COUNTY-WIDE DEBRIS-FLOW SUSCEPTIBILITY MAPPING IN COLORADO: PROGRAM OVERVIEW AND METHODS SUMMARY

In response to legislation passed in Colorado after severe flooding and debris-flows in September 2013, the Colorado Geological Survey (CGS) developed a program to map county-wide debris-flow susceptibility for 43 counties in 13 Priority Areas comprising the mountainous portions of the state. The maps are intended as planning tools to help state and local agencies, emergency managers, watershed coalitions, and other stakeholders identify where debris flows and mud flows might occur, and where detailed local studies may be needed. Mapping methodology consists of GIS source-area modeling, debris-flow runout modeling using Flow-R software, identification of fans from LiDAR data, and revision of computer generated outputs.

A source-area model was developed for the Front Range based on locations of debris flows and scarps mapped by CGS and the USGS after the September 2013 events, and from literature review. The model is divided into two zones: 1) mountains <2,897 m in elev., hogbacks, buttes and mesas, and 2) mountains >2,897 m in elev. Zone 1 sources are identified by slope (≥15° and ≤45°), plan curvature (<-0.5), and flow accumulation (within 10m of flow accumulation ≥999 m²). Zone 2 sources are identified by elevation (≥2,897 m), slope (≥15° and ≤ 45°), plan curvature (<-2), and flow accumulation (within 10m of flow accumulation ≥9,999 m²). Runout and spreading parameters were selected by varying parameters over a series of runout models originating from September 2013 source areas, and comparing the results to post-event aerial imagery. Runouts for both zones are modeled in Flow-R using the modified Holmgren algorithm with dh = 2m, Holmgren’s exponent = 6, and travel angle = 9°; runouts are constrained by velocity limits of 13 m/s in Zone 1 and 45 m/s in Zone 2. Computer-generated sources and runouts are manually revised as necessary based on review of digital terrain data, aerial imagery, existing geologic maps, and limited field observations.

Maps for the first Priority Area (Boulder and Larimer counties) have been published, maps for the second Priority Area (Jefferson, Douglas, and El Paso counties) are in progress, and planning is underway for the remaining Priority Areas. Anticipated challenges moving forward include availability of LiDAR data, and calibration of modeling parameters outside of the Front Range.