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Ecology: Reduce landslide risk by avoiding slide-prone areas | Series
POULSBO — On May 2, the North Kitsap Herald reported that the U.S. Geological Survey had identified 180 areas at risk for landslide in Kitsap County, and what happened when development occurred in some of those areas.
On May 9, the Herald reported on a Poulsbo condominium project that is proposed on a slope with a history of recent slides.
Today, the Herald shares the state Department of Ecology’s recommendations on ways you can minimize your risk. According to Ecology, the simplest way to reduce landslide risk is to avoid slide-prone areas in the first place. Simple practices such as building a house farther from the edge of a steep slope, maintaining native vegetation above the slope, or inspecting drains annually are important.
This information is from www.ecy.wa.gov/programs/sea/landslides/prevent/prevent.html.
Landsliding is a significant hazard along Puget Sound shorelines. Many factors contribute to slides, including geology, gravity, weather, groundwater, wave action, and human actions. Typically, a landslide occurs when several of these factors converge …
Our own actions, most notably those that affect drainage or groundwater, can trigger landslides. Clearing of vegetation, poor drainage practices, and onsite septic systems can all add to the water affecting the bluff.
Get professional help
Consult a professional geotechnical expert for advice on the landslide and on corrective actions you can take. If a failed drainage system aggravated the slide, it is important to fix the drainage system before fixing the slide.
Invest in prevention
To reduce the impact of landslides in the future, invest in prevention. Revegetate the area to prevent surface erosion. Inspect and repair all drainage systems. Contact your local emergency management office for more recommendations for your area.
Hydroseeding may not be enough
Hydroseeding involves the rapid planting of grass to reduce surface erosion. On steep slopes prone to erosion and gullying, hydroseeding may be ineffective. In addition, hydroseeding may hinder reestablishment of native, more erosion-resistant plants.
Learn about the geology and the history of your property. Talk to local officials, your neighbors, or visit the local library. Review geologic or slope stability maps of your area.
Do get advice
Get advice from a qualified geologist or geological engineer before buying a potentially unstable site or building your home. Although waterfront lots can be attractive sites, they often have severe natural limitations. They may also be subject to strict environmental and safety regulations.
Do leave a safe setback
Build a prudent distance from the top or bottom of steep slopes. Avoid sites that are too small to allow a safe setback from the slope. Allow adequate room for drainfields and driveways. Local setback requirements should be viewed as absolute minimums. Resist the urge to trade safety for a view.
Do keep plants
Maintain existing vegetation, both above and on steep slopes. Trees, shrubs, and groundcovers help anchor soils and absorb excess water. Get expert advice identifying and removing weeds.
Do maintain drainage
Collect runoff from roofs and improved areas and convey water away from the steep slope or to the beach in a carefully designed pipe system. Regularly inspect and maintain drainage systems.
Don't irrigate or put drainfields on a bluff
Avoid placing septic system drainfields or irrigation systems between a home and the edge of a bluff, where excess water or leakage could exacerbate slope instability.
Don't dump on a slope
Do not place clearing debris, yard waste, or fill material on a steep slope. Even small accumulations of debris can become saturated and precipitate a larger slide.
Don't change natural drainage
Avoid modifications of the ground that disrupt or alter natural drainage, unless based on the recommendations of a qualified geologist or engineer.
Don't cut into the slope toe
Don't cut into a steep slope or excavate the toe of slope.
Don't overlook slide hazards
Do not be lulled into complacency by the lack of recent slides. Landslides typically only occur every few decades on a given site, and in some cases are even less frequent, but may remain a serious risk when heavy rains occur.
BUYER BEWARE: A CHECKLIST
Consider the following checklist as a starting point. Seek professional advice from a qualified geologist before building or buying a homesite.
— Is the site on solid rock or loose till?
— Does the bluff appear solid and covered with stable vegetation such as mature Douglas firs or madrones?
— Does the site show signs of sliding or erosion?
— Do you see bare tree roots, sliding debris, or other signs of sliding?
— Can you see springs or ground water seeping from the side of the bluff?
— Are there signs of active erosion such as gullies etched into the side of the bluff?
— Do neighbors have seawalls, bulkheads, revetments, groins, or riprap on their properties?
— Have you asked neighbors about erosion problems?
— Is there enough room on the site for driveways, septic drainfields, and a safe setback from the bluff or slope edge?