Grant to Assess Sitka Landslide Risks

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Category: Local News

Created on Wednesday, 05 October 2016 23:12

By SHANNON HAUGLAND

Sentinel Staff Writer

The Alaska Division of Geological & Geophysical Surveys announced Tuesday that the agency has received funding for a comprehensive landslide hazard assessment of the Sitka area.

The landslide assessment, funded by the Federal Emergency Management Agency, will include landslide mapping and hazard modeling for about 25 square miles of Sitka, said De Anne S.P. Stevens, chief of the engineering geology section for DGGS.

It will cover the entire populated area of Sitka, including all of Harbor Mountain, the main part of Sitka, the Indian River Valley and out to Sawmill Cove, Stevens said.

“We’re hoping to do slope stability and landslide hazard analysis,” she said.

The interest in studying Sitka followed the Aug. 18, 2015, landslides here that killed three people on Kramer Avenue and caused about $1 million in damage, Stevens said.

The study will use the new lidar data set collected earlier this year under a partnership between the National Park Service, the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory and DGGS, a division of the state Department of Natural Resources.

Lidar is a technology in which laser beams are used to measure the elevation of the ground.

“It allows us to generate a 3-D topographic land level map from which we can strip all trees and vegetation and look at what’s going on underneath,” Stevens said. “It’s incredibly useful for analyzing any sort of land surface process. Things like geomorphology ... looking at unstable slopes, old drainage channels. There’s a lot we can see.”

She said following the slides last year, there was a sudden recognition of “how imminent this sort of hazard is to the community.”

“It’s been known for a while that the area is prone to slides,” she said. She noted that soon after the slides, the U.S. Forest Service flew over the area and counted a total of 45 new slides that resulted from that day’s heavy rainfall on Chichagof and Baranof islands.

“This struck close to home – everybody’s hyper-aware we need to understand the hazard, especially in urbanized areas. ... We really need to take it seriously,” she said.

Stevens sent an email Tuesday to the Sitka Geohazards Task Force to inform the group of the news about FEMA funding. The Geohazards Task Force was organized by the Sitka Sound Science Center after the 2015 slides, and included scientists from a number of agencies in Sitka and from outside the community, Stevens said.

“... We at DGGS are eager to work with all of you to ensure the best possible analysis for the benefit of the safety and wellbeing of the community and people of Sitka,” Stevens said in the email.

She said the community has been supportive of learning more about the hazards. In April the Assembly passed a resolution in support of communitywide mapping.

Sitka hired the geotechnical firm Shannon and Wilson to complete a study of the south Kramer slide neighborhood, and asked for a preliminary assessment of the area above Keet Gooshi Heen Elementary, but City Administrator Mark Gorman said it makes more sense at this point to allow the DGGS work to go forward before more studies are done. 

“By doing it comprehensively it doesn’t highlight certain neighborhoods, which may disadvantage and impact land values,” he said today. “I think it’s more even-handed to do it all at once. ... It will be easier to accept the fact that there is an inherent risk whether you’re living at sea level or living on a hill. It’s more responsible approach to do a comprehensive look at the community, rather than piecemealing it out.”

Stevens said she is looking at a two-year time line to complete the studies, and the results will be shared with the public as they come out.