Landslides are a risk to be taken seriously on Mercer Island, where many homes are built on or near slopes. In a typical year, Mercer Island will see between six and 15 landslides with resulting damage estimated to cost from a few thousand dollars for smaller slides to upwards of several hundred thousand dollars for those that are larger. The city cautions Islanders to be aware of landslide risks and take preventative measures.
The landslide “season” is from late winter to early spring, with January being the peak month for landslide activity. They are typically triggered by excess water and most include a contributing human factor such as roof drains discharging onto slopes, excavating near high risk areas, or broken pipes.
According to city building official Don Cole, recent weather events have potentially increased the risk on Mercer Island. Melting snow accompanied by significant rainfall and a freeze combine to dramatically increase water retention in the soil. Increased water in the soil adds weight and stresses slopes, increasing the chance of a “debris flow,” or mudslide.
The city monitors rainfall to help forecast landslides via the United States Geological Survey (USGS) website. In addition, the city completed a project in 2009 to map potential landslide hazard areas on the Island, which is available for viewing on the city website.
The city recommends that landowners become familiar with slopes on their property and those on adjacent properties and consider hiring a qualified geotechnical engineers to do a thorough site assessment if any risk is suspected. The typical cost of assessments ranges from $300 to $500. The following are examples of landslide prevention recommendations found in a geotechnical assessment:
• Direct drainage away from slopes, loose soils and non-vegetated surfaces.
• Monitor surface water that drains onto your property and look for trickles of flowing mud or areas of deposited silt, sand or mud.
• Look for signs of land movement such as cracks in a slope, progressively tilting trees, small landslides or debris flows.
• Avoid disturbing any slope by not cutting across it or digging into its bottom.
• Preserve natural vegetation and keep yard waste, or fill off of slopes.
• Check features such as rock walls and rockeries to ensure they have proper drainage.
• Perform periodic property inspections before winter and when safe during storm events.
• Be informed: standard homeowner insurance policies do not typically cover landslide damage.
Landslides happen extremely quickly and represent a risk to life and property. Building official Cole recommends that Islanders keep alert, consider consulting a geotechnical engineer, and never ignore landslide warning signs.
Questions regarding permit requirements, geologic hazard maps or related ordinances can be directed to Cole at don.cole@mercergov.org.
