Regional Hazard Mitigation Plan

The hazard

There is a cumulative 20% chance of a catastrophic Cascadia Subduction Zone or Seattle Fault earthquake affecting our region in the next 50 years. An estimated 75% of bridges within King County would be inoperable for at least one to three months immediately following a catastrophic earthquake.

• A major Cascadia Subduction Zone or Seattle Fault event would cause tens of billions of dollars in damages, hundreds of fatalities, and destroy regional transportation, communications, and utility networks.
• Current building standards distinguish between life safety codes and immediate occupancy codes. Life safety implies that the building will not collapse but may not be habitable. Immediate occupancy facilities are designed for use immediately after an incident. Most buildings are constructed to life safety standards.
• A major risk in King County is from unreinforced masonry structures exemplified by the historic, brick-façade buildings in Pioneer Square. Older buildings are generally most at risk, having been built to lower standards.
• Smaller, deep earthquakes occur approximately every 50 years, most recently in 2001 with the Nisqually Earthquake, and massive subduction zone earthquakes occur every 300-500 years, most recently in 1700.
• Areas with saturated soils, land made by filling in a waterbody, and soils of certain classes can be highly susceptible to liquefaction. This includes low-lying areas like river valleys and the land on which Pioneer Square and the Port of Seattle are built.

More information on geologic risk in Washington

Effective mitigation

• Preventing or reducing earthquake damage requires sustained investment in resilient buildings and infrastructure over decades. Appropriate building standards supplemented by a retrofit program for existing structures is the most cost-effective way to reduce risk.
• Structural retrofits to buildings and infrastructure to higher seismic standards can save lives and property and dramatically speed post-disaster recovery.
• Warning systems, such as Earthquake Early Warning, can reduce damage and save lives by providing seconds to drop, cover, and hold on and for water or power systems to temporarily shut down.
• King County, Washington State, and other local governments invest in seismic retrofits to public facilities, including buildings, bridges, roads, utility systems, and other essential services.

The hazard

• Floods can be caused by rapid snow melt or major rain events. Most of the funding for risk reduction projects in Washington is for flood hazards.
• The rivers in the south and east of King County are the highest risk areas, such as along the Green, the Cedar, and the Snoqualmie Rivers. Communities in these areas can sometimes be cut off for days by floodwaters.
• The last time floodplain maps were adopted was in the 1980s. A new set of floodplain maps are in review now.
• Climate change is expected to increase the frequency and severity of precipitation events in our region, which may change the floodplain even more and result in many more disasters.

Effective mitigation

• Property elevations and acquisitions can remove homes from the floodplain.
• Drainage projects reduce the risk to flooding caused by runoff.
• Major ecosystem restorations can have benefits of reducing flood risk and creating new open space that is beneficial to salmon and other wildlife. Levee setbacks are one of the most common projects that do this.
• Removing impediments to flood waters, such as undersized culverts, can lower upstream floodplains, while also improving fish passage.
• Washington State has some of the most effective floodplain regulations in the country, and King County is one of only 7 communities, and one of only three counties – all in Washington, receiving a “Class 2” rating from the Community Rating System in recognition of flood hazard mitigation and strong development regulations.

The hazard

• Landslides are ubiquitous to Washington and can occur in any steep slope area.
• Extreme winter events can produce hundreds to thousands of landslides in Western Washington in a single series of events.
• King County and WA DNR recently completed an updated landslide inventory for the county.

More information on geologic risk in Washington

Effective mitigation

• Slope stabilization measures, such as retaining walls or drainage improvements can reduce the risk of slope failure.
• Homes can be built on pylons or foundations to reduce susceptibility to slope failure and pressure on the slope from the weight of the house.
• The Critical Areas Ordinance requires any new construction in landslide-prone areas to build-in mitigation measures.

The hazard

• Known tsunami risk is relatively limited in King County, concentrated along Puget Sound, especially around the Port of Seattle.
• In Washington as a whole, this is potentially the deadliest hazard.
• More information on geologic risk in Washington.

More information on geologic risk in Washington

Effective mitigation

• Tsunami “safe havens” – buildings designed to withstand tsunami waves as evacuation structures
• Warning networks, including sirens, text alerts, and public information campaigns

The hazard

• Lahars from an eruption to Mount Rainier would threaten south King County, around the communities of Auburn, Algona, Pacific, and Enumclaw.
• A major eruption and lahar would also damage regional transportation networks.
• Ash would cause damage around the region and trigger significant health impacts.
• The 1980 Mount St. Helens eruption is one of the costliest disasters in Washington State history in lives and property.

More information on geologic risk in Washington

Effective mitigation

The only effective mitigation to lahar impacts is to avoid risk through development standards and to prepare through information campaigns and the establishment of warning systems and evacuation routes.

The hazard

• Severe weather, along with flood, is the most common disaster in our region and is the source of most federal disaster declarations.
• The largest sources of damage are utilities and transportation infrastructure.

Effective mitigation

• Measures that protect utility infrastructure are the most common severe weather mitigation strategies in King County.
  • Examples include underground power lines and installing backup generators at critical facilities.

The hazard

• Wildland fire risk is changing in King County due to climate change – the “return period” for fire will be much lower and fires will be more frequent.
• Development in the wildland urban interface – areas in which structures are adjacent or among heavy vegetation – is creating new risk in the county.
• Building codes are still catching up as communities work to assess their risk.
• Wildfire risk is most concentrated in the south and west of the county, where there are newer communities and more vegetated areas.
• After a fire the risk continues in the form of dangerous flash floods and mudslides as rain on recently-burned “hydrophobic” soils runs into area waterways and down steep slopes.

More information on living with wildfire

Effective mitigation

• Building codes and development standards to require use of inflammable construction materials and more space in between homes can protect structures and prevent the spread of fire.
• Neighborhoods designed for easy fire truck ingress and egress, by multiple routes are necessary to fight the fire and avoid tragedies like the Oakland Hills Fire in California.
• Defensible space – clearing 30 – 100 feet around a home or neighborhood is the best course of action for existing communities.
• Forest thinning and other forest-health measures can prevent some ignitions and help keep fires smaller.