13-2 Building Scalable Hazard Datasets From LiDAR: Standardized Mapping of Alluvial and High-Angle Fans in Fire-Prone Terrain

Session: Hazards and Resources Mapping in the Southwest US: Applications and Future Opportunities

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Post-wildfire debris flows in Colorado, including the 2021 events following the Grizzly Creek Fire, exposed significant limitations in conventional geologic maps, particularly in the representation of alluvial fan landforms that influence debris-flow routing and sediment deposition. Many depositional systems are generalized or omitted at quadrangle scale, constraining hazard assessment in rapidly changing fire-affected landscapes.

To address this gap, the Colorado Geological Survey developed a standardized, LiDAR-based GIS workflow to systematically delineate alluvial and high-angle fan landforms across multiple counties. Using 1-meter digital elevation models, slope rasters, convolution filters, and geomorphic interpretation in ArcGIS Pro, fan polygons were digitized and classified by mean surface slope (<=20° alluvial fans; >20° high-angle fans). Apex placement, distal toe constraints, and drainage context were incorporated to maintain spatial consistency across varying terrain settings. A structured QA/QC protocol included cross-review of 10% of polygons, resulting in modification of approximately 15–25% of initial delineations.

To date, more than 5,700 fan landforms have been mapped across eight Colorado counties, generating a regional-scale dataset designed for screening-level hazard assessment and integration with debris-flow modeling and land-use decision support. This approach demonstrates how high-resolution terrain analysis, combined with reproducible GIS criteria, can modernize legacy mapping and produce scalable hazard datasets applicable across fire-prone regions of the western United States.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

doi: 10.1130/abs/2026RM-14656

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