Dam Design and Construction

Labyrinth weir installed at Isabella Dam designed at Utah State University

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Labyrinth weir installed at Isabella Dam designed at Utah State University
(photo courtesy Utah State University)

The new emergency spillway installed at Isabella Dam in California is a unique labyrinth weir design developed at the Utah Water Research Laboratory at Utah State University.

The concept answers a fundamental question: How do you maximize the length of a line that fits inside a confined space? Options are to fold the line into a zigzag or curve the line. Or a combination of both. The idea that started at the Utah Water Research Laboratory decades ago is now a concrete reality in southern California.

Many dams feature a spillway with a weir, which allows water to flow out of the reservoir at controlled rates. Water reaches the top of the weir, then spills over and flows to the downstream side. But with a changing climate punctuated by extreme weather, civil engineers are increasingly concerned about the ability of conventional weirs to control the occasional, record-smashing outflow, USU said.

“Flow over a weir is proportional to its length. A longer weir has more flow than a short weir,” says Blake Tullis, a professor of civil and environmental engineering and associate vice president for research at USU. “In the event of a probable maximum flood, a short weir may be insufficient for high outflows. If water can’t flow through a spillway, it will build up in the reservoir and eventually overtop the dam, which could lead to a major failure.” That idea led to transformational change in spillway design.

Completed in 1953, Isabella Dam is about 40 miles northeast of Bakersfield. The reservoir is impounded by two earthen dams on the Kern River and Hot Springs Valley. Isabella Lake and its dams reduce flood risk for Bakersfield and the surrounding region and is a primary source for water users throughout Kern County. It also impounds water for the 11.95 MW Isabella hydro project.

At Lake Isabella, a 70-year-old service spillway controls outflow for the two earthen dams. The spillway can discharge about 120,000 cubic feet of water per second, far short of the estimated half-million required during a maximum precipitation event. In 2005, the U.S. Army Corps of Engineers began an extensive dam safety study at Isabella Dam, which identified important safety modification needs. In 2017, construction began on a new emergency spillway that defies conventional thinking.

Tullis’ father, Paul Tullis, first wrote about the concept of a labyrinth weir. Blake Tullis and his doctorate student at the time, Brian Crookston, took that original idea and turned it into reality when they published a paper about arced labyrinth weirs in 2012. Around that time, the Corps’ Sacramento District was looking for solutions for Isabella’s new spillway and turned to the Utah Water Research Laboratory for help.

David Serafini of the U.S. Army Corps of Engineers said the 1:45 scale model confirmed the spillway design and led to significant cost savings. Model testing revealed that with the zigzag shape, the labyrinth weir could accommodate unprecedented outflows. It also revealed that the spillway channel could be narrower than originally designed, meaning thousands of tons of granite in the spillway chute could be left untouched, reducing excavation time and costs.  “The labyrinth weir was a clear choice in order to reduce the width and the overall excavation footprint for a new spillway,” said Serafini, a lead engineer and principal in charge of the Isabella project.

The labyrinth weir arcs gently into the reservoir, zigzagging its way across the spillway channel. Stretched out to a straight line, it would be 3,100 feet long — nearly three times longer than the width of the channel in which it stands.

Tullis has long been a proponent of translational research, a term that describes practical solutions to complex, real-world engineering challenges. “One of our missions at the Water Lab is to carry out translational research that reduces risk and uncertainty for our clients,” he added. “And it provides a rich, educational experience for our students that they won’t find at other institutions.”