The construction site from across the Feather River channel (also known as the Thermalito Diversion Pool). At the direction of federal regulators, the spillway must be ready for releases from Lake Oroville by Nov. 1. (Dan Brekke/KQED)
That's the deadline for the army of construction workers laboring to rebuild Oroville Dam's main spillway to finish the first phase of the 18-month project -- now expected to cost at least $500 million. This year's work aims to ensure that the spillway is capable of handling large releases of water this winter, if necessary, and that the first step in reinforcing the erosion-prone slope serving as the dam's emergency spillway is well underway.
The result is a construction effort of vast scope and complexity.
"There's a lot of cool things going on here -- fast," Jeff Petersen, managing the project for contractor Kiewit Infrastructure West, told a pair of radio reporters during a recent site visit.
Kiewit, and Petersen, came to Oroville fresh from another big Northern California dam project -- the new auxiliary spillway at Folsom Dam, a structure that took more than a decade to design and build.
The Oroville project? With the coming rainy season as a deadline -- Oroville Dam got .69 of an inch of rain in last week's first storm of the season -- design, demolition and construction have been compressed into a few months.
"What's different is the design's being done fast. It's different for the regulatory agencies to approve it as fast as they did," Petersen said.
He notes that many parts of the project construction have been under discussion even as demolition and construction began earlier this year. One of those unknowns involves the massive chasms that eroded beneath the spillway and how much excavation would be needed to reach the competent bedrock upon which the new spillway must be built.
"We didn't know how deep this cut was going to go," Petersen said as he stood at the edge of one of those chasms. "So we had a rough plan, and then as you come out here and start doing the excavation and some more exploration, you say, 'Hey, we gotta get rid of this bad rock, let's take some more excavation out of this area.' "
he main spillway itself is a structure whose scale is nearly impossible to comprehend, even when you're standing in the middle of it. The chute is 3,000 feet long and 180 feet wide, with side walls that are close to 3 stories tall. The chute plunges downhill from a structure housing eight massive gates designed to control the release of water.
The system -- the flood control outlet, the spillway and the emergency -- are part of conventional dam design, allowing operators to manage the level of the reservoir behind Oroville Dam. What's not conventional is the scale: The dam is the tallest in the United States, rising 770 feet from the bed of the Feather River. And the reservoir behind it, Lake Oroville, is California's second largest (only Shasta Lake is bigger), with the capacity to store enough water to supply about 7 million California households for a year.
The Feather River watershed's potential for unleashing massive amounts of runoff is also vast, and the main spillway and emergency spillway are designed, in theory, to allow passage of more than 600,000 cubic feet per second without threatening the dam's integrity (under this scenario, the concrete spillway would discharge 277,000 cfs and the emergency spillway -- again, an unpaved hillside -- would handle about 350,000 cfs).
There's a caveat there: "in theory."
In practice, the highest flow the spillway has ever been called upon to handle was about 160,000 cfs, during the New Year's floods of January 1997. After that event, the structure, which had gone into service in 1968 and had already undergone extensive patching, required emergency repairs to cracked and eroded concrete and exposed rebar. A further top-to-bottom round of patching followed in 2009, followed by a less extensive job in 2013.
Which brings us to early February, when a large section of the invulnerable-looking chute disintegrated under flows of roughly 50,000 cfs. Subsequent flows devastated the concrete spillway structure. Water cascading over an ungated weir at about 12,000 cfs-- a trickle relative to what the Department of Water Resources says it could handle -- caused extensive erosion in the emergency spillway. That led to fears of a weir collapse and evacuation of 180,000 people downstream.
That spillway failure and crisis, in turn, spawned the current sprint to rebuild the structure.
isitors to the sprawling construction site actually see several spillways in various states of preparation or completion.
A portion of the original concrete chute -- the 730-foot-long top portion immediately below the spillway's giant gates -- is considered more or less sound and has undergone extensive patching. It will be demolished and replaced next year.
An 870-foot section of the chute adjoining that top piece and a 350-foot section at the bottom of the spillway, adjacent to the Feather River channel, are being completely rebuilt: demolished, excavated down to fresh rock, built up with layers of leveling and structural concrete. The new "finished" sections are being equipped with modern steel reinforcement, slab anchors, slab joints, water stops, drains and instruments to monitor future sub-slab flows.
The remaining piece of the spillway -- a 1,000-foot section in the middle -- suffered the worst damage in February, with high flows blasting away a deep core expanse of highly erodible rock and creating a series of deep chasms. That area, more than 200 feet deep in places, is being filled with hundreds of thousands of cubic yards of roller-compacted concrete, or RCC -- a stiff, drier-than-normal concrete mix that, as the name suggests, is packed into place by the same kind of roller equipment used in asphalt paving. The plan is to top that section with a 1-foot-thick layer of tougher RCC, which the Department of Water Resources and its expert consultants believe will stand up to flows of up to 100,000 cfs this winter.
Then, if all goes according to plan, that 1,000-foot spillway midsection and the patched section of old spillway at the top will be completed next year to the same standard as the rest of the structure.
The initial phase of the project to limit erosion in the emergency spillway -- a 1,700-foot-long wall of reinforced concrete piles sunk at least 35 feet below ground level and embedded at least 15 feet into strong rock -- is scheduled to be completed by January.
Click on an image below for a larger version that will also open up a slideshow of all the pictures in the gallery.