The Gibralter Road Landslide
This run-of-the-mill photo tells a story. It is a view of Gibralter Road which runs along the south shore of Fidalgo Island. A story is revealed by the left-tilting power pole, the meandering guard rail, the uneven road surface and possibly the tilting right-turn road sign. The next power pole down the road tilts to the right which becomes more apparent further around the bend.
On December 30, 1990, I was rousted out of bed at 03:30 in the morning. I found five members of the Mt. Erie Fire Department on my porch. They explained that a landslide was underway and that it was urgent that I leave the house immediately. I was instructed not to waste any time packing or preparing. Just get dressed and get out. Wow!
Like many, I presume, I thought a landslide was something that suddenly went ka-woosh. Luckily, this one didn't. Instead, it was a very slow and gradual movement of a piece of the earth that lasted several weeks. It was more of a creep than a plummet. The hump in the guard rail near the foreground marks the location of a crack that opened up right across the pavement. The asphalt separated about 8 inches (20 cm) and the far side was displaced up more than a foot (30 cm). I'm not sure if it was raised up or if the near side dropped. Maybe both happened. When I aimed a flashlight down the crack, I could not see the bottom. The narrow chasm seemed to go down forever. From here, the area of the slide extended about 1,500 feet (460 m) down the road to the southwest. On the far end, the slump block dropped about 6 feet (2 m).
The Washington Department of Ecology has mapped slide risk areas along South Fidalgo Island. The arrow locates the 1990 event. As a reference, Deception Pass is in the lower left corner.
There are more pieces of the story further down the road. In spots, a stair-step pattern of terraces is revealed in the profile of the hillside facing Skagit Bay. Gibralter Drive which climbs to the top of the bluff, appears to cut through one of these terraces.
Downslope from Gibralter Road, the Salmon Beach community appears to sit on another hillside terrace. These level areas along the bluffs bordering Puget Sound provide stunning views and have become highly coveted home sites.
This diagram from Be Safe Net illustrates the anatomy of the 1990 Gibralter Road landslide. It also shows how the hillside profile has formed over time. This was a slump or rotational landslide. Rather than a piece of the hill moving straight down, a circular section rotates. A terrace is left at the top and the toe of the slide may rise up out of the ground. In multiple or successive slide events, a step pattern forms along the bluff. It was a real eye-opener when geologists visiting our neighborhood pointed this out to us. Because they provided fairly level surfaces, both Gibralter and Salmon Beach roads probably run along such terraces.
Excessive rainfall and slope saturation are primary triggers for landslides. During November, 1990 we received a record 10.67 inches (27 cm) of rain. This is about half our normal rainfall for an entire year. Water saturated ground was likely the primary cause leading to the late December slide.
The concept of "earth tides" was also introduced as a possible precipitating factor. At the time of the event, we were experiencing an alignment of sun, moon and earth that was producing record low tides. Some geologists speculated that a gravitational pull on the land itself helped initiate the slump of the saturated hillside. Other experts pooh-poohed the idea, indicating that the force was too weak to make a difference. Whether true or not, the idea of this theory is interesting.
The most dramatic evidence of South Fidalgo landslide activity may be seen from the beach. The point in the photo marks the far southwestern end of the event where the slump dropped the most. Beyond the point is Dewey Beach which curves around to Yokeko Point and Deception Pass. There is a tilt in the surface of the point where the back side dropped about two meters. Also, the trees show a noticeable lean to the right which is further evidence of a change in grade. Over time, the trees will attempt to straighten themselves as they grow. Their trunks will become curved in the process.
The change in the tilt of the top of the point did not all occur in 1990. This has been an ongoing process over decades, perhaps centuries. This photo of the face of the point shows there is also a tilt to the right and reveals the nature of the soil along this part of Fidalgo. If I have done my homework, this is an example of glacial drift. The material is called Vashon Till which is compacted glacial sediment made of sand, rocks and clay. Some boulders can be seen eroding out of the cliff face. A thin layer of what passes for topsoil is visible at the top. Around much of South Fidalgo Island, this is the material in which many of us struggle to garden. It defies all efforts at soil improvement with organic material. This Vashon Glaciation material was deposited here between 15,000 and 18,000 years ago.
At the time of the landslide, other signs of the event could be seen. Sections of the bluff peeled off and fell into the bay, concrete bulkheads heaved and cracked and entire trees were dropped onto the beach. Over the past 20+ years, much of this evidence has been covered by regrowth of vegetation or cleared away by the tides. The large rock on the beach may be a glacial erratic either eroded out of the bluff or dropped near the spot by a melting glacier.
Trees can also provide evidence of past landslide activity. Back home in my own yard, the trunks of many of the big Douglas and Grand Firs are curved. This is not a normal growth habit. When the tree was young, the slope of the ground changed and the tree tilted. As the tree grew over the following decades, it gradually straightened itself leaving a curve in the trunk. On a slope, the trunk will typically curve into the hillside as the slump mass rotates. In my yard, some adjacent trees actually curve in different directions. This indicates that the ground tilted back and forth over the years. The mature trees are now nice and straight above the curved sections indicating the ground has stabilized for the moment.
When I built the house, I was required to include a geologist's report in the application for a building permit. The report indicated that unlike areas to the east and west, this property had stabilized. As predicted, my home was outside the 1990 slide area and the house was unaffected by it.
In the end, only one house in the slide zone was red-tagged and needed to be abandoned. It was literally split in half by the fault line of the slide. At another spot, a swimming pool cracked and drained. One family left ahead of the evacuation when they started hearing wall board nails popping out of the house's frame. Some of my neighbors reported that they had doors that were jammed shut because the frame had shifted. Others spoke of how their basement ceilings now had a wavy shape. It was quite an adventure for all of us.
One of the legacies of the 1990 landslide is several hundred feet of riprap seawall along the shoreline. Approvals by no less than ten agencies were required to build it. Shoreline erosion is a natural process that nourishes the beach (.pdf). It helps create habitat for fish and other marine life. The process should be disturbed as little as possible, and this kind of shoreline hardening is normally discouraged. But the slide initiated some alarming and rapid erosion along the beach front. To protect our homes, we were advised that a loose rock riprap seawall was a preferable (.pdf) alternative to concrete or other solid bulkheads. Riprap will absorb more wave action and allow beach material to accumulate. Upland sediments can continue to be delivered to the beach. It also has a more natural look that blends visually with the environment.
Soft armoring methods and site drainage improvements were also employed. The bluff was allowed to re-vegetate naturally with native plants. The accumulation of driftwood is welcomed by homeowners. These efforts appear to have stabilized the shoreline while preserving the character of the beach. The county made improvements in surface water management in the slide area. Open ditches were replaced with closed culverts. This provided controlled removal of storm water runoff rather than allowing it to soak into the ground.
If nothing else, the 1990 Gibralter Road Landslide was a learning experience. We became more aware of the ground under our feet and the processes that affect it. We were taught a little about our local geology and how shorelines work. We discovered the best ways we humans can fit comfortably, safely and naturally into this environment. Individual homeowners have become diligent Shore Stewards and remain mindful and informed about activities that can affect bluff stability. In this regard, the Guide for Shoreline Living (.pdf) is a handy booklet for homeowners.
Some persistent and heavy rainfall over the past month reminded me of this event. During rainy periods around here, we don't look to the skies any more. We look to the ground.
If you are like me and curious about these subjects, I can recommend a couple of good blogs to follow. They were also part of the inspiration for this post:
Reading the Washington Landscape
Weather Statistics for March, 2012
|Temperature||High 58.4° F||Low 29.1° F||Mean 42.5° F|
|Wind||High 33 mph||Average 3.3 mph||Dom Dir S|
Observed at South Fidalgo Island (See Climate page for complete climatological data)