Landslide of the Week – Hornby Landslides

May 26, 2009

Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Hornby Landslide, Glenoma Area, Lewis County

This is one of the more fascinating landslides that occurred during the January 2009 storm. Numerous debris flows and avalanches dotted the slopes above Mark and Jon Hornby’s farm.

Map of landslide in Glenoma Area, Lewis County

Map of landslide in Glenoma Area, Lewis County

Hornby Landslides

Hornby Landslides

Series of Events

At about 9:00 am on January 8, a large debris flow moved into Mark Hornby’s farm pasture, plugging a culvert and covering it with mud and debris. About a half an hour later, another landslide came off the slope, nearly striking Mark and his brother, Jon. I think this landslide also struck a bull and carried it a ways across the pasture. There full story can be found here.

View of debris flow deposit near house; DGER/DNR Photo

View of debris flow deposit near house; DGER/DNR Photo

The debris avalanche/flow in the middle of the clear cut slope is very shallow, less than 2 feet of incision in many places. The landslide incised down to bedrock in most places, which was probably why the landslide was so shallow. When it reached the pasture, the landslide turned into a short debris flow and then transformed into a hyperconcentrated flow and made its way across the Hornby’s Farm pasture. The landslide ponded against Highway 12, flowing to the Hornby’s driveway and then onto Highway 12. One of the oddities discovered in Glenoma was that many of the hyperconcentrated flows that reached the valley floor were entrapped into roads by snow berms from plowed snow.

Hornby Landslides; DGER/DNR Photo

Hornby Landslides; DGER/DNR Photo

January 7-8 Storm Summary

In December of 2008 and into January of 2009, cold air from British Columbia created an ideal condition for snowfall across Washington State. Snow accumulations preceding the storm were low in the Puget Lowland, with at only inches on the ground in most places. On January 7, a stream of moisture originating from around Kauai (Hawaiian Islands) flowed into Washington State, bringing warm temperatures and high amounts of rain, rapidly melting what snow remained in the lowlands and eating away at the snowpacks in the mountains. By January 8, the largest evacuation in the state’s history was under way, forcing more than 30,000 people living in the Puyallup River area to flee. The town of Orting, with a population of more than 26,000, was almost completely flooded. For the second year in a row, flood waters closed Interstate 5 in Centralia/Chehalis. In the rest of the state, rivers were also flooding—the Stillaguamish, Snohomish, Chehalis, Naselle, Hoko, Cedar, and Cowlitz were the most significant, peaking above the 100-year flood level.

King and Snohomish County were least affected, as a rain shadow from the Olympic Mountains shielded their low-lying areas. In the Puget Lowland, rainfall totals ranged from 1.5 inches in Seattle to 5 to 7 inches in southwest Washington and 3 to 6 inches in the northwestern counties. As the storm progressed into the Cascades, the higher elevation forced the clouds to release water as they moved over the mountains, leaving more than 20 inches of rain in two days. The rainfall saturated slopes, many already wet from melting snow, triggering debris flows and debris avalanches throughout most of western Washington. Areas sensitive to high-intensity storms, such as Glenoma, Concrete, and Van Zandt, were the site of numerous large debris flows, blocking roads, limiting emergency response, and destroying homes. In the end, more than 1,500 landslides were reported or recorded from Washington Division of Geology and Earth Resources (DGER) field and aerial surveys.

Logging and Landslides

DGER and AEG hosted a field trip in the Glenoma Area (field trip guild). The purpose of this stop was to discuss logging and landslides. Unfortunately, the conversation never got going very well. It certainly caught my attention when I was looking over the photos coming in. One of the first things that caught my attention was the prominent deep-seated landslide on the west side of the clearcut. It is difficult to see if it is active from a photo, but when I first saw it from the aerial photo I thought it probably had some recent movement (within the last 100 years, maybe?). In the subsurface, there are places of thick, mostly unconsolidated pumice. So, is this logging related? It is possible. Was it illegal? Probably not. I didn’t do any detailed ground survey of this area, but just at a general glance, I cannot think of any forest practice rules that they might have broke. Maybe we need to look at if the FP rules are protecting our slopes, especially in Lewis County.

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