GEOMORPHIC EFFECTS OF THE SEPTEMBER 2013 FLOOD IN FOURMILE CANYON, COLORADO, USING LIDAR AND FIELD STUDIES

This research uses the results of field measurements and remote sensing to reconstruct flood discharge, sediment transport, channel changes, and sediment budgets for a catastrophic flood along a 15 km reach of Fourmile Canyon, Colorado. Persistent rainfall from September 9 to 15, 2013 caused extensive flooding and local debris flows in the Front Range foothills west of Boulder, Colorado. Locally intense rainfall and persistent effects of recent wildfires probably contributed to the scale of the event, especially in catchments such as Fourmile Canyon, which heads in the alpine zone and flows into Middle Boulder Creek in the semiarid foothills. Field surveys focused on flood height and width indicators, point counts, and sieve analysis of gravel-rich deposits thicker than 0.5m. The availability of 1-meter resolution LiDAR from before (Aug. 2010) and after the flooding (Oct. 2013) provided an unusual opportunity to measure the geomorphic effects of the flood in the same areas as our detailed field surveys. Cross-sectional profiles allowed us to estimate peak discharge using the slope-area and critical-depth methods, as well as the competence and power of the event. Combining these approaches at 12 sites shows ~31,000 m³ of local deposition despite ~80,000m³ of net erosion from the valley floor, entrainment of boulders weighing over 3 metric tons, and estimated peak discharges of 58.5-97.9 m³/s. Comparison with other discharge estimates yields similar values (Jarrett, in press), but our combination of techniques characterizes the flood in greater detail. The mass balance of sediment in the canyon, the profile differences, and field observations all indicate that the flood event produced net erosion, mainly by channel widening, despite significant (>100m long, >1m thick) depositional zones throughout the canyon. Similar effects have been reported in other headwater catchments due to extreme floods such as Hurricane Irene in Vermont.