The Beach Road landslide area is “marginally stable” and still at risk of future slides, a team of geotechnical consultants concluded in an updated findings report published last week.
In the report, and in a public presentation on Monday, the consultants — from Oregon-based firm Landslide Technology — advised against building homes in the December 2020 landslide path and warned that sections of the hillside directly to the east and west of the path are still prone to sliding during significant rain events.
Out of 11 geologic hazards identified by the team, two stand out as having the greatest risk, the consultants said: an earthquake-triggered slide and a “global failure” of loose, weathered rock on both sides of the 2020 landslide area. That kind of rock, known as colluvium, is more likely to slide than the hillside’s bedrock, which the team said is stable.
They said for the hillside to liquefy and turn into a debris flow would take a “very unusual rainstorm” and a rise in groundwater of more than 10 feet. But some movement in the landslide area may occur “during moderate to large storms,” the report says.
The 2020 landslide occurred in the weakest part of the hillside, the consultants said, due to a variety of factors, including weathered bedrock, concentrated surface water, an accumulation of liquefiable fine-grained colluvium and high groundwater pressure from historic rainfall. The hillside initially failed at a point about midway up from the ocean, destabilizing the ground above and causing the slope, more than 2,000-feet long, to hurtle downward.
“Due to the high likelihood of hazard associated with the landslide area, we recommend avoiding construction of buildings within the active landslide limits, possibly including a buffer zone along the boundaries of the landslide or a hazard overlay where there is uncertainty whether the landslide may widen and whether debris flow paths/extent may run beyond the existing landslide impact area,” reads the report’s executive summary. “Limiting activities within the landslide area during periods of significant precipitation is also recommended.”
At Monday’s presentation, Landslide Technology senior engineer George Machan said he doesn’t think “complete stabilization is practically possible on this hillside” due to its complicated hydrology and the high expense of installing stabilization equipment. He said partial mitigation is possible by improving drainage mid-slope and constructing berms to deflect flowing debris.
While Machan advised against building homes in the 2020 landslide path, he said the residents who have property next to the path are in a difficult position. “It’s a community discussion to have. What’s the risk over there? Is it better to not build or to leave the homes there? It’s a difficult topic, and it’s not clean-cut,” Machan said.
Paul Graf, who lives down the road from the landslide “red zone,” asked Landslide Technology representatives about the risk level for residents near the end of the Beach Road.
Machan said the study focused only on the landslide path and the area adjacent to it. He said he couldn’t provide an answer without having analyzed the hillside above the rest of Beach Road but that similar geologic features are there and might suggest a similar level of risk.
Beach Road resident Michael Balise, who has expertise in geophysics (but not specifically landslides), said he thinks Landslide Technology “did a good job,” and he’s “in agreement with the majority of their conclusions.”
Balise noted that the firm was asked only to study the Beach Road landslide area but that slides, flows and rockfalls occurred all over Haines during the December 2020 storm event.
He said he’s concerned that labeling the area as at-risk could unfairly impact property values and business activity in the neighborhood.
“If we studied other areas (e.g. Mt. Ripinski, Lutak, up the highway) to the degree that Beach Road has been studied, we would no doubt find areas of equal or even greater geologic risk as compared to the risk at Beach Road,” Balise said. “It is not fair to the residents of Beach Road to apply any restrictions, warning signs, or anything else along these lines to the Beach Road area, unless we apply a similar level of scrutiny and similar restrictions to the rest of Haines.”
The consultants advised continuing to monitor for ground movement and groundwater pressure, to establish baseline conditions and improve understanding the effects of weather on the hillside.
“This may require retaining specialists to continue data acquisition, maintain the instrumentation, and interpret the results,” the report says. “A long-term goal would be to develop correlations between weather events, groundwater pressures, and ground movements. Threshold values to warn residents of potentially hazardous conditions may be determined once baselines and trends are established and used to evaluate the effects of weather and groundwater on slope stability.”
Landslide Technology will continue monitoring until its contract with the state expires in September. Machan said the firm is currently in “discussions about whether to continue our services or some further monitoring.”
A second town hall is scheduled for April 5. Borough staff requested that community members with questions email them to borough clerk Alekka Fullerton at [email protected] in advance of the meeting.