DERIVATIVE GEOLOGIC-HAZARD MAPS FROM DIGITAL GEOLOGIC-MAP DATA: FACILITATING LAND-USE PLANNING IN MOAB-SPANISH VALLEY, UTAH

The contiguous Moab and Spanish Valleys comprise a popular residential and recreational area in southeastern Utah. The local geology is relatively complex, resulting from the combined effects of late Paleozoic deposition of a thick evaporite sequence, late Paleozoic to Mesozoic diapirism, Cenozoic folding associated with regional compression, and late Cenozoic downcutting by the Colorado River system and associated structural collapse due to salt dissolution. Rock type, active geologic processes, and significant topographic relief contribute to a variety of geologic hazards, the most significant being expansive and gypsiferous soil and rock, stream and alluvial-fan flooding, debris flows, collapsible soil, soil susceptible to piping and erosion, rock fall, shallow ground water, and fractured rock.

Effective consideration of geologic hazards in land-use planning requires availability of detailed (1:24,000 scale) geologic mapping with particular attention given to Quaternary deposits. Such mapping was recently completed in the Moab-Spanish Valley area, but the potential for geologic hazards may not be readily apparent to non-geologists from the traditional map-unit descriptions. We used the digital geologic-map data to construct geologic-hazard maps, deriving many of the hazard areas directly from the geologic maps. Areas of potential hazard on our derivative maps correspond to geologic-map units having geologic-hazard implications. For example, talus deposits are areas of rock-fall accumulation and hazard, certain bedrock units and their associated soils contain soluble gypsum or expansive clays, and Holocene alluvial fans are areas of potential debris flows, alluvial-fan flooding, and collapsible soil.

The geologic-hazard maps present basic geologic-map information to land-use regulators, developers, and consultants in a form that is directly applicable to land-use planning. The hazard maps were derived relatively quickly from digital geologic data, and the resultant digital hazard-area files can be easily integrated into the local government's geographic information system and adopted by reference into geologic-hazards ordinances. Ultimately, this process facilitates the timely use of basic geologic mapping to help reduce risks and losses associated with geologic hazards.