Debris flow hazard evaluations for multi-hazard risk mapping in Sitka, Alaska

Debris flow hazard evaluations for multi-hazard risk mapping in Sitka, Alaska, Report of Investigation 2024-2, provides digital data used to develop interpretive maps and a geospatial database of historical debris flows, shallow-debris flow susceptible slopes, and simulated debris flow runouts for the City of Sitka, Alaska. On August 18, 2015, heavy rainfall and wind resulted in numerous debris flows in and around Sitka, Alaska. Four debris flows impacted roads and infrastructure in Sitka, and the southernmost of two flows at Kramer Avenue took the lives of three residents. In response to these events, the Alaska Division of Geological & Geophysical Surveys (DGGS), through a Community and Technical Partners Grant with the Federal Emergency Management Agency (FEMA), initiated a multi-hazard assessment of the Sitka area. The objective was to help better understand debris flow hazards, inform mitigation efforts, guide future development activities, and protect public safety in and around Sitka. We emphasize this study is a regional evaluation, not a site-specific assessment. It would be inappropriate to use the results for site-specific decision-making. Our final products show what computer models predict could occur based on the selected inputs and with the idealized assumption that the simplified input conditions occur, unvaryingly, throughout the entire model area. The reality is much more complex and can only be addressed with detailed site-specific studies. Our model is intentionally conservative to not underestimate the potential risk to life and safety, and this can be seen in areas where modeled potential debris flow runouts are longer than those observed from historic debris flows. These data and the interpretive maps and report are available from the DGGS website: http://doi.org/10.14509/30187.