Structural crack in Oroville Dam cause for concern
Recently, the area around the Oroville Dam in northern California was evacuated due to safety concerns relative to a crack in the infrastructure. Those fears were eventually allayed only as flood waters receded. Two Speed School professors, Drs. Thomas Rockaway and Omid Ghasemi-Fare, both of the Civil and Environmental Engineering Department, offer insight into the desired functionality of dams, the potential risks in long-term maintenance, and the existence of comparable concerns in our own back yard.
How Dams Work
Large dams are complex structures and understanding their intended purpose and basic components is critical in understanding the risks associated with these systems. A dam may serve a combination of functions including flood control, power generation, agricultural supply, drinking water, and recreation.
When dams are built for flood control, the intent is for them to temporarily hold back the flood waters and then gradually release the impounded water over time, preventing downstream flooding. If too much water is received behind the dam, then to prevent overtopping, the water is diverted around the structure through primary and auxiliary spillways.
Determining the amount of water to retain behind the dam is a challenging issue. When dams are used for flood control, the storage volume should be as large as possible. However, when dams are also used for drinking water or other purposes, then there is a balancing act between how much water you retain as compared to available storage volume. And while we may be able to identify wet and dry seasons, there is still much uncertainty associated with individual rain events.
The Oroville Damage
Located in the Sierra Nevada foothills, the Oroville Dam was completed in 1967 to help with flood control, power generation, and to maintain the water supply. In 2013, a crack required crews to repair the spillway, a problem that Kevin Dossey, a Senior Civil Engineer with the Department of Water Resources, reported to the Sacramento CBS affiliate as routine. He explains that “it’s common for spillways to develop a void because of the drainage systems under them.”
After heavy flooding in the area, additional damage to the main spillway resulted in the evacuation of approximately 180,000 residents in February of 2017.
Ghasemi-Fare explains that the purpose of the spillways is to regulate the level of water behind the dam “and are designed to transport huge amounts of water downstream during flooding events. If the spillways cannot pass the flood waters, the dam may be overtopped and fail.” In the case of Oroville, the main spillway was out of service, but the flood waters were able to bypass the dam using the auxiliary spillway.
The challenge, according to Rockaway, is that “overflow structures do not get used frequently. And when they are used, we generally have little warning and expect them to work perfectly. It is thus very important to be proactive with respect to maintenance.”
The failure of a dam is a catastrophic event, not only in terms of possibly lives lost, but in the environmental destruction to the surrounding area. For the Oroville Dam, the situation is still a cause for concern due to the anticipated rainfall.
Prevention is Key
For both Rockaway and Ghasemi-Fare, it’s about diligence in adhering to routine maintenance to prevent future incidents, not only in places like Oroville, but in our own back yard.
Dams, bridges and other structures are very complex. Each structure has its unique challenges and probable failure modes.
“We have to routinely perform inspections and provide maintenance in a timely manner so we can find the small problems before they become big problems," says Ghasemi-Fare. "Even if the structures are working properly, it doesn’t mean that we shouldn’t do the maintenance.”
Rockaway shares an optimistic appraisal of our regional vigilance to maintenance needs.
“When we build this infrastructure, it’s not a one-time investment. We have to be diligent and willing to meet the long-term maintenance obligations," he said. "As an example, when an inspection identified structural issues associated with the Sherman Minton Bridge, the response was immediate. With appropriate inspection and maintenance, these structures can have a long and useful service life.”