An analysis of cataloged December 2020 landslides near Haines, Alaska

Abstract

From November 30 to December 2, 2020, an atmospheric river event brought high winds, heavy precipitation, and unseasonably warm temperatures to Southeast Alaska. In a 48-hour period, weather stations located in the Haines, Alaska, area recorded record-breaking amounts of precipitation. This resulted in 160 landslides around the community, some of which cut off evacuation routes and access to the community's fuel supply, and caused power outages and evacuations. The largest of the landslides occurred along Beach Road on December 2, 2020; it destroyed or severely damaged four residences and killed two occupants. This report focuses on 58 of the landslides, chosen based on their proximity and impact to road corridors or private property. During field investigations in 2021 and 2022, I observed and described landslides, took in situ strength measurements, and sampled soils that I subsequently tested in the laboratory for engineering index properties such as soil classification, moisture content, and organic content. I mapped landslide extents and evidence of previous landslides using high-resolution lidar data. Using all of these data, I developed a landslide catalog of the 58 landslides, which contains information about location, impact on the road system in 2020, field observations, stratigraphy, laboratory test results, landslide classification, maps, and relevant photographs. Analysis of the collected data suggests that the most significant factor that contributed to the December 2020 landslides was the amount and intensity of precipitation. This precipitation exacerbated the preexisting condition of high slope angles in the surrounding area, and resulted in excess pore pressure in soil types that usually drain well. Anthropogenic factors, such as removal of vegetation and the toe of slopes, also likely played a role in the distribution of the landslides. Recommendations for further study based on results in this report are: 1) to date previous landslides in the study area to determine the frequency of these events; 2) to install additional weather stations in the Haines area for widespread real-time weather monitoring and studying effects of localized high precipitation and/or wind on landslide occurrence; and 3) to conduct additional strength testing on soil and bedrock within the failed areas.