Chilkat Valley News - Serving Haines and Klukwan, Alaska since 1966

Small landslide indicates instability in Beach Road area, EOC says

 

January 21, 2021

Courtesy of Haines Borough EOC.

The photo compares LiDAR images of the Beach Road landslide area collected in 2014 and 2020, after the slide occurred. Comparing the detailed images allows geologists to measure where slide material came from, where it ended up and in what quantity it moved. Dark green areas show where slide material came from and red areas show where the material was deposited. The information can be used to assess future slide potential in the area, and assist with search and rescue plans.

A small landslide occurred on the southeast flank of the Dec. 2 Beach Road slide path between Monday and Tuesday, according to a Jan. 19 Haines Borough press release. The movement occurred after Haines received roughly 2 inches of rain in a 12-hour period.

"The estimated size of the new slide area was about twenty feet wide by sixty feet high, which slid approximately two hundred feet downslope," the borough's statement reads.

Emergency Operations Center (EOC) spokesperson Liz Cornejo said the new slide was observed through routine photography monitoring of the Beach Road area-comparison between a Jan. 18 image of the slide path and an image taken on Jan. 19.

On Tuesday, Cornejo said the new movement doesn't change much when it comes to the risk assessment of the area. "It is not surprising and is an example of why the area remains evacuated. It is an unsafe and unstable area," she said.

The weather pattern leading up to the new slide movement was similar to the pattern observed in early December-substantial snowfall followed by a period of heavy rain-although in a much smaller volume.

In a Jan. 14 town hall meeting, the EOC team said geologists continue to monitor areas throughout the borough, keeping an eye out for signs that could point to increased landslide risk including increases in groundwater pressure, surface-water flow and erosion. Taking daily photographs of the Beach Road slide area is part of the ongoing monitoring.

The state is also beginning to collect and process LiDAR images of the Beach Road slide, and other areas of concern throughout Southeast, as part of the response effort in the wake of Gov. Mike Dunleavy's disaster declaration for the region.

LiDAR images are collected using lasers to measure very precise changes in topography and can image terrain under trees and snow. The LiDAR collection effort is still in its early stages, according to Alaska Department of Natural Resources (DNR) geospatial information officer Leslie Jones.

In December, "(DNR) collected LiDAR along potential landslide-susceptible slopes throughout the Haines Borough, as well as Beach Road slide area," Jones said. The collection, which cost about $85,000, was paid for by the state.

Jones said LiDAR collection in other Southeast communities has yet to begin, in part because the state still needs to secure a funding source and, in part, because snow cover makes winter a poor time to collect these images.

The LiDAR data will be used to "assist with change, risk and damage assessments" throughout the region, according to Jones.

In Haines, state geologists have been able to use preliminary LiDAR collected in December to compare with 2014 LiDAR images of the area collected by the Army Corps of Engineers.

The comparison between the 2020 and 2014 LiDAR allows geologists to measure where slide material came from, where it ended up and in what quantity it moved, which has a variety of practical applications, Cornejo said.

"The more that is known about the current slide, the more one is able to assess the potential of slide growth or new slides in the area," Cornejo said. She said understanding how the slide moved and where material ended up will also inform search and rescue plans once the effort resumes.

In the immediate aftermath of the slide, preliminary LiDAR data was used to inform evacuation decisions.

"LiDAR helps identify some smaller slides," especially those covered by vegetation, Cornejo said during the Jan. 14 town hall. She said a small slide identified above Mount Riley Road helped influence the decision to evacuate the Portage Cove area.

In an email Tuesday, Cornejo said a final, more detailed LiDAR product for the data collected in Haines in December is still a work in progress. Once finalized, it will be available to the public at https://elevation.alaska.gov/.

Jones said the hope is to collect more LiDAR data in Haines, along with other Southeast communities, in late spring or early summer if funding becomes available.

 
 

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