Posted on May 9, 2022
It’s the only protective infrastructure standing between Bakersfield and what the U.S. Army Corps of Engineers calls the “probable maximum flood.”
The possibly of such a cataclysmic event — a torrent of water overwhelming Isabella Dam with the potential to inundate much of Bakersfield and surrounding roads, industry and farmland — is considered exceedingly small, calculated at one in 4,100 years.
But even that remote chance, combined with seismic risks and evidence discovered in 2006 that water was seeping beneath the dam, led the Corps to place Isabella at the top of a national list of dams determined to be in critical need of attention.
The Army Corps named its gargantuan, multiyear, $284 million plan the Isabella Dam Safety Modification Project.
And though it may have seemed to some like it was never going to end, safety work on the dam will be finished by the end of this year, said the Army Corps. As it now stands, Kern County is already home to a dam that bears little resemblance to the smaller, more primitive structure that was built between 1949 and 1953.
“I want to give the Army Corps of Engineers a pat on the back,” said Kern River Watermaster Mark Mulkay, who acts as the liaison between water rights holders and the Army Corps of Engineers, which operates water releases from Isabella Lake.
“When this started in 2006, we were told they were looking at a 20- to 30-year process,” Mulkay said. “Now here we are in 2022 …”
And it’s almost finished.
Dam improvements
The safety modification project is complex, and includes a number of features, Program Manager Stephen B. Martinez told The Californian in an email.
“Dam safety modifications continue at the main dam, service spillway, new emergency spillway, new labyrinth weir, and auxiliary dam,” Martinez said.
Tasks completed since 2020 include the embankment — or reinforcement — of the upstream and downstream sides of the auxiliary dam, and completion of large-scale explosive blasts within the emergency spillway to mine materials used for those embankments of the main dam and auxiliary dam.
You heard right. Materials used to reinforce and strengthen the twin dams were produced on site, reducing the need to truck in load after load of materials produced at another site.
One of the most visible aspects of the project has been the construction of a much larger emergency spillway, which includes a new innovation known as the “labyrinth weir.”
Workers have removed 2.8 million cubic yards of rock material to make room for the new spillway. That’s nearly as much material as it took to build the Great Pyramid of Giza in Egypt.
“The emergency spillway is 82 percent complete,” Martinez said in late April. “The mining operation and excavation within the emergency spillway is 91 percent complete.”
The chance that the new spillway will ever be needed is quite small.
“They don’t want to fill and spill,” Mulkay said of the Army Corps. “That means you’ve lost control of the river. They just assume the emergency spillway will never get used.”
The huge new spillway would only be needed if a massive amount of snowmelt came roaring down the river, overfilling and overwhelming the lake. But should that unlikely event happen, the emergency spillway is designed to prevent the flood from overtopping the dam and causing its collapse.
Labyrinth weir
One of the most common safety issues older dams present to those living downstream is that their spillways are too narrow. Isabella has always been in that category — until now.
The modifications of Isabella included the creation of a much wider and deeper emergency spillway to supplement the existing service spillway.
Approximately 16 feet of material has been added to the elevation of the dam. In the event of a so-called 10,000-year flood — scientists call it a “probable maximum flood” — the emergency spillway should allow a controlled release of water rather than dangerous overtopping that could result in the dam’s collapse, engineers say.
But engineers have added an additional safeguard to Isabella’s modifications.
The crest of the emergency spillway now includes an innovative weir formed into an accordion-like design called a labyrinth weir. It’s one of the most important components of the model, Blake Tullis, an associate professor at the Water Research Lab at Utah State University, told The Californian in 2014.
That when team of scientists charged with designing a “fix” for the dams built a scale model one-third the size of a football field to help them understand how the weir could help engineers control unprecedented flood conditions at the dam.
“The weir controls how much water flows out of the reservoir,” Tullis said.
The zig-zag labyrinth construction maximizes the weir surface, and therefore its effect, in a confined space.
The three-story tall, concrete weir was built in the shape of an arc, which increases even more the weir surface between one side of the emergency spillway and the other.
The 28-foot tall weir was placed in a nearly 1,200-foot-wide opening to the emergency spillway. But if it were straightened out, the weir would stretch nearly 3,000 feet in length.
The 1:45 scale model helped engineers predict how the newly designed dam would handle a variety of water flows — from moderate to monstrous.
A full lake again?
If water is truly the lifeblood of Bakersfield, then the Kern River must be the region’s aorta, and Isabella Lake the city’s beating heart.
However the lake is also critical to the residents of the Kern River Valley, who depend on it to draw visitors to the region’s hotels, restaurants, retail shops and recreation and tourism industries.
But here’s the rub: Since 2006, during the valley’s peak tourism season, the lake has been subject to a “restricted pool.” That means it was allowed to reach only 361,250 acre feet, significantly less than the lake’s capacity of 568,000 acre-feet.
According to the Army Corp’s Martinez, that restricted pool is going away.
“The goal is to restore the reservoir to normal over the 2022 and 2023 runoff season,” he said, “but this will be conditional and dependent on available snow pack.”
Unfortunately, this year’s snowpack has little chance of reaching even the restricted level.
Miguel Chavez, hydrographic supervisor for the city of Bakersfield’s Department of Water Resources, said allowing Isabella to fill to its original capacity in wet years is important to all Kern River water interests, including the ag industry’s water storage districts and the city itself, which also holds significant water rights on the Kern.
“Kern River interests, in conjunction with the Kern River Watermaster, have worked vigorously since the implementation of the restricted pool to assist the U.S. Army Corps of Engineers with any testing or data needed to expedite the removal of the restricted pool,” Chavez said.
Now that it’s about to happen, those entities “are excited to have full capacity in the near future,” he said.
The increase in water storage will allow for a more gradual release of water from the dam in wet years, Chavez said.
Mulkay agreed, noting that bringing back full storage capacity to the reservoir would give water managers more flexibility in their water management strategies.
And the Kern River Valley will certainly benefit.
“I am sure there are many entities and communities surrounding Lake Isabella that are equally excited to have the lake filled in the future for recreational purposes,” Chavez said.
Matt Volpert, owner of Kern River Outfitters in Wofford Heights, said removing the restricted pool will positively impact the Kern River Valley — aesthetically and recreationally.
“By allowing the reservoir to hold more storage, a big water year will carry over for more than a single year,” he said, “which is what we’re currently working with due to the restricted pool.
“This saves us in back-to-back years of drought, giving rafting companies the ability to offer their regular schedule of fun Class IV whitewater trips.”
With the end now in sight, Martinez said the Sacramento District is proud of the work that has been done by employees with the Army Corps and other government agencies, private consultants and contractors who have supported the project with their expertise.
“The team is committed to completing the project safely and responsibly,” he said, and a USACE construction management presence at Isabella will continue through 2024 to oversee construction of the operations building and other remaining facilities.
It looks like a light flickering at the end of a very long tunnel.
Source: Bakersfield.com
