GSA Connects 2021 in Portland, Oregon

Paper No. 159-2
Presentation Time: 9:00 AM-1:00 PM


HAMMER, Morena, Department of Geology, Portland State University, 1721 SW Broadway, Portland, OR 97201 and BOOTH, Adam, Department of Geology, Portland State University, 1721 SW Broadway, Portland, OR 97201-3300

Post-fire debris flows initiated by overland flow in the Pacific Northwest (PNW) are largely undocumented. Typically, PNW debris flows mobilize from shallow landslides that result in a mud slurry of water and sediments traveling downhill under the force of gravity. However, due to the intensity and severity of the September 2020 fires in Oregon, we investigated the resultant erosional patterns, whether debris flows occurred, and if so, if they were initiated by overland flow rather than landslides. We monitored three catchments within the Riverside Fire, southeast of Estacada, Oregon. These catchments have experienced previous debris flows and landslides from significant precipitation and rain-on-snow events. At these catchments we have taken consecutive terrestrial lidar scans and collected hydrologic measurements to calculate approximate erosion magnitudes, patterns, and constrain a time frame for when the sediment was entrained. These hydrologic measurements include runoff monitoring and infiltration tests with a mini-disk infiltrometer. The terrestrial lidar differencing was performed by aligning point clouds of pre- and post-PNW wet season and subtracting them to understand millimeter to centimeter scale erosion that ensued across our monitoring sites due to rainfall and runoff. Produced from this differencing analysis are digital elevation models detailing areas of elevation change between November 20th, December 23rd, 2020, and May 26th, 2021. Between Nov. through May, one of the higher elevation sites (~70 m2) experienced ~9 mm of erosion. Major storms during this time occurred on Dec. 18th - 21st and Jan. 11th – 13th, precipitating 16 cm and 38 cm, respectively. To assist in determining if runoff occurred during these storms and to differentiate between their pathways, we developed a runoff response box model. According to the model, both events experienced a transition from saturation excess overland flow to Hortonian overland flow. Additionally, Infiltration measurements from May 26th indicate a mean hydraulic conductivity of 73 mm/hr (± 55 mm/hr) and 61 mm/hr (± 34 mm/hr) at the lower and higher elevation sites, respectively. With time we anticipate catchment recovery signaled by an increase (and stabilization) of infiltration, vegetation regrowth, and reduction in erosion magnitude.