GEOLOGICAL HAZARDS, VULNERABILITY, AND RISK ASSESSMENT USING GIS - MODEL FOR GLENWOOD-SPRINGS, COLORADO

Glenwood Springs, Colorado, lies at the junction of the Roaring Fork a nd Colorado Rivers, surrounded by the steep peaks of the Colorado Rock y Mountains. Large parts of the region have had intensive sheet erosio n, debris flows, and hyperconcentrated floods triggered by landslides and slumps. The latter come from unstable slopes in the many tributary channels on the mountainsides, causing concentration of debris in cha nnels and a large accumulation of sediment in colluvial wedges and deb ris fans that line the river valleys. Many of the landslide and debris -flow deposits exist in a state resembling suspended animation, ready to be destabilized by intense precipitation and/or seismic activity. D uring this century urban development in the Roaring Fork River valley has increased rapidly. The city of Glenwood Springs continues to expan d over unstable debris fans without any construction of hazard mitigat ion structures. Since 1900, Glenwood Springs has had at least 21 damag ing debris flows and floods; on July 24, 1977 a heavy thunderstorm spr ead a debris flow over more than 80 ha of the city. This paper present s a method that uses Geographic Information Systems (GIS) to assess ge ological hazards, vulnerability, and risk in the Glenwood Springs area . The hazards evaluated include subsidence, rockfall, debris flows, an d floods, and in this paper we focus on debris flows and subsidence. I nformation on topography, hydrology, precipitation, geomorphic process es, bedrock and surficial geology, structural geology, soils, vegetati on, and land use, was processed for hazard assessment using a series o f algorithms. ARC/INFO and GRASS GIS softwares were used to produce ma ps and tables in a format accessible to urban planners. After geologic al hazards were defined for the study area, we estimated the vulnerabi lity (V(e)) of various elements for an event of intensity i. Risk is a ssessed as a function of hazard and vulnerability. We categorized the study area in 14 classes for planning procedures; 7 classes defined as areas suitable for human settlement, and 7 classes defined as unsuita ble for building, and most effectively reserved for parks and forests.