GIS-BASED MULTIPLE NATURAL HAZARD ASSESSMENT OF SOUTHERN DAVIS COUNTY, UTAH: A PHYSIOGRAPHIC ²BOTTLENECK² ALONG THE WASATCH FRONT

Southern Davis County, Utah includes a physiographic “bottleneck”, a narrow strip of mostly developed land between the Wasatch Range (east) and Farmington Bay (west). Geologic processes affecting this densely populated region result in numerous hazards. These hazards must be addressed by community planners to determine proper zoning for their jurisdictions. While many research efforts have examined individual hazards in this area, none adequately integrated the results into a composite hazard assessment for the region.

The focus of this research was to develop a procedural model for a multiple natural hazard assessment to provide planners with a comprehensive view of the potential impact of a unique combination of hazards across their communities. The model consists of four phases. Phase 1 (Data Collection) utilized the previously published individual hazards research for surface fault rupture, ground shaking, liquefaction, rock fall, landsides, flooding, and debris flows. Phase 2 (Individual Hazard Assessment) involved the creation of standardized probability estimates used as input into the composite hazard combination rule. Phase 3 (Geomorphic Terrain Unit [GTU] Development) averaged the composite hazard rankings produced in the Phase 4, making it easier for planners to interpret the relative differences across the surface terrain zones reflecting the landform-generating geologic processes. Phase 4 (Multiple Natural Hazard Assessment) integrated the output from Phases 2 and 3 using an area-weighted combination rule to create composite hazard assessments at both the resolutions of raster cells and GTUs.

Area-weighted composite hazard rank scores had lower overall values than expected, but the relative differences between the values did provide useful information about how the composite hazard scores were related to the unique combinations of hazards associated with different sets of landforms. The sensitivity of the area-weighted combination rule was tested by comparing the differences in composite hazard scores at three different geographic scales. The results indicated that the composite hazard assessments for the larger area studies could be applied to smaller communities within this area that have the same geologic setting (i.e., the area weights for most hazards are similar).