GEOLOGIC HAZARDS AND ADVERSE CONSTRUCTION CONDITIONS IN THE ST. GEORGE – HURRICANE METROPOLITAN AREA, WASHINGTON COUNTY, UTAH

The St. George – Hurricane metropolitan area in Washington County, Utah, is one of the nation’s fastest growing regions. The population of Washington County has increased by more than 40 percent since 2000, and its current population of 126,000 is projected to reach 600,000 by 2050. As land well suited for development becomes increasingly scarce, urbanization has moved into less favorable areas where geologic hazards and adverse construction conditions are of concern. Consequently, the Utah Geological Survey (UGS) conducted a geologic-hazard evaluation of the St. George - Hurricane metropolitan area (366 square miles).

The purpose of the evaluation was to provide planners and other city and county officials with information on the kind and location of geologic hazards and adverse construction conditions that may impact existing and future development. Data for this study were compiled as a GIS-based map folio containing fourteen 1:24,000-scale geologic-hazard and adverse-construction-condition maps. Each map has an accompanying text document that provides information on the hazard or adverse condition in the study area. The maps are an aid for general planning to indicate where site-specific investigations are required. A GIS search application permits the maps to be queried by geologic hazard or adverse condition type and location, and produces a map and report on the hazards or adverse conditions of interest.

Geologic-hazard maps in the folio include surface faulting, liquefaction, flooding, landslides, and rock fall. Adverse-construction-condition maps include expansive soil and bedrock, collapsible soil, gypsiferous soil and bedrock, shallow bedrock, caliche, wind-blown sand, breccia pipes and paleokarst, shallow ground water, and piping- and erosion-susceptible soils. There is also a text document describing the earthquake ground-shaking hazard in the study area; however, data were insufficient to prepare a map.

The UGS has made the maps and search application available to Washington County, municipalities in the study area, and geotechnical engineering firms. To date, the maps have been used to evaluate a major pipeline crossing of the Hurricane fault, help administer a hill-slope ordinance in St. George, and evaluate the surface-fault-rupture hazard for three proposed freeway interchanges.