Fall Creek Dam is located on Fall Creek, a major tributary to the Middle Fork Willamette River, one mile upstream of Unity, Oregon and 25 miles upstream of Eugene and Springfield, Oregon. Fall Creek Dam is a 205-foot tall, 5,050-foot long rockfill earthen embankment dam with a concrete spillway, two spillway gates, and a regulating outlet. Fall Creek Dam is owned, operated, and maintained by the Portland District of the U.S. Army Corps of Engineers (USACE). Construction of the dam began in 1964 and was completed in 1965. Fall Creek Dam is part of a system of 13 multi-purpose dams in the Willamette Valley with the primary purpose of flood risk management and secondary purposes of recreation, irrigation, municipal and industrial water supply, fish and wildlife, conservation, water quality, and hydropower. Collectively, this system of dams is referred to as the Willamette Valley Project (WVP). Fall Creek Dam is one of four WVP dams that does not include a powerhouse.
During the winter months, the Willamette Valley Project reservoirs are maintained at their lowest elevations to allow for the temporary storage of rain and snow melt. When managing high flow events, the outflow from the system of dams is coordinated to reduce peak flows and river stages at downstream locations. In spring, USACE begins to fill the reservoirs, increasing the amount stored for conservation purposes and reducing the amount of storage available for flood risk management. During summer, stored water is used for recreation on the reservoirs, and some stored water is released in the river downstream to improve water quality, support fish and wildlife habitat, and provide water for
irrigation and municipal uses. During dry summer months, flows into the reservoirs are generally less than flows needed to meet minimum flow objectives, causing reservoir levels to drop. In fall, stored water remaining in the reservoir is drawn down to minimum levels in preparation for the flood season. Fall Creek Dam is located about 70 miles east of the Cascadia Subduction Zone, a megathrust fault along the Oregon Coast. The Cascadia Subduction Zone is capable of producing very large, long duration earthquakes. The last Cascadia Subduction Zone earthquake occurred in the year 1700.
High. USACE completed a routine risk assessment for the Fall Creek Dam in October 2014 that characterizes the risk associated with the dam to be High. USACE performs risk assessments as part of an ongoing dam safety program and to assist in the prioritization of investment for aging infrastructure. The risk assessments evaluate the life safety risks associated with the dams to determine if risk reduction actions are needed and, if so, what actions should be taken. The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely. The 2014 assessment concluded that the risk at Fall Creek Dam is driven by the high population downstream two possible, but very unlikely events: 1) an extreme earthquake occurring at the same time reservoir elevations are the highest or 2) an extreme flood event that fills the reservoir at a rate faster than the dam can pass water through the spillway.
An extreme earthquake could cause the rockfill earthen dam to settle and crack, resulting in water overtopping and flowing through the dam. It is difficult to predict the exact amount of settlement and cracking that could occur to the dam as a result of such an earthquake. The speed and depth of water flowing over and through the damaged dam could erode the soil and rock that forms the dam and cause significant flooding downstream. The strong shaking from an earthquake could also damage the spillway’s concrete structure and gates. The assessment also determined that fallen trees and woody debris floating in the reservoir could partially block the spillway gates during an extreme and unlikely rainfall event. The partial blockage of the spillway gates from the debris could reduce or prevent passage of flow through the dam, which would cause the reservoir to rise above and flow over the top of the embankment dam. As water flows over the top of the dam, the speed and depth of the water could erode the soil and rock that forms the dam. Because Fall Creek Dam is located upstream of Eugene and Springfield, Oregon, there is potential for devastating flooding to affect large downstream populations in urban areas and surrounding suburbs, as well as rural communities in the floodplain.
USACE is confident that the Willamette Valley dams are well-built, well-maintained, and will continue to significantly reduce flood risks for the region. However, the dams cannot eliminate potential for flooding. Even with the presence of the Willamette Valley dams, extreme rainfall and snowmelt events may result in flooding in areas downstream of dams. Flooding can be caused by high flows resulting from unregulated portions of the watershed and/or high flow that must be passed through the dam outlets and spillways when reservoir storage capacities are exceeded.
Risk Management Measures:
The likelihood is very low for an extreme earthquake or extreme flood resulting in a breach of the dam to occur, but the potential impacts of a dam failure are very high due to the large downstream population. Therefore, Fall Creek Dam will enter an advanced risk assessment called an Issues Evaluation Study (IES) starting in 2021 to further evaluate the dam’s performance during extreme seismic and flood events. The study will also determine whether short-term targeted measures (called Interim Risk Reduction Measures) or long-term modifications are necessary to reduce the risk. USACE regularly conducts routine inspections of its dams and Fall Creek Dam is equipped with instrumentation to monitor dam performance and seismic activity. Post-earthquake procedures are in place to inspect and evaluate earthquake damages and USACE conducts routine dam safety exercises with local Emergency Managers and first responders. Fall Creek Dam’s Emergency Action Plan (EAP) outlines actions to be taken during an emergency. USACE will update the EAP based on advanced assessment results and information from updated inundation maps. In addition, USACE will continue and increase its outreach to improve community awareness of flood risks and risks associated with the dam.