Natural Hazard and Risk Assessment Using Decision Support Systems, Application: Glenwood Springs, Colorado

The area of Glenwood Springs, Colorado (USA) was selected to evaluate the usefulness of a Decision Support System (DSS) to assist governments and communities in evaluating geological hazards, vulnerability, and risk. Large parts of the Glenwood Springs valley have had intensive sheet erosion, debris flows, and hyperconcentrated floods triggered by landslides and slumps. The latter come from slightly to extremely unstable slopes in the many tributary channels on the surrounding mountainsides, which cause concentration of debris in channels and a large accumulation of sediments in colluvial wedges and debris fans that line the valley. The semiarid climate, characterized by intense and short duration spring and summer thunderstorms and affected by occasional fast melting of heavy snow-pack, greatly decreases slope stability. Many of the “old” landslides and debris flows exist in a dormant state and are non active, but are ready to be destabilized by a change in ground-water content, seismic activity, and/or human activities. A DSS called Integrated Planning Decision Support System (IPDSS) is designed to assist an urban planner in organizing, analyzing, modifying, and reevaluating existing or needed spatial information within land-use planning activities. The IPDSS incorporates the Geographic Information Systems (GIS) software Geographic Resource Analysis Support System (GRASS) and engineering numerical models within a Graphic User Interface (GUI), which provides the user with comprehensive modeling capabilities for geological hazards, vulnerability, and risk assessment. The methodology that IPDSS follows for the evaluation of hazards takes into account the weight of each influencing factor within hazardous geologic processes. For each cell (based on the maximum resolution of the data), IPDSS interactive algorithms compute the following parameters: the related hazard, the vulnerability to geological hazards, and the risk. This DSS incorporates the following information: topography, aspect, bedrock and surficial geology, structural geology, geomorphology, soils (geotechnical data), land cover, land use, hydrology, precipitation (annual average and probable maximum), Federal Emergency Management Agency floodway maps, and historic data to assess hazards. IPDSS is designed to assess hazards, such as debris flows, subsidence, and floods, with probable maximum precipitation and seismicity as triggering factors for susceptibility scenarios. The regular items considered in vulnerability analysis are a) ecosystem sensitivity, b) economic vulnerability, and c) social infrastructure vulnerability. The risk is assessed as a function of hazard and vulnerability. This DSS application delineated excellent areas for future urban development, and provided insights into areas of high risk in residential, commercial, and assembly sites. Hazard-susceptibility maps were developed for debris flows, floods, and subsidence using algorithms which based on statistical analysis, weighted the controlling variables. A sensitivity analysis of these weightings was performed for the debris-flow map (Mejia-Navarro, 1995).