Rock on! Assessing Landslide Hazards in Hoonah

It’s an unassuming sight: a thin trickle of water running down a muddy slope into Hippoback Creek, but underneath the mud is critical information that will help protect Hoonah from future landslides. A team of geophysicists from the Sitka Sound Science Center and the University of Oregon came to town at the beginning of August to put boots on the ground and start understanding how and why landslides occur on Chichagof Island. Once they have a clearer picture of the physics beneath their feet, they’re hoping to translate it into a model that forecasts the level of landslide risk around town, allowing community members to assess the hazard and determine whether they need avoid certain areas during heavy rainfall.

After a series of lethal landslides in Sitka and Haines, community members in Sitka started thinking about ways to protect themselves and their city from geophysical hazards, and thus the KUTÍ project was born. The program has developed an early warning system by installing a network of sensors and developing a risk assessment model to predict how likely it is that Sitka will experience a landslide based on the conditions that have caused them in the past. Now with additional funding and more experience under their belts, they’re looking to expand the network to other towns where landslides pose a large risk to human safety and infrastructure, and Hoonah’s hoping to become a pilot for how the project might grow. While Southeast is already accustomed to heavy rains, understanding exactly how the land will respond and the impact that will have on people will become even more important as climate change alters how the region receives water. Many of the well-known landslides around Hoonah, for example, were caused by an atmospheric river in 2020, which is just one of the types of extreme weather that are predicted to be exacerbated by global warming in the future.

Despite being only 70 miles apart, the geology of Hoonah is fairly different from that of Sitka, and far less understood. The team spent several days hiking up landslides around town to understand what the ground is made of and how it’s put together so they can start to create a picture of how to best tailor their work to Hoonah. The soil and its behavior depend on what’s beneath it, with the composition and orientation of the rocks having a direct impact on how likely it is that landslides will occur. In the Hippoback slide, for example, the layers of rocks have been tilted nearly sideways, allowing water to seep into the rock and cause instabilities that lead to landslides. They also studied the scarp of the slide, where it broke off from the mountain and began traveling downhill, to understand the scale of problem at hand. Most slides on Chichagof island are debris flows, which are when a yard or two of soggy dirt breaks off and catches more material as it runs downhill. While this type of landslide occurs more often than the deep-seeded ones they have in Haines and other places on the mainland, they’re usually smaller and therefore less lethal.

The next step of the process is for the KUTÍ team to review what they learned in Hoonah and get a better understanding of what the community wants to see from the project. While initial interest in Hoonah was quite positive, they want to tailor the information they gather to match what residents would like to know and what areas they’re most concerned about. Ted Elliott (AYS) said he thinks “one or two [sensors] over town would be a great idea, above the residents so if something goes wrong, we’re prepared for it.” Other ideas included in Gartina Basin, where the municipal water supply and hydropower comes from or on key hunting roads to ensure access to subsistence resources. The landslide stations will include at a minimum soil moisture sensors, a precipitation gauge and a small well to understand how much water is being stored underground, but may include additional weather sensors to address community concerns . Keep an eye out for future meetings with the landslide team to play a role in keeping Hoonah as safe as possible!