Lessons Learned From the 2017 Oroville Dam’s Damaged Spillway

Lessons Learned From the 2017 Oroville Dam’s Damaged Spillway

Spillways are overflows for excess water; they serve an important purpose in maintaining safety and preventing serious flooding. There are a couple key lessons we can learn from the 2017 Oroville Dam emergency that occurred in California. Images of water free flowing from California’s damaged Oroville Dam are shocking, to say the least. As powerful winter storms hit the area, there were many concerns that the overflowing dam would flow downstream into the town of Oroville, causing serious damage and harm.

Constructed in 1698, Oroville Dam is the tallest spillway in the US with a crest at 275m above sea level. So, needless to say, it was rather alarming when engineers noticed a hole in the concrete spillway. The damages were caused by excessive rainfall after several years of drought. Right away, over 3,000 employees of the California Department for Water Resources were assigned to emergency response work on the spillway.

How the Emergency Unfolded

When it rains a lot, water flows through the primary spillway into the auxiliary spillway, which grants added capacity in the instance of an emergency or flood. Damages to Oroville Dam were noted in February and the spillway was closed for inspection before it was slowly reopened under controlled water flows. Rainwater didn’t let up, flooding the reservoir, until the massive 275m crest was infringed and water reached the emergency (auxiliary) spillway. This was the first time this happened in the entire history of Oroville Dam.

The auxiliary spillway was simply a channel cut into the hill, and so it didn’t take long before major erosion took place and undermined the concrete dam.

At this point, water was flooding down so quickly that 180,000 residents were ordered to evacuate their homes. Engineers working onsite described the situation as incredibly stressful and hectic. In between storms, engineers and other team members raced around the clock to make repairs and seal up the hole found in the auxiliary spillway using massive amounts of rocks and gravel brought in by helicopters.

What the Experts Have to Say

An Independent Forensic Team (IFT) was brought onto the scene after the worst of the emergency calmed down. The goal was to identify what went wrong and how to prevent it from happening again in the future. Their findings were that a small chunk of concrete in the main spillway chute came loose due to fast moving water. From there, erosion formed under the chute and additional concrete slabs were taken out by the water. This is how the first hole engineers noted formed.

The spillway should have been able to withstand this level of water, it had in the past. Yet, this time it created an issue because of unnoticed and unrepaired corrosion to the reinforcement joints. Furthermore, anchors embedded in concrete slabs were not correctly encased in grout, allowing corrosion to occur. Experts surmise that a series of shallow voids had formed over time, thus allowing erosion to expand at the rate it did.

So, What Lessons Can We Take Away from This?

According to the experts, no amount of maintenance could have solved the problem because the issues were visually undetectable. The ITF report stated that physical inspections are not always sufficient when identifying all potential risks. Instead of simply inspecting the dam, the entire spillway needed to be reviewed, but that wasn’t part of the normal assessment tasks.

Lessons Learned:

  • Older infrastructure that has not been updated or overhauled is at a much greater risk.
  • Concrete contained large aggregate size, thus making it more susceptible to cracks and issues with spalling.
  • Poor drainage design led to a reduction in collector drain capacity with no filter system
  • Over 50% of foundations were not correctly treated

At the end of the day, this goes to show how important redesigns are. Construction is much more advanced today than it was 50 years ago. Because of that, we can create far more efficient systems that are less likely to encounter these types of issues.

The goal is to address issues before they have a chance to get out of hand. That’s why regular maintenance inspections, repairs, and re-designs are essential for spillways. Kleenco Construction offers full-service water management including spillway repair. We’d love to help you find the best solution for your project.