USING LIDAR DIFFERENCING TO BETTER UNDERSTAND POST-FIRE DEBRIS FLOWS FOLLOWING THE 416 FIRE NORTH OF DURANGO, CO

In the aftermath of the 2018 416 fire near Durango, CO, which burned 54,130 acres, debris flows caused millions of dollars in damage. Despite the magnitude of the events, there has been no analysis of the source of the debris, whether from the hillslopes, the channels, or both. Here we identify geomorphic changes following the fire by comparing two lidar elevation datasets: a pre-fire dataset from 2015 and a post-fire dataset from 2018. The two datasets overlap along the foot of the Hermosa Cliffs, where the most destructive debris flows occurred, providing an opportunity to perform change detection by differencing them. First, point clouds were processed in LAStools to remove vegetation. The resulting bare-earth point clouds were aligned in Cloud Compare then rasterized using LAStools. The elevation models of the two rasters were finally subtracted in ArcGIS. Although noise from misalignment remains, there was significant incision in nearly all streams draining the burn area, where the vertical incision reached up to around 5 m. Overall, minimal changes were detected on the hillslopes. Thus it seems that the debris stored in the channel beds was critical in bulking up the post-fire flash floods into debris flows. Field checking revealed similar results, indicating that despite the imperfect alignment and disparities in point density and registration between the two different point clouds, lidar differencing can be used to accurately determine geomorphic changes after fires.