Geologic-Hazard and Adverse-Construction-Condition Maps for the St. George–Hurricane Area, Washington County, Utah
By W.R. Lund, T.R. Knudsen, G.S. Vice, and L.M. Shaw
Geologic hazards and adverse construction conditions included in the report:
- Surface Fault Rupture
- Earthquake Ground Shaking (Text document only, no map prepared.)
- Rock Fall
Adverse Construction Conditions
- Expansive Soil and Rock
- Collapsible Soil
- Gypsiferous Soil and Rock
- Shallow Bedrock
- Wind-Blown Sand
- Breccia Pipes and Paleokarst
- Soil Piping and Erosion
- Shallow Ground Water
About the Authors
William Lund has 36 years of experience as an engineering geologist—7 years with geotechnical consulting firms in Arizona, California, and Oregon, and 29 years with the UGS. He is a former deputy director of the UGS, presently is the Geologic Hazards Program Senior Scientist, and is manager of the UGS’ Southern Regional Office in Cedar City.
He is a Licensed Professional Geologist in Utah, Registered Geologist in Arizona, and Certified Engineering Geologist in Oregon, and is a past president of the Intermountain Section of the Association of Engineering Geologists, Utah Geological Association, and Dixie Geological Society. In addition to the geologic-hazards mapping project described in this article, other recent projects include seismic-hazard evaluations of the Hurricane, Sevier, and Washington faults.
Tyler Knudsen is a geologist with the UGS Geologic Hazards Program in Cedar City. His principal responsibilities are to identify, investigate, and publish information on Quaternary faults, landslides, and other potential geologic hazards in southwestern Utah. Tyler has been instrumental in the completion of the comprehensive St. George–Hurricane geologic hazards mapping project and is currently studying the seismic potential of the Washington fault near St. George.
Upcoming projects include ArcGIS Geographic Information System-based geologic hazards mapping in Zion National Park and the Cedar City– Parowan Valleys. Prior to joining the UGS in early 2006, Tyler worked for a St. Georgebased geotechnical consulting firm, and completed a master’s degree in geology at the University of Nevada, Las Vegas.
Garrett Vice was formerly a geologist for the UGS. He worked as a co-investigator on the St. George–Hurricane geologic hazards project for several months prior to attending graduate school at the University of Nevada, Reno. Garrett will graduate from UNR in June with a master’s degree in geology, and is looking forward to a promising career in the oil industry.
Lucas Shaw is a GIS Analyst for the Geologic Hazards Program. Since signing on with the UGS in 2004, he has worked to get GIS technologies and information into the hands of expert and novice alike.
His work for the Hazards Program includes landslide susceptibility analysis, emergency response preparedness, and custom GIS application development. He has also improved the GIS administration, data management, and GIS integration with Internet map services at the UGS.
Lund, W.R., Knudsen, T.R., Vice, G.S., and Shaw, L.M., 2008, Geologic hazards and adverse construction conditions, St. George-Hurricane Metropolitan Area, Washington County, Utah: Utah Geological Survey Special Study 127, variously paginated, 14 plates, scale 1:24,000.
Surface Fault Rupture
Expansive Soil & Rock
Gypsiferous Soil & Rock
Breccia Pipe & Paleokarst
Wind Blown Sand
Piping & Erosion
Shallow Ground Water
Southwestern Utah’s warm climate and beautiful scenery have made the St. George–Hurricane area one of Utah’s fastest growing regions for more than two decades. As land well-suited for development becomes increasingly scarce, urbanization has moved into less favorable areas where geologic hazards and adverse construction conditions become more of an issue for development. In these areas, timely geologic information early in the planning and design process is critical to avoiding or mitigating geologic hazards and other geology-related construction problems. The Utah Geological Survey (UGS) is nearing completion of a Geographic Information System (GIS)-based study of the St. George–Hurricane area that will help planners and other local-government officials determine where geologic hazards and/or adverse construction conditions may exist, and consequently where detailed, site-specific studies are needed prior to development.
This study encompasses 366 square miles in Washington County. We established the study-area boundary in consultation with Washington County, the Five County Association of Governments, and cities and towns in the study area. The study area includes most remaining developable land in the St. George–Hurricane area, including large tracts of vacant land that will undoubtedly be developed in the future. Principal communities in the study area are Hurricane, Ivins, La Verkin, Leeds, Santa Clara, St. George, Toquerville, and Washington. Growth in these communities over the past six years has been phenomenal.
The principal products of this study are 14 geologic-hazard and adverse-construction-condition maps (1:24,000 scale) with accompanying text documents. Each map covers a different hazard or adverse construction condition, and the text documents provide background information on the data sources used to create the maps, the nature and distribution of the hazards or adverse construction conditions, and possible mitigation measures. An additional text document discusses earthquake-induced ground shaking; however, data are insufficient at this time to prepare a ground-shaking-hazard map.
The UGS and Utah Automated Geographic Reference Center developed a new GIS search application for this project. The GIS format permits the powerful organizational and analytical features of computer-based databases and maps to be used in the identification, characterization, and mitigation of geologic hazards and adverse construction conditions. The application allows the user to quickly search the maps to identify geologic hazards and adverse construction conditions, and then create a custom map and explanatory text for a particular area. The user can search by property tax ID number, by specifying a point defined by latitude and longitude or Universal Transverse Mercator (UTM) coordinates and then defining a radius around that point, by clicking on an individual parcel, or by drawing a polygon around an area of interest.
Although we compiled data for this study from many sources, the chief sources of information were nine new digital 1:24,000-scale geologic quadrangle maps covering the St. George–Hurricane area prepared by the UGS Geologic Mapping Program. These maps provide the basic geologic data necessary to derive geologic-hazard and adverse-construction-condition maps. This new geologic information allowed us to update and expand on an earlier 1983 UGS geologic-hazard report for the St. George area, both in terms of the number and kinds of hazards and adverse construction conditions considered and the extent of the area covered.
On an annual basis, the most widespread and damaging geologic hazard in southwestern Utah is flooding, and the most troublesome construction condition is expansive soil and rock. The devastating floods of January 2005 on the Santa Clara and Virgin Rivers provided ample evidence of the destructive power and life-threatening nature of flooding in the area. Many buildings and other structures throughout the area have experienced cracked foundations and walls, as well as other kinds of structural, architectural, and landscape damage from expansive soil and rock. Because of their wide distribution, frequent occurrence, and damage potential, floods and expansive soil and rock will remain the principal geology-related issues with which planners, developers, and residents must contend in the future.
Landslides and rock falls are of increasing concern as land well-suited for building in lowland areas becomes scarce and development moves near or onto hillsides. Some clay-rich bedrock in the area is weak and susceptible to landslides, especially when wet. The close correlation of existing landslides with weak bedrock units provides ample warning that development on slopes underlain by landslide-susceptible bedrock must proceed with caution. Southwestern Utah has a history of damage to buildings and other facilities from rock falls, and favorable conditions for rock fall are widespread in the study area. Rock-fall damage is likely to increase as development moves into those areas unless effective mitigation measures are implemented.
Although large, damaging earthquakes are rare in southwestern Utah, active faults capable of producing earthquakes as large as magnitude 7 exist in the area. Hazards associated with such earthquakes (ground shaking, surface fault rupture, landslides, rock falls, and liquefaction) have the greatest potential for catastrophic property damage, economic disruption, and loss of life of any geologic hazard in the area. Because of their great destructive potential, the effects of large earthquakes must be reduced through land-use planning, adoption and enforcement of modern seismic building codes, and disaster preparedness planning and drills. Moderate earthquakes similar to the magnitude 5.8 St. George earthquake in 1992 must also be considered because they are more common than large earthquakes, are capable of doing significant property damage, and may be life threatening.
The remaining geologic hazards and construction conditions addressed in this study are typically localized in nature, and while the problems associated with them are rarely life threatening, they are potentially costly when not recognized and properly considered in project planning and design.
|Population data for principal communities in the St. George – Hurricane area|
|Community||Estimated 2006 Population||2000 Census Population||Percent Increase|
|Population information provided by Five County Association of Governments (2006).|
Survey Notes, v. 40 no. 2, May 2008