Anyone who contemplated the wreckage of the Oroville Dam’s main spillway back in February — either while water was pounding down the shattered concrete structure or when the flow was stopped later and the enormity of the damage was fully visible— probably had this thought cross their mind: “That is going to be tough to fix.”
Officials with the California Department of Water Resources were apparently thinking something similar. They got in touch with researchers at Utah State University as part of the process to figure out just how to approach the job of rebuilding the 3,000-foot-long concrete chute.
The department hired the university’s Water Research Laboratory to create a scale model of the spillway to help assess its condition after it breached and broke apart and to test concepts for its reconstruction.
“When we were contacted back in February, DWR had no idea what was feasible in this construction season,” Michael Johnson, an associate professor of hydraulic engineering at Utah State, said in an interview Thursday.
“They realized they may have to run this thing again this year, before it’s finished. So part of our work was evaluating conditions for this coming season,” Johnson said.
To do that, Johnson and a team of fellow researchers, engineers and technicians built a 1:50 scale model — essentially, a replica that’s 1/50th the original’s size — of the wrecked spillway. To create the very realistic 3-D model, Johnson says, the team used lidar (light detection and ranging) data from the Department of Water Resources.
The lab has since created a version of the model that depicts an intact spillway. The purpose of Spillway Model 2.0 is to test design features under consideration for the rebuilt structure.
Among those features being examined on the model: aerators for the surface of the concrete chute that are designed to prevent or dampen some of the destructive effects of water that prototypes suggest can move down the steeper sections of the spillway at 130 feet per second — nearly 90 mph.
The Department of Water Resources and its contractor on the project, Kiewit Infrastructure West, have outlined a spillway rebuilding plan stretching over two construction seasons.
In the first season, which began in late May and is slated to last through next November — later if weather and reservoir conditions allow — crews will demolish and rebuild most of the lower section of the damaged concrete chute.
At the same time, workers will undertake a second massive project to reinforce that unpaved hillside designated as the dam’s emergency spillway. A 1,730-foot concrete weir at the top of the slope and adjacent to the main spillway is designed to allow uncontrolled flows down the slope from Lake Oroville to the Feather River. The slope eroded rapidly when water flowed over the emergency weir in February, threatening to undermine the weir and unleash a catastrophic flood.
To try to stem erosion in the event the emergency spillway is pressed into service again, DWR’s plan calls for building a huge “cutoff” wall on the slope beneath the weir. The water agency has said that work should be finished by November.
In the second construction season, contractors will rebuild the upper portion of the main spillway chute and build a massive concrete “splash pad” below the emergency weir — another step intended to prevent erosion.
Utah State’s project is not the first time hydraulic engineers have created a model to test the design of the Oroville Dam Spillway.
If you’ve dipped your toe into the history of the dam since February’s crisis — wondering, for instance, exactly what the engineers who designed the complex had in mind when they decided to create an emergency overflow down a bare hillside — then you may well have stumbled onto a document labeled HYD-510.
The U.S. Bureau of Reclamation produced the 189-page paper in 1965 to summarize the results of spillway design testing it had performed at the request of the California Department of Water Resources, which had puzzled over how to configure the structure.
Just as a reminder, major dams need spillways to regulate the levels of the reservoirs behind them. They allow excess water to flow at a controlled rate through or around dams and prevent reservoirs from flowing over the top of the dam itself. In the case of an earthen embankment dam like Oroville, an overtopping event could erode the dam, undermine its structural integrity and lead to collapse — a calamity.
The design of the spillway at Oroville presented some special challenges, though not necessarily unique ones, because of the sheer size of the dam (you’ve read by this time that it’s the nation’s tallest) and the reservoir it would create as it held back the waters of the Feather River watershed (the reservoir, Lake Oroville, is the second-largest in the state; when it’s full, it holds enough water to supply about 7 million California households for a year).
Now back to HYD-510.
The report describes the process of testing conducted at a Bureau of Reclamation lab in Denver, and it’s full of details that might never occur to the casual spillway-watcher.
For instance, how much water the main spillway and the nearby emergency spillway were designed to handle, and under what conditions. One of the big areas of inquiry was how different configurations of the the channel outside the massive spillway gates would affect the speed and turbulence of the flow heading into the spillway’s concrete chute.
More than 50 years and one major spillway crisis later, Utah State’s researchers are revisiting many of the same problems.