HSC : June 2025 Meeting Looks at Preliminary Landslide Risk Results in Hoonah

In attendance : Jackson C., Jeromy G., Thomas M., Ian J., Marlene D., Stephanie H., Jackie D., Julian N., Chloe, Josh R., Nick M., Brad S., Elizabeth F., Erica D., Bill M.

The Hoonah Stewardship Council members met with Khuti partners from Oregon State University and Sitka Sound Science center to review preliminary and new results of Hoonah’s landslide risk. These results will be ground-truth by Khuti members during a 5 day field trip in July.

Let us know your thoughts on landslides and data sharing about landslides here : https://docs.google.com/forms/d/1P6f0KnAVAF8mW3VGN4at2CEdqTYjebguQ9j_raTsohM/edit respond by July 15,2025 to be entered in a drawing for a $250 giftcard!

The Model

Khuti researcher Nick Matthews explains his model for analysis. What is a model? A good analogy is a puzzle. If you have more pieces of a puzzle put in the right place the picture becomes clearer. In a model the data is the puzzle piece. The more data you have the more likely you can predict with accuracy what the final “picture” looks like. In this case, what can the data we have tell us about landslide risk in Hoonah?

Nick presented the draft model, built from:

1. LiDAR data (2015) – this is accurate data that can show ground topoography
2. Tongass NF Landslide Inventory Map (years 1950s/60s – post 2020)
3. Focused on landslides that start in a pocket of soil and either do shallow slides or channelized debris flows

Small landslides are most common; most frequent type is about the size in area of a 3 bedroom home

Q: where do landslides intersect with humans (buildings, roads, etc) and subsistence resources (streams)?

Next Steps : Nick presented the preliminary draft of this model and we need to test the model against reality with field visits in July.

Preliminary Results

• Did not see much risk above Hoonah proper based on landslide inventory
• Saw overlap of landslide risk with roads outside Hoonah
• Want to adapt road damage models from Oregon to Hoonah

Community Questions/Feedback

1. during visit to Hoonah, where will study first? In town and then out, or out of town and then in town?
   • Nick says they want guidance from HIA and locals, about priorities, because the study can start anywhere
   • Josh says prob start in town because that is where impacts would be greatest; they want to visit areas outside town to calibrate the LiDAR and model, and they want to add more recent slides to the inventory; but they want to focus where impacts are most critical; also want to visit shallow and channelized examples in the field
2. When will new LiDAR be available? Model can be updated when that happens
   • Spring 2026 is current estimate
3. A lot of construction on Eagle Drive, eventually they will sell more properties up there and she sees dirt being moved and the water is starting to change how the water is coming down; want to see some study focus on Eagle Drive
   • Josh: affect of roads on slides; can affect where slides initiate and where water goes
4. 2020 Dump Road landslide is modeled but not in inventory but happened and covered the road, he thinks
5. Community members describes the Spasski deep slide
   • the models they are working on will not capture the deep slides where the side of the mountain collapses
6. Are the deep slides more likely to be earthquake triggered?
   • Josh: earthquakes tend to rattle ridges more than lower elevations; this looks like it could be that type; need to do dendrochronology and carbon dating
7. Comment in room: focusing on town is really important first, and then main USFS roads like Game Creek are key for recreation and subsistence
8. 2020 landslide took 3 years to clear the road; Pavlof Lake/Freshwater Bay area
9. A lot of topography around Hoonah town has not been affected by landslides since the glaciers retreated, based on LiDAR, which is a really good thing
   • Do we have to worry more about areas where it has never slid before, given the AR events; areas that have had soil deposition over thousands of years, it seems like it could be a high risk area, even the highest, or not?
   • we do tend to see reactivation in old debris flow channels; like an avalanche, where clearing out instability means expected stability for a time, but then a recurrence interval; the debris flow channels re-fill with material; expect to see debris flows in the future where they have been before
   • gentler slopes around Hoonah may take longer than steeper rock faces to give way
10. We have a lot of shale and the water collects into the shale; I noticed slides in the graveyard and other areas; will you be able to look at that?
    • I want to see that before commenting too much
    • shale tends to weather rapidly when exposed and that can destabilize the material above it; slaking (disintegration of shale) happened in Wrangell, and that can build up soil pockets through weathering, which can help guide the water as well