The town of Blenheim is planning to reroute Cole Hollow Road before part of the roadway plummets down a 100-foot ravine.
About a dozen homes were cut off from civilization in August 2011 when the typically peaceful Cole Brook — fueled by massive tropical storm rainfall — tore the road apart just west of Route 30.
That lower portion was repaired after the disaster, but officials two months later noticed a small crack in the road a half-mile up the hill had turned into a major fissure. Part of the roadway broke off in October, and now it sits as a landslide threat because it can’t be fixed without major cost.
Crews were able to fill the hole and get the narrow road passable temporarily, and town Highway Superintendent Gerald Felter is optimistic the roughly $400,000 project will be covered by state and federal disaster aid.
The landslide on Cole Hollow Road is but one of several in hard-hit Schoharie County that followed the soaking from tropical storms Irene and Lee.
Felter was helping a flooded victim move belongings out of their Route 30 home after the flood last September when a slope on the nearby mountain gave way, sending tons of debris cascading onto the state highway and tearing out power lines. That landslide — followed soon by two smaller ones nearby — is now visible from online satellite imagery.
The Schoharie County Soil and Water Conservation District is confronting three other high bank slope failures that took place along the Little Schoharie Creek, Line Creek and Platter Kill, stream program manager Peter M. Nichols said. Surveys and design work are under way for restoration projects along all three waterways as part of the federal Emergency Watershed Protection program.
There are two other “significant” high bank failures overlooking an unnamed tributary off Dave Brown Mountain Road in Blenheim, Nichols said.
He said it’s not a mystery or marvel. When the base of slopes get too wet, they give way.
“Large high bank failures such as this are common after a significant flood event due to fluvian entrainment at the toe of the high back slopes,” Nichols said.
He said entrainment occurs when soils that make up the base of a steep bank start to move and the slope collapses under its own weight after the base material washes away.
Nichols said slope failure will continue without aggressive stabilization methods, such as re-armoring the toe at the bottom of the slope.
That would be an extremely expensive option in the case of Cole Hollow Road, where Felter said engineers would have to design pilings or other reinforced structures at the bottom of the 100-foot slope and work upward from there. Instead, Felter said, the plan is to carve a new path into the mountainside — avoiding a historic cemetery and a private septic system — and create roughly 950 feet of new road.
Engineer Francois Vedier from Cobleskill-based Lamont Engineers said it’s impossible to tell how long Cole Hollow Road will hold until it slides away.
“It’s failing, no doubt about it. It’s moving,” he said.
He said post-flood landslides can be expected in some areas of the state where clay comprises part of the soil structure.
“A lot of the hills have clay layers that are trapped between other types of soils, and clay is a very slippery material when it gets wet. We see it all the time,” Vedier said.
He said the goal is to complete designs and environmental reviews in time to get the road re-routed during the next construction season.