Landslide Season Starts with Heavy November Rainfall
City of Seattle Offers Two Free Landslide-awareness Seminars

For immediate release: 11/10/08
For more information, Contact: Alan Justad, 206-233-3891 or Bill Benzer, (206) 684-7845

SEATTLE — With last week’s combination of sustained rainfall and falling leaves blocking storm drains, the City of Seattle is encouraging residents to take preventive measures to protect themselves and their property from landslides.

Heavy rain can increase the chance of landslides due to soil saturation, which reduces slope stability. This year, the rainy season began in earnest on Oct. 31, and just over five inches of rain has fallen since then. Experts say the risk of local landslides has increased to the point that they are likely if Seattle gets further prolonged and heavy rainfall.

The risk of landslides occurring increases and diminishes over the winter based on the amount of rainfall and the duration of the storms.

Property owners on slopes and landslide-prone areas are advised to clear drains on their property and storm drains near their property. Property owners are also asked to take other simple measures, such as: checking downspouts, to make sure they are functioning and routed to a safe location; and inspecting sloping areas of their property for indications of slope movement (such as cracks in the ground, and newly tilting trees) and erosion.

If a landslide damages your property and you have an immediate concern for your safety, leave the premises and call 9-1-1. Seattle property owners with structures that may be affected by or endangered by a landslide should contact a Geotechnical professional for an evaluation.

To help residents understand how they can better manage their landslide-prone property, the City of Seattle has setup two free public landslide awareness meetings. The meeting presentation will include a discussion of the causes of landslides and proper drainage for sloping sites. This will be followed by a question and answer session and time for one-on-one discussions with professionals in the field.

The free landslide awareness meetings will be held at the Northgate Community Center Multipurpose Room on Nov. 22, and at the South Seattle Community College Judge Warren & Nobie Chan Education Center on Dec. 6. Both meetings will be held from 10 a.m. to noon. They will have the same format and information.

The Northgate Community Center is located at 10510 5th Ave. NE (across from the Northgate Mall). The South Seattle Community College is located 6000 16th Ave. SW.

The most common type of landslide in Seattle is the shallow colluvial slide, accounting for about 68 percent of local slides. Colluvium is the weathered soil and debris that accumulates on steep slopes. Its formation is part of the natural weathering process. As the colluvium on steep slopes becomes saturated with winter rains it’s weight can exceed its strength and slides occur.

The landslide record that was documented in the Seattle Landslide Study indicates that 84 percent of landslides that have occurred in the city have some human influence such as excavating and filling on steep slopes, broken pipes and uncontrolled storm water.

Since the devastating landslide season of 1996/1997 the city has taken steps to prepare to respond to slides and construct mitigation projects to reduce the potential for slides to occur. City field crews from Seattle Public Utilities, the Seattle Department of Transportation and Seattle Parks are ready to respond to flooding and landslide calls 24/7. Each city department has geotechnical engineers on staff to respond to slides and evaluate public safety.

The city also has an ongoing program to construct projects that either repair damage that has occurred or correct drainage issues that may contribute to slides. Since 1996/1997 the city has competed numerous projects.

Many of the same techniques that the city uses to reduce the chance of a slide occurring can be used by our citizens on a smaller scale. The best option is to avoid slides in the first place. People who live on steep slopes should check their drain systems and make sure that roof drains are routed away for steep slope or discharge in a safe location. It is also important to periodically inspect the steep slope portion for signs of slope movement or erosion such a as newly leaning trees and cracks opening up in the ground close to or on the slope.

For more information about the free landslide awareness meetings, please visit the Department of Planning and Development website.

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Seattle Public Utilities