USING MULTITEMPORAL LIDAR TO BETTER UNDERSTAND POSTFIRE DEBRIS-FLOW HAZARDS IN WESTERN OREGON (Invited Presentation)

The geology, relief, and precipitation in the Columbia River Gorge (CRG) result in a debris-flow prone landscape with many recorded historic events during the past 100 years. In 2017, the Eagle Creek Fire burned ~50,000 acres in the western section of the CRG on the Oregon side. Airborne lidar was collected before the fire in 2009 and after the fire in 2018. In January 2021, an atmospheric river (AR) triggered debris flows in the Eagle Creek burn area. Sixteen debris flows were inventoried through field work performed during the two weeks following the storm. Post-event lidar data was collected in December 2021. In January 2022, one year later, another AR soaked the same burned area. This event triggered many debris flows; 17 of which were identified in the field. Again, post-event lidar was collected in June 2022. These four lidar datasets were differenced to examine the hillslope and channel processes occurring during AR storms in the postfire landscape. The lidar difference mapping revealed over 200 mass wasting events had occurred during the January 2021 storm. This landslide inventory and local precipitation measurements indicate that the fire: (1) lowered the necessary terrestrial water input (TWI) threshold to trigger debris flows, (2) increased the temporal frequency of debris flows, (3) switched the debris-flow initiation process from a mostly infiltration-dominated process (shallow landslides on hillslopes) to a mostly runoff dominated process (in channel), and (4) decreased the required upslope contributing area for debris-flow initiation sites. We examined the upslope contributing area above the postfire debris-flow initiation sites and found approximately 70% of the postfire debris-flow initiation sites included >50% of the contributing areas is composed of slopes ≥ 25° with a differenced normalized burn ratio (dNBR) ≥ 270. This indicates that steep, burned areas above potential initiation sites promote postfire debris-flow initiation. The high resolution lidar change mapping resulted in the ability to perform detailed, spatially continuous analysis along a debris-flow path. This data improved the ability to understand where and why debris flows grow or deposit in the context of geologic and geomorphic factors like surficial soils, channel gradient, and confinement. These advances will improve the ability to model future debris-flow inundation zones.