Expert says drainages ran together, triggered slump
As immediate measures to prevent further erosion of the slumping, Mount Ripinsky hillside, the Haines Borough will extend two drainage pipes, redirect an uphill drainage eastward and pursue an impermeable liner for a roadside ditch, manager Mark Earnest said last week.
In the future, the borough may also encourage or require retention of trees on the hillside, Earnest said, following up on a presentation to the borough assembly by PND Engineers on Sept. 25.
Mike Hartley of PND attributed the slump to heavy snow and rainfall, redirection of surface water drainages in a way that concentrated runoff in one area, unstable soils and removal of trees in two areas. “A lot of water was put into that soil.”
Hartley told members the hillside has slid before and there is no foolproof fix.
“I don’t think there’s a clearcut way to completely resolve the potential movement of this slope. It’s going to move over time. It’s just a question of can you control water to minimize the risk to the community,” he said.
A horseshoe-shaped cleavage of the hillside in January cracked Lutak Road, broke a sewer line, forced the relocation of utilities, undermined a home and raised questions about the future of a new subdivision.
PND also called for a more thorough evaluation of drainage systems on hillside streets and roads.
Earnest told the assembly last week that mitigating the slumping would be “quite costly,” and that leaders may want to add it to the top of their capital priorities list to the Alaska Legislature. “It’s going to be beyond what the borough can afford.”
Hartley suggested the borough should immediately do a comprehensive review of drainages uphill toward Highland Estates. “If anything could be done uphill to redirect this flow out of this area, that might be a more economical fix” than remedies at the bottom of the hill, he said.
Hartley said a planned subdivision by Jan Van Dort of Juneau “needed to be rethought in terms of how it is being developed.”
“That’s already being done,” said assembly member Norm Smith.
Hartley said the hillside was comprised of marine silts and clays that have gained elevation through glacial uplift.
Large volumes of surface water saturate sands and gravels, descending in the soil to the clay layer. A water-softened clay layer reduces the shear strength of soil, causing it to move, including through mudflows, he said.
Earnest said he’d ask PND for a scope and fee proposal to survey drainages along the uphill roads, including creeks, culverts and discharge pipes from homes along Barnett Drive, Mathias Avenue, Oslund Drive, and Young Road.
“You dig drainage ditches and you’re now taking sheet flow, concentrating it and running it into a culvert, but now you’ve got a focused (flow),” Earnest said.
Oceanview Drive residents have directed water around their properties that was directed toward their properties by property owners on the hillside above them, Earnest said.
The borough also may consider a PND recommendation to build ditches down at the clay layer to collect and divert groundwater before it softens the clay. When it’s not saturated, the clay layer is stable, Earnest said.
Victoria Moore, whose house at the bottom of the hillside sustained considerable damage, said she wished property owners would have had more time with Hartley than just the audience before the assembly.
Hartley’s report didn’t address a water service line that was leaking when the ground shifted, she said. She was also critical that Hartley attributed the cause so heavily toward extreme snow and rain events.
“It’s like, ‘Okay, so we’re at the whims of God. Take you your chances. Sorry,’” she said.
She pointed out that record floods in November 2005 and snowfall in 2006-7 that set the previous town record for total accumulation didn’t affect her property.
She said she’s encouraged public officials will be looking at the bigger picture of drainage on the hillside. “We need to look at, ‘Is that hillside overdeveloped?’ Or if there’s going to be development, how do we ensure proper drainage?”