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Earthquake considered more likely than in previous version of emergency mitigation plan

With forest fires rampant in California, hurricanes frequent in the southern United States and severe winter storms an annual occurrence in the Northeast, much of the most recent notable weather events in the United States have taken place far away from the City of Wilsonville's purview. SPOKESMAN FILE PHOTO - The draft plan states that Wilsonville has a moderate probabillity and vulnerability to a flood.

But, as the 2017 Columbia Gorge fire indicates, risks still exist across the Northwest and in Wilsonville. In turn, the City is in the process of updating its addendum to the Clackamas County Natural Hazard Mitigation Plan and recently released a draft of the plan. The plan, which is effective for five years once adopted and will be the third iteration, considers the city's risks, current infrastructure and potential future improvements.

One major tweak from the previous plan is that the risk for a Cascadia Subduction Zone earthquake was upgraded from a low probability to a moderate probability. Based on an Oregon Department of Geology and Mineral Industries regional impact analysis cited in the draft plan, there would be between 199 and 315 casualties and over $4.4 billion in damages in Wilsonville if a subduction zone earthquake were to take place.

However, according to Public Works Operation Manager Martin Montalvo, Wilsonville's buildings are better prepared for an earthquake than many other cities in the region because its infrastructure was developed more recently. Also, due to upgrades, the Wilsonville City Hall building and Willamette River Water Treatment Plant could be occupied as an emergency shelter in the event of a large earthquake and the City also plans to seismically retrofit the public works and police building.

"The age of infrastructure plays into your vulnerability. A lot of city facilities have had seismic improvements incorporated into the design and material type that we've chosen in the last 30 years versus what other communities have; that can play into the resiliency of the community," Montalvo said. "We're a little bit more resilient as a result of having newer infrastructure."

However, Montalvo added: "The scale of the event can negate a lot of that."

Montalvo also said Wilsonville's topography is an advantage over more hilly communities like Lake Oswego and West Linn but it also has greater risk for liquefaction (the process of a substance turning into a liquid) than areas located away from rivers that have more basalt soil.

Additionally, Inza R. Wood Middle School is noted as having a high collapse potential (over 10 percent likelihood) in case of an earthquake and Wilsonville's Clackamas Community College campus has a very high collapse potential (100 percent likelihood).

Montalvo said the plan helps identify which buildings need prioritization for seismic improvements.

"Once the plan is adopted by the state and accepted it allows everything outlined as vulnerabilities to be eligible for federal grants to address natural hazards that have been identified," he said.

The analysis also determined that the City has a moderate probability and a moderate vulnerability during flooding episodes of the Willamette River. During the worst flood in Wilsonville history, in 1996, three homes flooded and Wood Middle School also experienced flooding.

"In the past flooding has occurred along the Willamette River, in Coffee Creek Wetlands, and at choke points that can back up during heavy precipitation events," the draft plan reads.

To mitigate the effects from floods, the City has developed codes for flood-prone areas and regularly inspects stormwater facilities, among other measures.

"One of the mitigation strategies is to minimize development within those areas to lessen the impacts of the flooding hazard," Montalvo said.

Also, the plan gives the city a high probability and a low vulnerability to a drought, a high probability and moderate vulnerability to extreme heat and a moderate probability and moderate vulnerability to a wildfire.

"The City of Wilsonville has not experienced any life-threatening consequences from the few historical extreme heat events, although changes in climate indicate that the area should expect to see more extreme heat events," the draft plan reads.

The draft plan states that most fires in Wilsonville have been ignited near railroads and I-5, most of which have been contained quickly. To prevent fire hazards, the City prevents density in some forested areas and requires property owners of open lots to cut grass or the City will do it.

"The probability for a forest fire is greater than it had been in previous iterations," Montalvo said, citing better data and the increasing salience of climate change.

The plan also calls for the City to construct the French Prairie Bridge with emergency vehicle access to provide better emergency response across the Willamette River and develop public education programming to inform citizens about mitigation tactics.

To review the draft plan, visit the City's website at

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