New insights into debris-flow hazards from an extraordinary event in the Colorado Front Range

[摘要] Rainfall on 9–13 September 2013 triggered at least 1,138 debrisflows in a 3430 km2 area of the Colorado Front Range. The historicalrecord reveals that the occurrence of these flows over such a large area in theinterior of North America is highly unusual. Rainfall that triggered the debrisflows began after ~75 mm of antecedent rain had fallen, a relatively low amountcompared to other parts of the United States. Most flows were triggered in responseto two intense rainfall periods, one 12.5-hour-long period on 11–12 September, and one 8-hour-long period on 12 September.The maximum 10 min. intensities during these periods were 67 and 39 mm/hr. Ninety-fivepercent of flows initiated in canyons and on hogbacks at elevations lower than awidespread erosion surface of low slope and relief (<2600 m). These flows wereon steep (>25°), predominantly south- and east-facing slopes with upslope contributingareas <3300 m2. Flows with the largest scars and longest travel distancesoccurred at elevations above 2600 m on steep slopes with contributing areas >3300m2. Areal concentrations of debris flows revealed that colluvial soilsformed on sedimentary rocks were more susceptible to flows than soils on crystallinerocks. This event should serve as an alert to government authorities, emergencyresponders, and residents in the Front Range and other interior continental areaswith steep slopes. Widespread debris flows in these areas occur infrequently butmay pose a greater risk than in areas with shorter return periods, because the publicis typically unprepared for them.

[学科分类] 地质学

[关键词] Fire safety;fatal fires;smoke alarms