The Oroville Dam in Northern California is the highest in the country at 770 feet – four stories taller than the more celebrated Hoover Dam – but few outside the Golden State had ever heard of it before February of 2017.

That’s when the dam made headlines worldwide after its primary and emergency spillways suffered significant damage just days apart, prompting California officials to evacuate thousands of downstream residents for fear the emergency spillway would let go. Video footage of the hillside next to the main spillway being washed away under a punishing torrent of water aired for days on newscasts across the globe.

In an amazing turn-around, however, construction crews last year put the finishing touches on the second phase of the Lake Oroville Spillways Emergency Recovery Project, a massive $1.1 billion overhaul of the 50-year-old dam, meeting a key November 1 deadline as California braced for its rainy season.

“More than 700 construction workers, many of them from Butte County and other parts of Northern California, literally worked day and night to make incredible progress during the 2018 construction season,” said Tony Meyers, the project manager for the California Department of Water Resources.

TRC played an integral role in the ambitious endeavor, providing construction management support and performing contract administration, claims support, scheduling review, blasting inspection and round-the-clock field inspections (including drone documentation) for the California Department of Water Resources – all to help bring a major infrastructure project to fruition in less than two years.

“It’s incredible what the department did in a very short amount of time,” said Agnes Weber, senior vice president of TRC’s Construction Services West group. “The manpower, the materials – plus they had an incredible safety record.”

TRC was on-site from the start. Staff from Vali Cooper & Associates – acquired by TRC in 2018 – were among the first called in under an on-call construction management contract the company has held with DWR since 2008.

“I remember being out there just a few days after repair work started,” said Weber. “It was 2 a.m. and it was raining, cold and wet. It looked like a Mad Max movie. There was heavy equipment and dredging taking place in the river and bright lights lighting up the night. It was amazing.”

Within 20 months, the chaotic work site was replaced by a new reinforced spillway chute designed to withstand the onslaught of water from above as well as new emergency spillway buttressed by a new retaining wall and featuring a staircase-like splash pad to prevent erosion.

 

What Happened

In 2017, Northern California was hit by record rainfalls that caused water levels in Lake Oroville to steadily rise through December and January. Then, in early February, the area got hit with 13 inches of rain in just five days.

On February 7, DWR increased the outflow on the Oroville Dam’s main spillway to keep up with the rising water levels. But workers quickly noticed a “substantial disturbance” in the flow of water halfway down the spillway. When they closed the gates completely an hour later, they discovered a 250-foot gash in the spillway chute.

With the rain continuing, however, officials were left with no choice but to cautiously reopen the gates and closely monitor the situation.

By February 11 the water level was high enough to crest the emergency spillway for the first time in the dam’s history. But the heavy flow of water caused extensive erosion – erosion that was “rapidly advancing.” Fearing catastrophe, public safety officials ordered residents of the Feather River basin below the dam to evacuate the area. Within 48 hours, 188,000 people had packed up and gotten out of harm’s way.

In the meantime, dam personnel were forced to crank up the flow to the main spillway to unprecedented levels. This allowed the lake’s water level to slowly decrease and residents to return to their homes. But when the gates to the main spillway gates were finally closed on February 27 and officials could survey the damage, they discovered the deluge had turned the gash in the spillway chute into a 200-foot deep, 1,100-foot long chasm and washed away much of the adjacent hillside.

The Fix

Repairs began almost immediately.

“When the project got underway, there were just a few trailers on top of the dam,” said Weber. “Over the next several months more and more kept arriving, eventually turning the site into a small city. We quickly ramped up our staffing levels and during the peak had about 60 people on site.”

The first phase of the project kicked off just a few weeks after the near-catastrophe, with work crews making temporary repairs to the main chute and the emergency spillway. That work was performed at a breakneck pace and was completed later that fall.

In May of 2018 work began on Phase II, with workers demolishing the 3,055-foot long main spillway and erecting a much beefier replacement. The original spillway floor was 2.8 feet thick. Its replacement is 7.6 feet thick and anchored to bedrock with bolts up to 25 feet long. The chute is 178.5 feet wide – equivalent to a 12-lane highway – and framed by walls up to 34 feet high.

At the same time, work crews began permanently repairing the emergency spillway. There they built a 1,450-foot long retaining wall anchored to bedrock as well as a 1,850-foot long steel reinforced concrete splash pad that resembles a stairway to reduce the massive force of the water cascading down from above. At 750 feet wide, the splash pad is large enough to contain 25 full size football fields.

On April 2, the main spillway got its first test. Due to impending storms and growing snowpack, dam operators performed the first controlled release of water from the Oroville Reservoir, sending a gentle cascade of H²0 down the spillway. With everything going smoothly, the flow was gradually increased to 8,300 cubic feet per second (cfs) and then 10,800 cfs. Over the coming days the flow was dialed up to 25,000 cfs, with the torrent of water splashing over the four giant concrete teeth at the bottom of the spillway and creating a frothy churn in the Feather River below.

The Oroville Dam was back in action and better than ever.

“We feel honored to have been part of such an important project for the state of California,” said Mike Broadwater, TRC’s senior vice president of construction management, who served as TRC’s quality control manager on the project. “Restoring the Oroville Dam to its former glory will help the state safely manage its water supply for decades to come.”

Author: Doug Hanchett, Corporate Communications Manager, TRC