The residents along Perkins Lane don’t have to be reminded that there is still a month left in Seattle’s landslide season.
Thankfully, 2012 has been a moderate year for snow, precipitation and storms. But residents in this area know they have to remain vigilante. After all, this roadway that cuts below Magnolia Bluff, remains the poster image for Seattle’s landslide problem.
After a week of snow and rain in late December of 1996, the saturated land above the southeastern tip of Magnolia’s Perkins Lane gave way in early January, 1997, beginning a series of mudslides that over a number of months would end in the total destruction of a group of homes located below.
However, Perkins Lane wasn’t alone. The wet winter of 1996-1997 proved that many other areas of town were equally fragile. Leading the list was West Seattle. There, mud and debris flowed down onto homes and condominiums along Alki Avenue Southwest, causing thousands, if not millions of dollars in damages. Also in danger are homes on Capitol Hill and Madrona. In fact, in many areas of Seattle where there is a steep hillside, there is the possibility of a landslide.
In the years since then, Seattle has learned plenty about landslides and spent tens of millions of dollars developing sophisticated drainage systems, walling off crumbling hillsides and reinforcing parks and streets. The result is a much safer environment for homeowners throughout the city.
But, still, there is no stopping Mother Nature when the circumstances are right. And despite a recent dry spell, experts predict the upcoming rain and landslide season to be no different than in years past.
“Traditionally, we usually start to see landslides kick in around December,” Bill Laprade, senior vice president of the geotechnical consulting firm Shannon and Wilson, said. “I like the way Seattle has framed the landslide season. They say it’s from Halloween to April Fools Day, which is a great way to remember it.”
Laprade said that November tends to be the rainiest month, which Seattleites saw firsthand during the storms that occurred around Thanksgiving weekend.
“There were some pretty bad storms right before December, which led to a period of landslides,” he said. “But the ground dried out pretty quickly.”
Now, with the onset of late winter storms, groundwater levels may rise, particularly in areas prone to landslides.
Historically speaking, property in West Seattle, Queen Anne, Magnolia and Madrona faces the highest risk of landslides, though every incident depends on a number of different factors.
“All neighborhoods in Seattle that have steep slopes have the potential for slides,” Laurel Harrington, the drainage and dam safety manager for Seattle Public Utilities, said.
Many factors to landslides
In addition, blame a lot of Seattle’s troubles on its topsoil. Often, it is made up of sandy soil on top of clay, which is a good recipe for landslides. When the soil becomes saturated, the ground, whether on top or within the incline, can slide off the clay, break loose and flow down the slope.
But there are many other reasons why Seattle is prone to landslides.
“It’s very rare that a landslide is caused by one factor,” Laprade said. “It’s usually a number of contributing factors that go into it.”
Adding to the presence of steep slopes and soil, human influence also impacts the chance of a landslide.
A major concern for landslide experts remains the construction projects that occur throughout the rainy season. Whether roadside improvements or work on a new building, tearing up the ground and changing the land’s natural layout tends to mobilize sediment. And particularly during the wintertime, the moving of sediment–in addition to heavy rainfall–leads to shifting ground, and in many cases, a landslide.
“In some parts of the state, construction starts shutting down in the middle of October,” Laprade said. Yet given the number of construction projects active during the month of November, Seattle is not one of them.
On a smaller scale, even home improvement projects can impact the odds of landslide activity, both positively and negatively.
Removing trees to improve a view may loosen soil within the property. And if located on an at-risk hill–a slope with an incline greater than 40 degrees–severe storms can cause the loose soil to saturate, and subsequently slide away.
Explaining a rough rule of thumb used by geotechnicians for years, Harrington said that typically, possible landslides occur if it rains two inches in one day, or three inches over the course of two days.
“Essentially the ground needs to be saturated and there needs to be sufficient additional rain that can’t be absorbed,” she said.
Despite the inevitable nature of some landslides, city officials have spent millions of dollars reinforcing hillsides and developing drainage systems. Among the city’s success stories in the Queen Anne area include the shoring up of a number of hillsides. Across the street from the intersection of 11th Avenue West and Galer Street, on Queen Anne’s western slope, visitors can see one of the areas that has been rebuilt to protect against landslides.
Soldier pile retaining walls made of steel and concrete protect the steep hillside. Plastic piping in the hillside collects water in the soil and drains it off to an underground cistern. A similar system was built along at least one other hillside in the neighborhood.
In Kinnear Park, a few blocks south, Laprade showed how past landslides had impacted the park. One slide came flowing down the hillside and did considerable damage, including ending up in and around a daycare business at the bottom of the slope along Elliott Avenue West.
To prevent future landslides, construction crews created a 20-foot-deep trench to capture the water in the soil in Upper Kinnear Park. That water is then transported through pipes off the hillside and down to the bottom of the slope. He said much of the hillside had to be rebuilt with vegetation as protection.
Laprade, who has worked on more than 500 landslide sites in his career, said an important factor in deterring landslides is to keep the runoff off of the slopes. The Seattle Public Utilities have built drainage systems on a number of streets along the slopes of Queen Anne and Magnolia that run the water through piping to the bottom of the slope and reducing the chances of saturation.
Harrington said residents have a number of ways to protect their property against major damage.
“Homeowners can reduce erosion potential,” which Harrington said could lead to a landslide, “by keeping the slopes vegetated with deep rooted plants.”
Other methods include terracing property to decrease its slope, and installing soil reinforcements such as a retaining wall.
Seattle’s Department of Planning and Development also advises that irrigation systems should be turned off during the winter, especially during storms, and checked periodically to ensure proper functioning.
Yet the most important step residents should consider is to control and direct water accumulation on the ground. In doing so, homeowners can prevent massive amounts of runoff from sliding down a hill, which in turn decreases the risk of the slope falling away.
In the event of a landslide, particularly in which there is an immediate concern for safety, the city advises homeowners to first call 9-1-1, followed by contacting the Department of Development and Planning and Seattle Public Utilities.
The U.S. Geological Survey, the national organization regarding Earth science, also advises residents to watch for potential flooding, and to check building foundations in areas near a landslide zone.
Following a landslide, homeowners must also be prepared to foot the bill for ensuing damage. Most insurance policies do not include landslide repairs, and though city officials will inspect property damage, they only restore public lands, such as parks and sidewalks.
With this added cost in mind, experts advise residents in landslide-prone areas to examine their property, and take the necessary precautions.
The following are some landslide Do’s and Don’ts supplied by the Seattle’s Department of Planning and Development:
1. Maintain drainage system (pipes, ditches, etc., on your property and keep street drains free of leaves and debris).
2. Direct stormwater away from steep slopes, if possible.
3. Perform periodic inspections of property before winter and during storms keeping safety as the no.1 concern.
4. Check weep holes on walls and keep them open.
5. Be alert during and following storms.
6. If you have an irrigation system, shut it off and check it out seasonally.
7. Keep fill and yard waste off slopes.
8. Leave stumps in the ground on slopes.
9. Call a professional, if you have questions or a problem!
1. Don’t direct storm or other water onto a slope.
2. Don’t denude vegetation on slope without a re-vegetation plan.
3. Don’t cut into the toe (or bottom) of a slope.
4. Don’t remove tree stumps from slopes.
5. Don’t install a permanent irrigation system in landslide-prone areas.
6. Don’t put fill or yard debris on a steep slope.
Signs of a future landslide: Following a heavy rain, look for severely saturated soil or pools of water. Also check for newly eroded surfaces, the deposit of silt, sand or mud, as well as flows of water coming to and from the property.
If you are in immediate danger: Dial 9-1-1. To report landslide debris blocking a street, call SDOT Street Maintenance at (206) 386-1218. To report drainage complaints, call SPU Field Operations at 206-386-1800.
For more information onlandslides, visit www.seattle.gov/dpd/Emergency/Landslides/default.asp