Recorded on: May 19, 2017
Air Date: May 22, 2017
Residents of Western Oregon owe much to a series of federal flood control dams built within the last 75 years. The US Army Corps of Engineer’s Willamette Valley Project includes 13 dams that regulate 27% of the total flow of the Willamette River, much of it at the southern end of the valley. Each of the eight Army Corps operated dams in Lane County is estimated to have prevented hundreds of millions or billions of dollars in flood damage.
Completion of the youngest of the dams in the late 1960s marked the end of a devastating cycle of lowland flooding that inundated substantial portions of the valley’s largest cities. With the end of the periodic floods came a perception of safety and the opening of new land for development. The Ferry Street Bridge portion of Eugene became a center of residential and commercial development. In Springfield, the city government recently began projects intended to spark a similar level of development in the formerly flood-prone community of Glenwood.
Approximately forty-five years after completion of the first Willamette Project dam, and about twenty years after the final dam was completed, geologists made a profound discovery about the seismic character of the Pacific Northwest. Rather than being a relatively inactive component of the Ring of Fire, the region was now determined to be regularly subject to enormous earthquakes, albeit with a recurrence period of several centuries. Using geological markers, the last major event was discovered to have occurred in the year 1700, just a few generations before permanent European settlement and historical record keeping began.
Those who built the Willamette Project dams did not enjoy the benefits of this latter-day scientific knowledge, and design standards were in accord with a much lower threat level. Seismic design of dams and reservoirs typically includes accounting for the nature of soil underneath the dam; identifying the presence of land forms that could slide into the reservoir; evaluating the likelihood that ground shaking will generate waves in the reservoir-called seiche, and calculating the required strength of the dams’ structural features.
In Lane County, the Willamette Project dams are the embankment type, resilient structures made of rock fill or other material. This type of construction is thought to have advantages over solid structures in a seismic event. However, embankment dams are vulnerable to over topping, which can set in motion a process leading to release of the entire reservoir. Seismic shaking without over topping can also damage an embankment dam and render the facility serviceable until lengthy repairs are completed.
How do those who operate and finance the Willamette Project dams judge the acceptable risk of operating these important flood control facilities in light of geological knowledge that emerged decades after design and completion? When, if ever, is major reconstruction or complete replacement of a dam in order? Are the seismic upgrades completed to date sufficient? Do dams get weaker with age?
The probability of a mega-earthquake in the Cascadia Subduction Zone is quite high-over one-third in the next 50 years. An earthquake-caused catastrophic dam failure and inundation of populated areas has been described as an “extremely low probability, high consequence event”. How safe do we want to be?
Erik Peterson is Operations Project Manager, Willamette Valley for Portland District of the US Army Corps of Engineers.
Ray Weldon is Professor in the Department of Earth Sciences at the University of Oregon and a risk consultant for dam operators in California. He is a member of the National Hazard Mapping Team.
