GEOLOGIC HAZARDS OF THE ZION NATIONAL PARK GEOLOGIC-HAZARD STUDY AREA, WASHINGTON AND KANE COUNTIES, UTAH

Zion National Park (ZNP) receives more than 2.5 million visitors annually, and is subject to a variety of geologic hazards that may affect park development and visitor safety. To provide the National Park Service with geologic-hazard information for future park management, the Utah Geological Survey conducted a geologic-hazard investigation of a portion of ZNP. The ZNP Geologic-Hazard Study Area is a 154-square-mile area that encompasses Zion Canyon, Kolob Canyons and Kolob Terrace, the Zion-Mount Carmel Highway corridor, and all currently developed and high-use areas of the park.

Results of this investigation include nine 1:24,000-scale GIS-based geologic-hazard maps that cover flooding and debris flows, rock fall, landslides, surface faulting, liquefaction, collapsible soil, expansive soil and rock, gypsiferous soil and rock, and soil piping and erosion. Accompanying text documents describe the hazards and provide background information on data sources, the nature and distribution of the hazards, and possible hazard-reduction measures. The text documents also include a discussion of earthquake-induced ground shaking, but data were insufficient to prepare a ground-shaking-hazard map.

The maps are intended for general planning use to indicate where site-specific geologic-hazard investigations are necessary. Site-specific investigations can resolve uncertainties inherent in the 1:24,000-scale maps, and help increase safety by identifying the need for special engineering design or hazard mitigation.

The most widespread geologic hazard in ZNP is flooding. Eight individuals lost their lives between 1950 and 2008 due to flooding, and floods and debris flows have repeatedly damaged park facilities. Rock falls have resulted in three deaths and property damage in the park. Landslides are common where clay-rich bedrock crops out on slopes, and have damaged park transportation corridors and dammed streams. Collapsible soil is the most prevalent soil-related geologic hazard and at least one park building has sustained significant damage due to soil collapse. Large earthquakes are rare events in southwestern Utah, but earthquakes have the greatest potential for causing catastrophic property damage and loss of life. Faults in the region are capable of producing earthquakes of magnitude 6.5-7.0.