Oroville – In the nearly 50 years since the Oroville Dam was completed, construction methods have changed.
Chico State University construction management professor Chris Souder consulted on the Folsom Dam auxiliary spillway project which began construction in 2008 and is on pace to be completed in October.
He also got a tour March 1 of the work going on at the Oroville Dam through the engineering contractor Syblon Reid out of Folsom. Souder said he wanted to take his college students to show them what their future careers might entail, but the Department of Water Resources wouldn’t allow it.
FORCE OF WATER
Although Souder isn’t an inspector or maintenance crew worker, he thinks visual inspections every one to two years should be enough with additional x-ray or sonar inspections that can indicate “caving, hollowing, undermining, basically if voids are present” every five to 10 years.
“There should be some or all these tests after a spillway has been used for an event,” he said.
“It’s pretty much common sense. It needs to be inspected and maintained so that there are no cracks in the concrete. Normal weathering is going to make that concrete more susceptible to that damage.”
And the water flowing down it can cause a lot of damage.
“At Folsom they directed the concrete has to be extra straight and strong,” he said. “Anything that the water can grab onto at that velocity, it will tear apart.”
That was illustrated when DWR continued releasing water through the spillway after spotting the hole. The release was an attempt to lower the lake level and not allow water over the emergency spillway. The water ripped away much of the main spillway below the hole and tore into the hillside on both sides of the structure.
Souder said the “baffles,” the enormous concrete blocks at the bottom of the Oroville spillway, were designed to slow the water down, which lessens the destructive force. The Folsom spillway has larger, steel-plated baffles and an extra element designed to slow water down.
“Folsom stepped the slabs every 7 to 30 feet once the slope was more than 30 degrees and depending how steep the slope,” Souder wrote. “There were almost 70 steps to slow the water.”
BUILDING IT TODAY
If Souder were in charge of building the Oroville Dam spillway using modern methods, he said he would build it thicker with more reinforcing steel and anchor it to the rock.
The Folsom spillway is a minimum of 42 inches thick. It’s thicker at some points where the rock is uneven, but the thinnest spot on the Folsom spillway is 42 inches.
The Oroville Dam spillway concrete is only 15 inches at its thinnest by design.
After seeing the construction up close, Souder said the original work could have been better reinforced, but it was well-built at the time.
“We learn from mistakes and time,” he wrote in an email.
The construction management professor recommended building the spillway with concrete 3 to 5 feet thick (36 to 60 inches) and with a crisscrossing “double mat” of reinforcing steel bars (rebar) an average diameter of one inch (rebar size No. 8) spaced 6 to 9 inches apart.
The original work had reinforcing steel, but Souder said it could have used more.
“There is reinforcing in the existing concrete, however, it is extremely light. It appears they barely reinforced the top and bottom double mats and what they did place has a spacing no closer than 12 inches,” he wrote.
The rebar in the original construction appeared to be about 3/4 inches in diameter (size No. 6), but Souder couldn’t get close enough to the broken pieces at the bottom of the spillway to see it up close.
Souder would also anchor the spillway concrete to the rock underneath.
The construction professor said the Oroville spillway was on a different type of rock than the Folsom spillway so it would be more difficult to anchor, but lengths of 8- to 16-foot-long rebar with hooks at the end are used to attach slabs of concrete to the rock underneath in modern building.
“Whether Oroville rock is/was competent enough to anchor to is a mystery, but if it is not, then you go deeper,” he wrote.
Although the concrete slabs made 50 years ago are essentially the same as they would be now — albeit thinner and with less rebar — Souder said he was surprised how many expansion joints were used compared to construction joints in the current structure.
Construction joints connect a concrete section with reinforcing steel extending from the end and fits it like a 3-D puzzle into the slab next to it.
The reinforcing steel rebar inside an expansion joint doesn’t extend into the next concrete slab, allowing the concrete at the joint to expand and contract from temperature and moisture changes, which can actually help prevent cracks, especially in large flat expanses of the material.
Souder said both joints are used together, but he thought there should be more construction joints and fewer expansion joints to strengthen the structure.
One other difference clear to Souder is the size of the side walls on the edges of the spillway.
In his experience working at the Folsom Dam spillway, Souder said the walls were a minimum of 30-feet tall and up to 80-feet at the bottom.
The Oroville spillway’s walls are only about 20-feet high.
WHAT’S NEXT?
Department of Water Resources acting director Bill Croyle said in a late February press conference the water agency was still trying to determine whether it will repair the broken spillway or build a new spillway.
He speculated the cost could be between $100 million and $200 million, but said it was too early to make a closer estimate.
The department hasn’t released any plans and apparently has only a rough timeline of when it will actually begin either fixing or replacing the broken spillway. Construction work at the site to date has been focused on clearing debris.
“As DWR further assesses damage to the flood control spillway, we will be able to better determine recovery efforts — experts are currently developing next steps and as current information is evaluated, the scope and magnitude of needed repairs is vetted and refined,” DWR public information officer Lauren Bisnett wrote in an email to this newspaper.
“Looking ahead, DWR’s goal is to have a fully functional spillway before next winter begins, and we’ll be working round-the-clock through spring, summer, and fall to make that happen. Within the next few weeks, DWR intends to have narrowed the approach for next winter to several options.”
NATIONWIDE PROBLEM
Another Chico State professor pointed out that while the break in the spillway is unique, aging dams are a problem across the country.
Steffen Mehl is the chair of the university’s Civil Engineering Department. Mehl’s background is in hydraulics and hydrology, and he also teaches courses in fluid dynamics.
“The best designs out there aren’t going to last if you don’t maintain them,” Mehl said.
The last “report card” from the American Society of Civil Engineers gave dams nationwide a “D” and noted that California has 678 dams that the engineer society considers “high hazard.”
Still, Mehl said the spillway endured more than 130,000 cubic-feet per second during storms in 1997 and came through without problems.
“It is uncharted a little bit,” he said. “I don’t think they saw this coming.”
