Petersburg plans for landslides

You can’t do much to stop a landslide. But in Petersburg, an Alaska Sea Grant-supported effort will help residents better understand which areas are most at risk.

“We’ve seen a lot of shallow landslides in Southeast Alaska over the last 10 years or so, some of which turn into debris flows that can be very destructive or fatal,” said Nick Mathews, a postdoctoral researcher supported by Alaska Sea Grant and the Sitka Sound Science Center. “This is a primary hazard in the region, so we are working to develop a model that simulates them to try and predict where they might happen in the future.”

Destructive landslides have been increasing in Southeast Alaska in recent years, a consequence of warming oceans, which can generate more heavy rain events. The U.S. Geological Survey recorded 162 significant landslides in Southeast Alaska between 1990 and 2024, including fatal ones in Sitka, Haines, Wrangell, and Ketchikan.

Downtown Petersburg itself is in a flat, safe area, but landslides are still a major concern for its 3,000-plus residents. The only highway out of town is wedged between steep mountains and the sea, as are the lines that transmit hydroelectric power to the community. A landslide on October 31, 2022, cut off both.

“The Halloween landslide showed the kind of impact these events can have,” said Sunny Rice, Alaska Sea Grant’s Marine Advisory Program agent for Petersburg. “In addition to damaging properties, they can cut people off from critical infrastructure like our hospital, school, airport and post office.”

It was a fatal 2023 landslide in nearby Wrangell that spurred Rice and a group of Petersburg homeowners to start exploring what would be needed to improve local landslide predictions. As Rice describes it, there are two important questions to address, “Where are landslides likely to occur, and then when are they likely to occur?” She said answering those questions requires maps of likely landslide routes, and reliable weather data to know when local rainfall is likely to trigger an event.

The group dealt with the weather issue first. They received a $2,000 grant from the Petersburg Community Foundation and installed three weather stations in a high-risk landslide area. Data from one of these stations are available online through local public radio station KFSK. Rice said she is continuing to search for more funds to further improve local weather data.

Rice then teamed up with the Sitka Sound Science Center (SSSC) to secure a $200,000 award from the National Sea Grant College Program’s Disaster Preparedness for Coastal Communities program. The funding is enabling Mathews to build predictive landslide risk maps for the Petersburg area as well as for Ketchikan. The work is an extension of Mathews’ involvement in the National Science Foundation-funded Ḵutí Geohazards Project, which is creating landslide awareness products for the Southeast communities of Yakutat, Klukwan, Skagway, Hoonah, Craig and Kasaan.

The predictive maps combine data on topography, hydrology, soil depth, and soil shear strength to pinpoint probable landslide trajectories. Mathews plans to validate the maps by comparing them to known landslide paths documented by the U.S. Forest Service. “We can compare our products to those landslides that have actually occurred to say, how is this model performing?” he said. “And from there, there are knobs you can turn within the model to make it work as best you can.”

Mathews’ final step will be to research historic landslides and cross-reference them with local weather conditions at the time of their occurrence, drawing on methods developed by a collaborator at the University of Oregon. This will enable him to calculate the precipitation thresholds likely to trigger landslides.

The question remains of how best to disseminate the maps and findings. Rice said one likely outcome is that the maps will be shared online and hard copies made available in Petersburg as well. Rice would then distribute information to Petersburg residents on how to cross-reference the maps with National Weather Service rain predictions and local weather data to inform their decision making.

“The maps are there and the basic weather information is there,” she said. “Now we’re trying to figure out how best to get it into people’s hands.”