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

GSA Today
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Abstract

Rainfall on 9–13 September 2013 triggered at least 1,138 debris flows in a 3430 km2 area of the Colorado Front Range. The historical record reveals that the occurrence of these flows over such a large area in the interior of North America is highly unusual. Rainfall that triggered the debris flows began after ~75 mm of antecedent rain had fallen, a relatively low amount compared to other parts of the United States. Most flows were triggered in response to 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-five percent of flows initiated in canyons and on hogbacks at elevations lower than a widespread erosion surface of low slope and relief (<2600 m). These flows were on steep (>25°), predominantly south- and east-facing slopes with upslope contributing areas <3300 m2. Flows with the largest scars and longest travel distances occurred at elevations above 2600 m on steep slopes with contributing areas >3300 m2. Areal concentrations of debris flows revealed that colluvial soils formed on sedimentary rocks were more susceptible to flows than soils on crystalline rocks. This event should serve as an alert to government authorities, emergency responders, and residents in the Front Range and other interior continental areas with steep slopes. Widespread debris flows in these areas occur infrequently but may pose a greater risk than in areas with shorter return periods, because the public is typically unprepared for them.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title New insights into debris-flow hazards from an extraordinary event in the Colorado Front Range
Series title GSA Today
DOI 10.1130/GSATG214A.1
Volume 24
Issue 10
Year Published 2016
Language English
Publisher Geological Society of America
Contributing office(s) Geologic Hazards Science Center
Description 6 p.
First page 4
Last page 10
Country United States
State Colorado, Wyoming
Other Geospatial Colorado Front Range