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<br />002807 <br /> <br />Debris-flow Hazard - a case history <br />(G1enwood Springs, Garfield County, Colorado) <br /> <br />Introduction <br /> <br />Glenwood Springs is a small mountain city in Garfield County. It is located <br />in a narrow canyon at the junction of the Roaring Fork and Colorado Rivers. <br /> <br />Identification of Debris-Flow Hazard <br /> <br />Awareness of the debris-flow and flooding hazard in G1enwood Springs occurred <br />early on in the town's history. However. identification and analysis of the <br />source areas was not begun formally until the late 1970's. <br /> <br />Land-Use and Development History <br /> <br />Most of the city is located on debris fans. These fans have historically been <br />very active. At least 18 major damaging debris flows occurred between 1900 <br />and 1981. <br /> <br />Geologie Evaluation <br /> <br />Debris flows are the most serious type of landslide hazard to affect Glenwood <br />Springs. Debris flows are slurries of rock, soil, organic matter, water, and <br />air that flow rapidly (usually) down pre-existing drainage channels until they <br />are deposited in fan-shaped cones where the channels enter the main valley <br />floor. Debris-flow damage results from impact, burial, and flooding. <br /> <br />Debris flows are extremely common in Glenwood Springs and appear to be the <br />dominant form of flood event for most of the smaller watersheds of the re9ion. <br /> <br />It is the local geology that causes the frequent debris flows at Glenwood <br />Springs. The mountain slopes surrounding the city consist of sedimentary <br />rocks whose weathering products are highly susceptible to debris-flow activity. <br /> <br />Sudden failure of the accumulations of soil and debris can be triggered by <br />wetting due to snowmelt and thunderstorms. Also, debris fans and aprons are <br />very prone to hydrocompaction and require special drainage and foundations <br />even where not sUbject to debris flooding. <br /> <br />- 82 - <br />