Exhumation By Landslide-Initiated Debris Flows in the 2013 Colorado Front Range Storm

Abstract:

What is the role of rare events in the exhumation of steep landscapes? We use the “millennial” storm that hit the Front Range, Colorado, USA, in September 2013 to explore this question in a semi-arid landscape. More than 250 mm of rain fell over a ~100 km swath of the Front Range in a 5 day period; totals in some areas exceeded average annual precipitation. The storm triggered over 1300 landslides and debris flows in four major Front Range watersheds (Coe et al., 2014).

We created a DEM of difference in the 102 km² area of overlap using aerial LiDAR surveys, acquired in August 2010 by the Boulder Creek CZO and in November 2013 by FEMA. The study region covers the Boulder Creek watershed from the middle of the Rocky Mountain Surface to the western edge of the Plains, and encompasses Boulder Canyon, Fourmile Canyon, and the 26-km² 2010 Fourmile Canyon Fire. Precambrian crystalline rocks underlie most of the area, although the eastern margin includes sedimentary rocks in hogbacks along the mountain front. We computed site characteristics and volumes for 120 failures.

Within the crystalline terrain, most failures occurred at or near the bedrock interface at 0.5-1 m depth, often near the ridgelines downslope of bedrock outcrops. Failures occurred on slopes of 25-40°, and show no slope aspect bias. Failures evolved into debris flows that scoured chutes from the initiation site down to the master stream, traveling at up to 10 m/s. We saw little evidence of deposition; most debris was entrained in the flooding master streams and exited the mountain front. Evacuated sediment volumes represent several hundred years of exhumation within the source basins, based on published long-term erosion rates calculated from ¹⁰Be concentrations. We infer that, even in this semi-arid environment, debris flows initiated by rare shallow landslides are a dominant process for evacuating sediment from steep channels and delivering it to the plains.