Eugene Weekly : News : 2.14.08

Slippery Slopes
Developers want to build homes on a landslide

Her driveway is cracking, a hawthorn tree is tilting and unstable earth means she may have to put thousands of dollars of repairs into her garage. Lisa Warnes, who lives across the street from the proposed development of the Amazon Headwaters, says her own property shows signs of ground movement — a potential landslide indicator.

In order to build houses on the Green property portion of the disputed Amazon Headwaters, the developers will first have to find a way to deal with the landslides on the property. Warnes, who founded Vision for Intact Ecosystems & Watersheds (VIEW) to protect the Amazon Headwaters area, would rather see the fragile ecosystem protected.

Powell Valuation, the appraisal firm hired by the city to estimate the worth of the Green and Beverly properties, valued the Green property at approximately $3.8 million dollars. The appraisal, done at the city’s request as it ponders exercising eminent domain to acquire the sites, assumes that 108 houses could be built on the 39.9 acres. The owners have not been able to obtain a planned use development (PUD) permit from the city to develop either of the properties due to concerns about unstable slopes.

According to a LIDAR (Light Detection and Ranging) earth scan of the properties done by Sky Research of Ashland, the Green property has a large debris flow on it. A debris flow is a type of landslide: a river of rock, earth and other debris saturated with water.

Derek Cornforth, founder of Portland-based Landslide Technology, which specializes in complex landslide evaluations and remediations, says “prior landsliding” is an indicator of future landslides. He says in order to detect previous debris flows, LIDAR provides “very accurate mapping, even if there’s strong tree cover.”

The Federal Emergency Management Agency (FEMA) says debris flows can move “rapidly, striking with little or no warning at avalanche speeds. They also can travel several miles from their source, growing in size as they pick up trees, boulders, cars and other materials.”

Cornforth says, “It can be difficult to convince people to put upfront money” into preventing landslides. “Landslide work tends to be reactive rather than proactive.”

The Powell appraisal also made note of the landslides, relying on a November 2007 report by Foundation Engineering which said the land is developable but requires the construction of a “shear key” to prevent further landslides. A shear key is usually made of gravel and rock and extends down into the soil (20-25 feet in this case) to buttress the slide. It includes drains to keep water from building up behind the key.

The appraisal says that “actual costs for this shear key were not available.” Cornforth says the cost of a shear key “depends on the amount in cubic yards of excavation and the amount of more costly rockfill, which is the most common way to build a shear key.”

A shear key that was built to protect a road and house near the Clackamas River in Oregon City cost $900,000 to construct. According to a paper given at Geological Society of America conference by staff from Landslide Technology, “Costs for treating a large landslide can range from hundreds of thousands of dollars to many millions.” It estimated the cost to remediate an 800 foot wide and 60 foot deep slide at up to $4 million dollars in 2002. Warnes says the landslide on the Green property could be as wide as 300 feet, according the LIDAR images.

The appraisal also says that the developer’s proposal for the site, which would be called Green Valley Glen, “in the opinion of the engineers, does not properly mitigate the potential landslide activity in and outside the subject.”

The appraisal states that “development will be much more costly and risky” than “typical hillside development practices.” It “could activate dormant landslides.”

The Green site also contains a small area of wetlands, the appraisal says, which would be packed with fill dirt in order to be developed. While the appraisal of the Beverly property doesn’t note any landslides, it does say that the ownership of one of the parcels included in the development proposal, the 5.2 acre tax lot 200, is “unknown,” and quotes Steve Nystrom, Eugene’s head planner, as saying, it “should not have been included in their original development plan.” The appraisal says, “There is a possibility that Lane County or the city of Eugene own the land.”

In January the city of Eugene agreed to spend $100,000 for an option to buy the Green property and formed a committee to look into ways to purchase the land aside from eminent domain.

EDITOR’S NOTE: This is the second in an ongoing series of stories about urban, suburban and rural landslides in Oregon.