The Mount St. Helens volcanic eruption 37 years ago could endanger as many as 50,000 residents of Washington state if new infrastructure isn't built to contain catastrophic flooding, a new study released Friday by the National Academies of Sciences said.
The study was commissioned by the U.S. Forest Service to examine the risks associated with the hundreds of tons of volcanic ash that dammed nearby Lake Spirit, creating a risk of massive flooding.
The study says 50,000 lives would be in danger if the volcanic debris erodes and floods the nearby area. It recommends creating an updated plan to contend with the risks and examine options for upgrading, or adding, new infrastructure to avert a disaster.
"The 1980 eruption of Mount St. Helens radically changed the landscape surrounding the volcano in southwest Washington state," the report's summary read. "The eruption sent an avalanche of debris into the North Fork of the Toutle River and blocked the drainage of Spirit Lake, causing a dangerous rise of lake waters. Should the debris blockage — which is functioning as a dam — fail, 50,000 people could be put at risk of catastrophic flooding and mud flows."
The eruption is often called the most powerful in U.S. history and was the first in the continental U.S. since 1915. The eruption was so intense it caused the entire north face of the mountain to fall away, resulting in massive mudslides as an 80,000-foot plume deposited ash across 11 states. The eruption accounted for the deaths of about 60 people.
The region is prone to chronic flooding "which is exacerbated by heavy sediment loads coming off the mountain," the study said.
More alarming is that the area could experience more earthquakes in the future. "Recent insights about the likelihood of a Cascadia Seismic Zone earthquake affecting the Mount St. Helens vicinity warrant greater examination," the study said.
The study recommends that the federal government assist in updating outdated data from 30 years ago on the risks, while simultaneously looking for ways to rebuild the post-eruption flood-prevention infrastructure.
"In the 1980s, engineering measures were implemented to manage catastrophic and chronic flooding risks — including a tunnel to drain and help manage water levels in Spirit Lake, and a sediment retention structure to prevent sediment from flowing downstream," the summary said. "The tunnel now requires major repairs, and the sediment retention structure is nearing its capacity."
It suggested that a concrete spillway, similar to those built for manmade dams to prevent flooding, may need to be constructed.
"It is likely that the first attempt to apply the decision framework will be related to decisions regarding management of water levels in Spirit Lake," the summary said. "Options to consider could include, for example, constructing a dry spillway as a backup outlet, or installing a second modern drainage tunnel that would provide redundancy and flexibility. The viability of these and other options is best quantified through an analytic deliberative process as outlined in the report."
The study's authors are planning to hold a public hearing early next year in Washington state to discuss the findings and go over the study's recommendations.