LiDAR-Based Mapping of Alluvial and High-Angle Fans for Post-Wildfire Geohazard Assessment in Colorado, USA

Debris flows are rapid mass movements of water-laden debris that flow down mountainsides into valley channels and eventually settle on valley floors. The risk of debris flows can be significantly increased after wildfires. Following the destructive 2021 debris flows in Glenwood Canyon, the Colorado Geological Survey (CGS) initiated a LiDAR-based alluvial fan mapping project to improve geologic hazard delineation of alluvial and high-angle fans in response to developing wildfire-ready watersheds. These landforms, shaped by episodic sediment-laden flows, pose significant risks and are often misrepresented on conventional geologic maps. CGS delineated fan-shaped landforms with improved precision using 1-meter resolution LiDAR-based DEMs, DEM-derived terrain metrics, hydrologic analysis, and geospatial analysis tools within the ArcGIS Pro platform. Our results reveal previously unmapped or misclassified alluvial or high-angle fans in areas undergoing increasing development pressure, where low-gradient terrain indicates a high hazard potential. This work highlights the critical role of high-resolution LiDAR data, geospatial analytical techniques, and systematic QA/QC protocols in refining hazard awareness. The resulting dataset supports proactive land-use planning and wildfire resilience by identifying areas prone to debris flow and flood hazards. Although not intended for site-specific design, these maps serve as a critical resource for prioritizing geologic evaluations and guiding mitigation planning across Colorado’s wildfire-affected landscapes.