ASSESSMENT OF MULTIPLE GEOLOGIC HAZARDS FOR RECREATIONAL CABINS ON NATIONAL FOREST SYSTEM LANDS IN CALIFORNIA, 2017-2021

One of the devastating effects of wildfires on National Forest System (NFS) lands in California has been the destruction of recreational cabins. Since 2017, 401 cabins have been totally destroyed on the Stanislaus National Forest (2017 Donnel Fire), Cleveland National Forest (2018 Holy Fire), Sierra National Forest (2020 Creek Fire), and the El Dorado National Forest/Lake Tahoe Basin Management Unit (2021 Caldor Fire). These fires have completely destroyed the structures, leaving only concrete foundations and metal remnants of appliances and other metal framing that were built into the cabins. District Rangers are faced with decisions to allow the cabin owners to rebuild their structures. One of the considerations in the decisions to rebuild are geologic hazards posing a threat to life and property at the cabin site. Geologic hazards have been assessed at the cabin sites and include debris flows, rock falls, deep-seated landslides, and floods. Geologic hazards have been assessed by mapping landforms using digital aerial photography, Lidar and high resolution UAS images. Deep-seated landslide hazards to cabin sites have been assessed by considering the activity level of a landslide and the location of the cabin sites relative to the location of a landslide. In one case, the activity level of deep-seated landslides has been determined by using InSar surface deformation data. Flood hazards have been assessed by considering the location of a cabin site relative to the flood plain of a stream or river. In some cases, the Army Corp of Engineer’s HecRAS Model was used to assess cabin sites within the 100-year flood plain. Once geologic hazards are identified, potential remediation of the hazards can be considered in the decision to allow the cabins to be rebuilt. In some cases, the hazards are too high to mitigate.