Bolt Creek Fire: Post-Wildfire Debris Flow Risk Assessment and Barrier Design on US 2, Near Grotto, WA

Post-wildfire debris flow hazards created by the Bolt Creek Fire warranted rapid response from the USDA Forest Service, Washington Geologic Survey, and Washington State Department of Transportation (WSDOT), to analyze and mitigate the risk of debris flows reaching US Route 2 (US 2). The 2022 Bolt Creek Fire burned over 14,000 acres of forest along US 2 between approximate mileposts 40 and 50, creating debris flow hazards within the preexisting drainage network upslope. The Bolt Creek Fire burned area along US 2 is on the western slopes of the North Cascades, where the surface geology is composed of alluvial soils, Mesozoic metasedimentary, and Tertiary intrusive rock. The incised drainage network extends from US 2 approximately 4,000 feet upslope to the crests of Baring Mountain and Grotto Mountain. Post-wildfire assessments by the USDA Forest Service, USGS, and WGS provided specific points of concern for WSDOT with elevated debris flow risk, based on burn severity, and mapped alluvial fans intersecting US 2 and the town of Grotto, WA. WSDOT provided rapid response, by completing site specific evaluations of each point of concern. Evaluating available catchment and sediment delivery potential, WSDOT identified two locations with the highest risk of debris reaching US 2. To reduce the risk of debris flow impacts to the highway, WSDOT used hand calculations along with GeoBrugg Inc.’s dimensioning tools DEBFLOW and SHALLSLIDE to calculate estimated static and dynamic loads that would be generated in the event of a debris flow in these two locations. Using the results of the analysis, WSDOT recommended construction of post-supported debris flow (or shallow landslide) barriers (western and eastern) that will withstand impacts of up to 1570 psf (75 𝑘𝑁 𝑚2 ). Rapid response, design, and construction times are required to construct the debris flow barriers in a timely manner, considering the estimated 1–to-5-year timeline in which the post-fire debris flow risks are the greatest.

Language

  • English

Media Info

  • Media Type: Digital/other
  • Features: Illustrations; Maps; Photos; References; Tables;
  • Pagination: 26p
  • Monograph Title: Proceedings of the 72nd Highway Geology Symposium (HGS 2023)

Subject/Index Terms

Filing Info

  • Accession Number: 01892722
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Sep 11 2023 11:42AM