MONITORING SPATIAL PATTERNS OF POST-WILDFIRE EROSION WITH TERRESTRIAL LIDAR

The Fourmile Canyon wildfire burned in September 2010, 15 km west of Boulder, Colorado. We used repeat terrestrial LiDAR surveys to monitor the erosional evolution of a small 5500 m² basin within a high-intensity burn zone. The study area includes a ridge top and a concave hillslope with a colluvial channel. Repeat terrestrial LiDAR scanning revealed erosional depths and spatial patterns of post-wildfire erosion, and provided more spatial information than traditional methods such as erosion pins.

The first LiDAR survey was conducted approximately three weeks after the wildfire and prior to any rainfall. Four additional surveys were performed between major rain storms during a two-year period following the wildfire. LiDAR results show that after the first major summer rain storms, the majority of erosion occurred within existing channels. However, subsequent erosion appeared to be independent of the landscape position, occurring in patches on the hillslopes and within the channel. The change in process from focussed channel erosion to spatially distributed erosion appears to be related to the change in surface material roughness with time.

Immediately following the wildfire, LiDAR data shows that the hillslopes had few roughness elements, and boulders within channelized drainages were clustered in distinct groups. After the first major rain storm, the channels incised, and a boulder/cobble lag was exposed, creating a more uniform distribution of roughness elements in the channel. The median size of surface roughness elements within the channel increased from 50 cm to 55 cm, but more importantly, the channel connectivity was disrupted after the initial erosion. After the initial incision, the distance within the channel between boulders decreased from a maximum distance of 12 m to less than 5 m. With the exposure of large grains in the channels, the volume of sediment available for transport decreased and long reaches for channelized flow erosion were eliminated. Consequently, no long-term rilling or gully erosion was sustained within the basin. This study suggests that the spatial pattern and mode of erosion is highly dependent on the sediment availability and surface roughness.