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

Racehorse Creek Landslide, Whatcom County

On January 6th, 2009, a Pineapple Express (actually to have formed off of Hawai’i) flowed into Washington State, hitting the northern counties first and moving into the southern counties. Whatcom and Skagit Counties were first to report landslides late on January 6th as warm rains melted away snow and thawed the ground. As the rain continued, a small rain on snow type event occurred, spawning over 1,500 landslides. Debris flows and debris avalanches were the most common landslide to have formed from the storm event and the majority of the landslides occurred on the flanks of the Cascade Mountain Range.

January 2009 storm map with an incomplete landslide inventory

January 2009 storm map with an incomplete landslide inventory

This map shows the storm intensity overlain with landslide initiation points, primarily from DNR/DGER aerial surveys after the storm and reported landslides from public and private folks.

One of the largest landslides during the January 7-9th storm occurred along Racehorse Creek in Whatcom County.

Racehorse Creek Landslide; DNR/DGER Photo

Racehorse Creek Landslide; DNR/DGER Photo

The landslide occurred in two major components, the main debris avalanche and near lateral debris flows. The main debris avalanche is over 160,000 square yards and moved a significant amount of trees into Racehorse Creek. The debris flows scoured into the ground, removing timber in its way, also reaching Racehorse Creek. Once in the swollen waters of Racehorse Creek, the moved debris moved downstream, forming a massive logjam.

Logjam formed by the Racehorse Landslide, looking towards Kendall Creek; DNR/DGER Photo

Logjam formed by the Racehorse Landslide, looking towards Kendall Creek; DNR/DGER Photo

Logjam formed by the Racehorse Landslide, looking towards Racehorse Creek; DNR/DGER Photo

Logjam formed by the Racehorse Landslide, looking towards Racehorse Creek; DNR/DGER Photo

The size of the landslide has caused many to scratch their heads as to what possibly might have triggered this landslide. Some point to an earthquake as a possible trigger (one did occur on January 6th, 2009), others, natural factors of erosion and saturation. Or, as is common, a combination of saturated ground, erosion of the toe and a bit of shaking from an earthquake.

Whatcom County Earthquake Map; DNR/DGER

Whatcom County Earthquake Map; DNR/DGER

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Hyak Landslide

June 3, 2009

One of the more talked about landslides from the January 7-8th storm was the landslide that occurred at Hyak. The landslide started at the Summit at Snoqualmie ski area and moved into the Hyak community.
This landslide got a lot of press and originally, it was thought that is might be an avalanche. I remember having a discussion about this at a NOAA/NWS video conference meeting. Although, looking at the photos, it seemed that instead of the snow scraping up the soil beneath it, the slope gave way and moved the snow along. Something like a debris/snow avalanche.

Hyak Landslide Location Map

Hyak Landslide Location Map

The landslide occurred at approximately 11:40 a.m. Wednesday, January 7, 2009. Heavy rains (probably at about its elevation limit before turning into snow) from the storm had reached the summit earlier, warming the hillside and inundating it with rain.

Here at DNR – Division of Geology and Earth Resources (Washington Geological Survey), we were in emergency mode. We mobilized all of our geologist and sent them into the field to document landslides, but more importantly, sent them to check on residences that were impacted from landslides and to make sure they were safe from future landslide movement. Unfortunately, I was in the office, directing geologists to specific areas and creating updated maps of where we had located landslides or had damaged houses or blocked roads. I sent one of our geologists on the east side to get to Hyak and investigate what had happened and determine if it was an landslide or a snow avalanche. Plus, it did damage a handful of houses and the hillside had the potential to continue moving.

Aerial photo of the Hyak Landslide - DGER Photo

Aerial photo fo the Hyak Landslide - DGER Photo

Oblique aerial view of the Hyak Landslide - DGER Photo

Oblique aerial view of the Hyak Landslide - DGER Photo

Our geologist arrived I think late on January 7th and found numerous other crews investigating the landslide. Talking to them and doing some investigation herself, it was determined that it was most certainly a landslide and further, the slope was completely saturated. The scarp and material had woody debris within it and within the scarp, casts of old logs could clearly be seen, most with water gushing out the casts. It turns out that the slope had been modified about 40 or so years earlier and it appears they incorporated woody debris into the material. Over 40 years, the wood deteriorated and probably allowed water to more easily infiltrate into the subsurface, probably to the contact between the fill and rock/soil.

According to Matt Cowan, Fire Chief of the Snoqualmie Pass Fire and Rescue, the landslide impacted eight houses, one which was pushed off its foundation, the other lightly damaged. Two people were injured.

(Photo from Hyak Flickr site)

May 27 photo of Hyak Landslide

May 27 photo of Hyak Landslide

The hillside might still pose a threat to future failures. If woody debris exists in the subsurface then continued weakness still exists. I am not sure if the ski resort is planning on regrading the hillside to make the slope usable to skiing, although I cannot imagine that they will abandon the ski area. Unfortunately, we have inherited a lot of legacy problems from early land-use modifications (from the early 1900’s to 1970’s) that still plague us today. They are rarely recognized as a hazard, but their results can be deadly.

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.

We tried getting into the Pilchuck headwaters yesterday to no avail. Roads were washed out and gates were locked. We did see a number of landslides on the Sultan Basin Road heading up to Olney Pass. These landslides bare the mark of a large rainstorm event and almost certainly moved during the January 7-8th, 2009 storm event

Sultan Basin Road Landslides

Sultan Basin Road Landslides

The picture has a backdrop of 2003(? I think it was later than that) Snohomish County LiDAR.

The most interesting of these landslides is a debris avalanche at the bridge crossing at Olney Creek. It was probably dealt a one-two punch, the swollen Olney Creek was probably eating away at the bank (and probably has been for years) and the saturated ground allowed enough driving forces to overcome the resistive forces. It also moved a good amount of timber into the creek, which might cause a problem down the road by creating a debris dam behind the bridge.

Sultan Basin Rd Debris Avalanche.  Photo by Carol Serdar

Sultan Basin Rd Debris Avalanche. Photo by Carol Serdar