It's the only significant structure standing between Bakersfield and what the U.S. Army Corps of Engineers calls the "probable maximum flood."
The 61-year-old earthen dam at Isabella Lake, 40 miles northeast of the city, has been the target of years of study, evaluation, planning and public comment since Army Corps officials concluded the twin dams face a triple-threat: seepage, earthquakes and overtopping during a rare, cataclysmic flood.
Now a team of scientists charged with designing a "fix" for the dams has built a scale model one-third the size of a football field to help them understand how the newly designed dam will handle a variety of water flows -- from moderate to monsterous.
Nathan Cox, a hydraulic engineer for the Army Corps' Sacramento District, said the 1:45 scale model, built at Utah State University's Water Research Laboratory in Logan, Utah, helps engineers predict how planned modifications to the dam will perform under real-world conditions.
"The model is important because it's one of our primary design tools to verify that the design does work how we want it to work," he said.
One of the most common safety issues older dams present to those living downstream is that their spillways are too narrow, said Blake Tullis, an associate professor at the Water Research Lab at Utah State.
That's why the proposed modification of Isabella -- construction is expected to begin in 2017 -- will include the construction of a much wider emergency spillway to supplement the existing service spillway.
In the case of a 10,000-year flood -- scientists call it the "probable maximum flood" -- approximately 16 feet of material being added to the elevation of the dam, combined with the emergency spillway, would allow a controlled release of water rather than dangerous overtopping that could result in the dam's collapse, engineers say. Such a collapse has the potential to inundate much of Bakersfield, as well as surrounding roads, industry and farmland.
The crest of the emergency spillway will include an innovative weir formed into an accordion-like design called a labyrinth weir. It's one of the most important components of the model, Tullis said.
"The weir controls how much water flows out of the reservoir," he said.
The zig-zag labyrinth construction maximizes the weir surface, and therefore its effect, in a confined space, Cox added.
"We put an arc (shape) in the weir," he said, which increases even more the weir surface between one side of the emergency spillway and the other.
"We fit a 3,000-foot long structure inside an 800-foot-wide space," he said.
Many of these and other concepts used in the model came from research -- a master's thesis and a doctoral dissertation -- generated by two grad students from Utah State University, Nathan Christensen and Brian Crookston.
According to Mike Ruthford, the Army Corps' lead engineer for phase 2 of the dam's modification project, the model in Utah will allow engineers to tweak the design as new data demands. The "additional level of complexity" at Isabella, he said, means designers have had to think beyond the conventions of dam construction.
Computer modeling, combined with what scientists learn from the physical model, will contribute more to their knowledge than either method alone.
Not only that, the model could result in a reduction in the overall cost of the project, which is estimated at between $400 million and $600 million.
The primary goal of the model is not cost savings, Tullis said. It's to verify the design of the dam. It's about science and safety. However, cost reductions often result from these projects.
Saving money on the final cost would be gravy, if that can be said of anything that costs a half-billion dollars.