reconnaissance-level geotechnical-engineering investigation necessary; detailed engineering geologic and geotechnical-engineering investigation may be necessary. Geologic evaluation and reconnaissance-level geotechnical-engineering investigation necessary, detailed geotechnical-engineering investigation generally not necessary. U.S. Geological Survey, 2008, USGS frequently asked questions: Online, <http://landslides.usgs.gov/learning/faq/>, accessed September 30, 2008.
Transcript of S ZION NATIONAL PARK GEOLOGIC-HAZARD STUDY AREA LANDSLIDE HAZARD - Utah · 2015-12-15 · For...
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LANDSLIDE HAZARD ZION NATIONAL PARK GEOLOGIC-HAZARD STUDY AREA
T 38 ST 39 S
37 30'
113 7' 30"
2010
DISCUSSION
USING THIS MAP
SYMBOLS
Landslide is a general term covering a wide variety of mass-movement landforms and processes involving thedownslope transport, under gravitational influence, of soil and rock material en masse (Cruden and Varnes,1996; Neuendorf and others, 2005). The term includes both deep-seated and shallow mass movements (Crudenand Varnes, 1996). The moisture content of the affected materials at the time of landsliding may range from dryto saturated.Landslides can be both damaging and deadly. The U.S. Geological Survey (USGS) estimated that in the UnitedStates, landslides on average cause $1-2 billion in damage and more than 25 deaths annually (USGS, 2008).Giraud and Shaw (2007) compiled mapping of approximately 14,000 landslides statewide in Utah. Anderson andothers (1984) estimated that the total direct costs of landslides in Utah in the abnormally wet spring of 1983exceeded $250 million. The 1983 Thistle landslide, Utah’s single most destructive landslide, is recognized, bothin terms of direct and indirect costs, as the most expensive individual landslide in North American history(University of Utah, 1984; Schuster, 1996; USGS, 2008).Rock and soil units susceptible to landsliding underlie parts of the Zion National Park Geologic-Hazard StudyArea. Historical landslides have disrupted transportation routes, houses, commercial sites, and public utilitieswithin and adjacent to the study area (Black and others, 1995; Lund and Sharrow, 2005; Lund and others, 2007).For additional information about the landslide hazard in the Zion National Park Geologic-Hazard Study Area, referto the Landslide Hazard chapter in this report.
ByWilliam R. Lund, Tyler R. Knudsen, and David L. Sharrow
Plate 3Utah 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
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South
LeePass
NaguntMesa
BuckPastureMountain
Creek
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Kolob
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ShuntaviButte
GregoryButte
KolobArch Langston
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BEAR CANYONTRAP
KO
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NorthGuardian
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ad
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Fork
Left
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Blue
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Goose
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Kolob
Creek
DeepCr
eek
Coalp
itsWa
sh
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Scoggins
Altar ofSacrifice
The WestTemple
MeridianTower The
Sentinel
MountKinesava
Wash
Hube
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Creek
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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
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Death Point
Tucupit Point
Paria Point
Beatty Point
Top
Mountain
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WEST
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gs
Wash
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ECHO
CANYON
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Temple ofSinawava
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The Grotto
Zion CanyonVisitor Center
ZionLodge
Little
Creek
Grapevine
Wash
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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
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INGT
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OKA
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IRON COWASHINGTON CO
Neag
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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
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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
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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 landslide hazard, and provides a basis for requiring site-specific hazardinvestigations. Site-specific investigations can resolve uncertainties inherent in generalized geologic-hazardmapping and help ensure safety by identifying the need for hazard mitigation.This map identifies areas, based on previous landslide history, material characteristics, and slope, where site-specific slope-stability conditions (such as material strength, orientation of bedding or fractures, ground-waterconditions, erosion or undercutting) should be evaluated prior to development. The level of investigation neededat a given site depends on the relative hazard and the nature of the proposed development (structure type, size,use, and placement; required cuts and fills; and changes in ground-water conditions). A valid landslide-hazardinvestigation must address all pertinent conditions that could affect, or be affected by, the proposed development,including earthquake ground shaking. This can only be accomplished through the proper identification andinterpretation of site-specific geologic conditions and processes (Blake and others, 2002). Nearby conditions thatmay affect the site must also be considered.The analysis of natural and modified slopes for static and/or seismic stability is a challenging geotechnicalproblem. Blake and others (2002, p. 3) consider the following steps required for a proper static and seismicslope-stability analysis.
The stability calculations are then carried out using an appropriate analysis method for the potential failuresurface being analyzed. A seismic slope-stability analysis requires consideration of each of the above factors forstatic stability, as well as characterization of:
Blake and others (2002) consider all of the above factors vital for a proper slope stability analysis, but note thatsome factors are more easily characterized than others. They identify two factors—subsurfacestratigraphy/geologic structure and soil shear strength—as particularly challenging to accurately characterize.Accordingly, landslide-hazard investigations must be interdisciplinary in nature and performed by qualified,experienced geotechnical engineers and engineering geologists working as a team. Utah Geological SurveyCircular 92 Guidelines for Evaluating Landslide Hazards in Utah (Hylland, 1996) presents minimum standards forperforming landslide-hazard evaluations. Turner and Schuster (1996), and Blake and others (2002) provideadditional guidance for evaluating landslide hazards. Local jurisdictions may adopt more stringent requirementsfor slope-stability investigations, as they deem necessary, to meet local needs and conditions. The UGSrecommends that the following site-specific investigations be conducted for each of the landslide-hazardcategories.
37 27' 30"
1. Surface topography,2. Subsurface stratigraphy,3. Subsurface water levels and possible subsurface flow patterns,4. Shear strength of materials through which the failure surface may pass,5. Unit weight of the materials overlying potential failure planes.
"Accurate characterization of:
Design-basis earthquake ground motions at the site, andEarthquake shaking effects on the strength and stress-deformation behavior of the soil, includingpore pressure generation and rate effects.”
1.2.
MAP LIMITATIONSThis map is based on 1:24,000-scale UGS geologic mapping, and the inventory of landslides obtained from thatmapping and shown on this map reflects that level of mapping detail. Some smaller landslides may not havebeen detected during the mapping or are too small to show at that scale. Therefore, site-specific geotechnicaland geologic-hazard investigations should be preceded by a careful field evaluation of the site to identify anylandslides present. The mapped boundaries of the landslide-hazard categories are approximate and subject tochange as new information becomes available. The landslide hazard at any particular site may be different thanshown because of variations in the physical properties of geologic units, ground-water conditions within a mapunit, gradational and approximate map-unit boundaries, and the generalized map scale. Small, localized areas ofhigher or lower landslide hazard may exist within any given map area, but their identification is precluded bylimitations of map scale. This map is not intended for use at scales other than the published scale, and isintended for use in general planning and design to indicate the need for site-specific investigations.
HAZARD REDUCTIONAs with most geologic hazards, early recognition and avoidance are the most effective ways to mitigate landslidehazards. However, avoidance may not always be a viable or cost-effective option, especially for existingdevelopments, and engineering techniques are available to reduce potential landslide hazards. Techniques formitigating landslide hazards include, but are not limited to, care in site grading; proper engineering, construction,and compaction of cut-and-fill slopes; careful attention to site drainage and dewatering of shallow or perchedground water; construction of retaining structures within the toe of slopes; and use of mechanical stabilizationincluding tiebacks or other means that penetrate the landslide mass to anchor it to underlying stable material.Other techniques used to reduce landslide hazards include benching, bridging, weighting, or buttressing slopeswith compacted earth fills, and installation of landslide warning systems (Keller and Blodgett, 2006). However,some geologic units, for example the Petrified Forest Member of the Chinle Formation, may be too weak tobuttress, and may continue to move upslope of the buttress (Francis Ashland, UGS, written communication,2007).Where development is proposed in areas identified on this map as having a potential for landsliding, werecommend that a phased site-specific investigation be performed early in the project design phase. A site-specific investigation can establish whether the necessary conditions for landsliding are present at a site; if theyare, appropriate design and construction recommendations should be provided.
UTAH GEOLOGICAL SURVEYa division of Utah Department of Natural Resourcesin cooperation withNational Park Service
Approximate meandeclination, 2009
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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
REFERENCESAnderson, L.R., Keaton, J.R., Saarinen, T. F., and Wells, W.G., II, 1984, The Utah landslides, debris flows, and
Ashland, F.X., 2003, Characteristics, causes, and implications of the 1998 Wasatch Front landslides: Utah
Black, B.D., Mulvey, W.E., Lowe, M., and Solomon, B.J., 1995, Geologic effects, in Christenson, G.E., editor, The
Blake, T.F., Hollingsworth, R.A., and Stewart, J.P., editors, 2002, Recommended procedures for implementation of
Cruden, D.M., and Varnes, D.J., 1996, Landslide types and processes, in Turner, A.K., and Schuster, R.L., editors,
Giraud, R.E., and Shaw, L.M., 2007, Landslide susceptibility map of Utah: Utah Geological Survey Map 228DM, 11
Hylland, M.D., editor, 1996, Guidelines for evaluating landslide hazards in Utah: Utah Geological Survey Circular
Keller, E.A., and Blodgett, R.H., 2006, Natural hazards – Earth’s processes as hazards, disasters, and
Lund, W.R., and Sharrow, D.L., 2005, Field Trip 7 – Engineering geology and geologic hazards Las Vegas, Nevada
Lund, W.R., Sharrow, D.L., and Knudsen, T.R., 2007, Engineering geology in the Rocky Mountain West – effective
Neuendorf, K.K.E., Mehl, J.P., Jr., and Jackson, J.A., editors, 2005, Glossary of geology (fifth edition): Alexandria,
Schuster, R.L., 1996, Socioeconomic significance of landslides, in Turner, A.K., and Schuster, R.L., editors,
Turner, K.A., and Schuster, R.L., editors, 1996, Landslides – investigation and mitigation: Washington, D.C.,
University of Utah, 1984, Flooding and landslides in Utah ─ an economic impact analysis: Salt Lake City, University
floods of May and June 1983: Washington, D.C., National Academy Press, 96 p.
Geological Survey Special Study 105, 49 p.
September 2, 1992 ML 5.8 St. George earthquake, Washington County, Utah: Utah Geological Survey Circular88, p. 2-11.
DMG Special Publication 117 – Guidelines for analyzing and mitigating landslide hazards in California: LosAngeles, Southern California Earthquake Center, 125 p.
Landslides – investigation and mitigation: Washington, D.C., National Academy of Sciences, National ResearchCouncil, Transportation Research Board Special Report 247, p. 36-75.
p. pamphlet, scale 1:500,000, digital video disk.
92, 16 p.
catastrophes: Upper Saddle River, New Jersey, Pearson Prentice Hall, 395 p.
to Zion National Park, Utah: Association of Engineering Geologists 48th Annual Meeting, Las Vegas, Nevada, 33p., compact disk.
geologic practice, in, Lund, W.R., editor, Field guide to geologic excursions in southern Utah: Utah GeologicalAssociation Publication 35, 39 p., compact disk.
Virginia, American Geological Institute, 779 p.
Landslides – investigation and mitigation: Washington, D.C., National Academy of Sciences, National ResearchCouncil, Transportation Research Board Special Report 247, p. 12-35.
National Academy of Sciences, National Research Council, Transportation Research Board Special Report 247,673 p.
of Utah Bureau of Economic and Business Research, Utah Department of Community and EconomicDevelopment, and Utah Office of Planning and Budget, 123 p.
EXPLANATIONVery High: Existing landslides (susceptibility category A).
High: Areas where the Petrified Forest Member of the Chinle Formation and the overlying MoenaveFormation (susceptibility category B) crop out on slopes greater than 15 percent (8o).Moderate C: Areas where susceptibility-category C geologic units crop out on slopes greater than 20percent (12o).Moderate B: Areas where the Petrified Forest Member of the Chinle Formation crop out on slopes lessthan 15 percent (8o).Low: Areas where susceptibility-category D geologic units crop out on slopes greater than 30 percent(17o).
Landslide Susceptibility of Geologic Units
A
SusceptibilityCategory Geologic Units1 Comments
B
C
D
Existing landslides
Petrified Forest Mbr., Chinle Fm.; Moenave Fm.where above slopes of Petrified Forest Mbr.
Existing landslides are considered the most likely units in which new landslidesmay initiate (Ashland, 2003).
1See chapter 1, figure 1.4, in accompanying text for complete geologic unit names.
Harrisburg Mbr., Kaibab Fm.; Shnabkaib Mbr. andred members, Moenkopi Fm.; Moenave Fm. notabove slopes of Petrified Forest Mbr.; KayentaFm. above Springdale Sandstone Mbr.; TempleCap Fm.; Carmel Fm.; Cedar Mountain Fm.
The Petrified Forest Member consists chiefly of bentonitic clay, which is expansiveand has low shear strength. This unit includes the greatest number of landslides inthe study area. Numerous landslides have also formed in the overlying MoenaveFormation where the Petrified Forest crops out on lower slopes.These bedrock units contain varying amounts of gypsum, shale, claystone,mudstone, siltstone, or a combination of these rock types that impart weak shearstrength to the units, at least locally, and make them susceptible to landsliding. Theseunits contain the second greatest number of landslides in the study area, which oftenoccur as landslide complexes.These geologic units either contain a higher percentage of stronger rock types, cropout on slopes too gentle to generate landslides, or generate failures that are too smallto map at 1:24,000-scale. As a result, they exhibit few or no mapped landslides.Landslides identified within these units typically result from mass movement orerosion in underlying, weaker geologic units.
Remaining bedrock and unconsolidated geologicunits exclusive of the Navajo Sandstone. Slopefailures in the Navajo are limited to rock falls andfin-collapse mass wasting which are discussed inthe Rock-Fall Hazard section of this study.
Map Location
1 0 10.5 MILE
1000 0 1000 2000 3000 4000 5000 6000 7000 FEET1 0 10.5 KILOMETER
SCALE 1:24,000
Landslide-HazardCategory Recommended Site-Specific Study
Very High
High
Moderate
Low
Detailed engineering geologic and geotechnical-engineering investigation necessary.Predevelopment stabilization recommended for historical and geologically young (latePleistocene or Holocene) landslides.Detailed engineering geologic and geotechnical-engineering investigation necessary.Geologic evaluation and reconnaissance-level geotechnical-engineering investigationnecessary; detailed engineering geologic and geotechnical-engineering investigation maybe necessary.Geologic evaluation and reconnaissance-level geotechnical-engineering investigationnecessary, detailed geotechnical-engineering investigation generally not necessary.
U.S. Geological Survey, 2008, USGS frequently asked questions: Online, <http://landslides.usgs.gov/learning/faq/>,accessed September 30, 2008.
