Keep an eye out for landslide risks after record autumn rains

This autumn has been one of the most consistently-rainy in western Washington records, meaning you should keep a sharp eye out for landslides.

Prolonged rain increases the chances of shallow landslides on hillsides and other steep slopes. According to the National Weather Service’s Seattle office, western Washington has seen more days of rain in October and November 2016 than ever.

Sunday 45th day w/rain in #Seattle in Oct&Nov ( 25 in Oct, 20 in Nov ), a new record. Last year Oct&Nov had 20+ rain days, 1950.#wawx pic.twitter.com/hHohfDvDqo

— NWS Seattle (@NWSSeattle) November 28, 2016

As that rainwater infiltrates the ground, soils can weaken and slide. Water adds significant weight to the slope as it seeps into the ground, becoming groundwater. Water lowers the strength of the material which can make it less able to withstand the force of gravity. Water also reduces friction, making it easier to move material downhill. These processes help to explain why landslides are much more common during the rainy season, and especially common during or right after large storms.

The geology of western Washington — steep slopes and soils — make this landslide country but with the right conditions, steep slopes in eastern Washington are vulnerable, too.

A landslide on the Whidbey Island bluffs in 1997.

Many of the Puget Sound bluffs were built by glacial till pushed upward by Ice Age glaciers, making their soils more porous and responsive to slides.

Warning signs of an impending landslide

If you live on or near a steep slope, here are some warning signs of potential slope instability:

• Cracks forming in your yard, driveway, sidewalk, foundation or in other structures.
• Trees on slopes, especially evergreens, start tilting.
• Doors and windows suddenly become more difficult to open or close.
• Water begins seeping from hillsides, even during dry weather.

If you see any of these early signs of a potential landslide, immediately contact your city or county.

DNR/NWS map shows daily landslide hazards

DNR and the National Weather Service have teamed up to show the people of Washington when and where landslide risks are elevated on our Shallow Landslide Hazard Map uses Weather Service precipitation data

This site provides daily hazard levels for precipitation-induced shallow landslides in Washington State. Hazard levels are calculated using a model based on antecedent and predicted rainfall. Hazard levels are graphically portrayed for counties, subdivided into National Weather Service forecast zones. The model is intended for use as a shallow landslide forecasting tool for use by city and county emergency managers.

The map is not predictive, nor does it forecast the potential for deep-seated landslides. If a landslide occurs, evacuate the area and contact your local emergency management agency.

Find out more about why landslides happen and why Washington is one of the most landslide-prone states in the country here. You can also find mapped landslides on our Geologic Information Portal and Geologic Hazard Maps page.

Increasing landslide knowledge

DNR is also working to better locate and map historic, deep-seated landslides, which are the top predictor of where future slides may occur.

In 2015, the Washington State Legislature mandated that the Department of Natural Resources Division of Geology and Earth Resources collect, analyze, and publicly distribute detailed information about our state’s geology using the best available technology – lidar.

Landslides in the Cedar River Valley south of North Bend are obscured by vegetation in aerial photos, at left. Lidar was used in the image on the right to strip away trees and show land forms and past slides in the valley.

Before lidar, geologists used aerial photographs, topographic maps, and field surveys to catalog landslides. This method is problematic in Washington because of the density of vegetation that often obscures features.

Landslides often have characteristic topography that is obvious once it’s visible. Slump blocks, hummocky topography, scarps, sag ponds can all identify a landslide. With lidar, a geologist can go over large tracts of land to find landslides quickly and more accurately than using aerial photographs alone.

DNR is now collecting, inventorying, processing, and storing existing lidar data. New airborne lidar collection is underway, starting with areas at higher risk for landslides that do not already have high-quality lidar data.

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