3-D modeling key to innovative labyrinth spillway

Inter-District staffing provides dam safety solution

By Amy Echols, Portland District Public Affairs Office and John Prettyman, Sacramento District Public Affairs Office

This illustration shows the future location and footprint of the emergency spillway and labyrinth weir in relation to the existing main and auxiliary dams. The new spillway and weir are designed to function only during extreme storm events to safely pass large volumes of water, prevent dam failure and minimize impacts to downstream Bakersfield.(Illustration provided by John Prettyman, Sacramento District)

This illustration shows the future location and footprint of the emergency spillway and labyrinth weir in relation to the existing main and auxiliary dams. The new spillway and weir are designed to function only during extreme storm events to safely pass large volumes of water, prevent dam failure and minimize impacts to downstream Bakersfield.(Illustration provided by John Prettyman, Sacramento District)

The U.S. Army Corps of Engineers’ Sacramento District is facing one of its toughest engineering projects with help from Portland District: upgrading Isabella Lake Dam in southern California. Earthquakes, seepage and extreme storms could all potentially cause the dam to fail, resulting in catastrophic damage to nearby Bakersfield, with economic impacts nationwide.

Designing an emergency spillway to increase the dam’s capacity to safely release large volumes of water and significantly reduce risks to the large downstream population is well underway. But the local topography and limited space to construct the spillway made traditional engineering solutions ineffective. As explained by Sacramento District engineers, this project required some solutions from outside the handbooks.

Laurie Ebner, hydraulic engineer with the U.S. Army Corps of Engineers’ Portland District, and David Serafini, geotechnical engineer with the Corps’ Sacramento District, observe modeling of 506,000 cubic feet per second flow of water through a 1:45-scale model labyrinth weir and emergency spillway. This flow is equivalent to the largest storm predicted to hit the region. (Photo courtesy of John Prettyman, Sacramento District)

Laurie Ebner, hydraulic engineer with the U.S. Army Corps of Engineers’ Portland District, and David Serafini, geotechnical engineer with the Corps’ Sacramento District, observe modeling of 506,000 cubic feet per second flow of water through a 1:45-scale model labyrinth weir and emergency spillway. This flow is equivalent to the largest storm predicted to hit the region. (Photo courtesy of John Prettyman, Sacramento District)

A dam safety risk assessment determined that Isabella Lake Dam was threatened by the risk of failure from overtopping due to a spillway unable to handle the maximum estimated peak flow. Sacramento District evaluated a wide range of potential spillway modifications, including Tainter gates found on many Portland District dams.

Several years of initial alternative testing provided a front-runner design for further evaluation: the innovative, horizontal labyrinth weir. The 3,000-foot labyrinth weir will channel flow into an 800-foot wide spillway. The location and size of the spillway and its chute had to support the design objective of passing peak flows, while avoiding some seismically sensitive locations. Engineers further optimized the design within the limited space by aligning the labyrinth pattern of the weir crest along a curve instead of a more typical straight line.

To help ensure the labyrinth weir could provide maximum flow capacity in the limited space available, Sacramento
District sought the specialized, seasoned expertise of Portland District hydraulic engineers Liza Wells, Laurie Ebner and Sean Askelson to conduct 3-D Computational Fluid Dynamics modeling for the Isabella project.

This modeling uses numerical methods and algorithms to study the physics of fluid flow. In particular, Portland District uses CFD modeling to provide information about how water passes through and near structures as part of the design process. To accurately model flow, specialized software divided the volume of water in the reservoir forebay, spillway and spillway chutes into about 2.5 million computational cells. Computations for each cell provided information on velocities, water surface elevations and pressures, depending on what was happening to the cells around it. For example, one model simulated a portion of the reservoir forebay, confirming flow patterns as water approached the labyrinth spillway. More detailed modeling simulated flow over the weir to ensure the weir’s performance.

Isabella Reservoir is located on the Kern River in southern California, one mile upstream of the town of Lake Isabella and approximately 42 miles northeast of the city of Bakersfield (with a metropolitan area population exceeding 500,000). (Illustration provided by John Prettyman, Sacramento District)

Isabella Reservoir is located on the Kern River in southern California, one mile upstream of the town of Lake Isabella and approximately 42 miles northeast of the city of Bakersfield (with a metropolitan area population exceeding 500,000). (Illustration provided by John Prettyman, Sacramento District)

The trio from Portland also recommended that Sacramento District use a composite modeling approach that combines physical and CFD modeling to balance the benefits and limitations of both technical tools. Engineers often use physical scale models as highly visual tools for observing design performance, validating flow capacity or evaluating features for design projects like Isabella’s labyrinth weir.

The physical model footprint for this 800-foot weir could have become impractically large to appropriately model the flow physics. Portland District’s composite modeling approach, however, supported the building of a cost-effective and technically sound 1:45-scale physical model by Utah Water Research Laboratory staff at Utah State University.

“The flexibility of CFD modeling allowed us to reproduce detailed flow patterns and test flows under a variety of conditions that can’t be practically modeled physically,” said Askelson. “And while CFD modeling has its advantages, it has not frequently been used for design of this type of labyrinth weir, so confirmation of the design through physical modeling was critical to the process.”

Askelson explained that seeing the physical model in action helped validate CFD modeling results and identified areas where CFD modeling could further improve the weir’s design. “Another advantage to the CFD model is that we can look at any model results and analyze them differently in the future.”

As the composite modeling progressed, Ebner and Askelson frequently met with the Sacramento District project team and researchers in Utah to make sure the modeling tools worked together successfully.

“Marrying the results from both CFD and physical modeling provided the design team an important level of technical confidence in our decisions,” said Ebner. “This hybrid approach also offered engineers a chance to learn from each other’s specific experience and specialized disciplines and added significant value to the project.”

After testing and modifications, engineers confirmed that raising the existing Isabella Lake Dam 16 feet and constructing the new labyrinth spillway would allow reservoir flows to meet dam safety objectives.

Ebner and Askelson anticipate playing consulting roles as the design moves into construction. They explained that even with the best design, the excavation and concrete construction phases of the project may require hydraulic modeling to help adjust the alignment of the project.

“The Isabella Lake Dam project demanded staff to function at the highest possible level. They operated in a very positive manner, with unusual technical ability, in the demanding atmosphere of a constrained project schedule,” said Sacramento District Commander Col. Michael J. Farrell. “The team’s performance continues to receive positive feedback from counterparts at the Corps’ Risk Management Center, academia and subject matter experts in private industry.”

And with many Corps dams in need of additional spillway capacity, the methods developed by the Isabella Composite Modeling Team may be applied readily to significantly benefit other projects. Wells, newly appointed as chief of Portland District’s Hydraulics and Hydrology Branch, added praise for Ebner and Askelson: “They are a model themselves for the success of effective regional and national knowledge sharing and collaboration that will serve the Corps well in the future.”

Modeling is an essential part of the Corps’ engineering and design phase and allows engineers to test the design against extreme storms and improve it before construction begins. Engineers used the best from both physical and computer models to validate the effectiveness of the labyrinth design that will translate better into the full-scale construction, ultimately saving both time and money. (Illustration provided by John Prettyman, Sacramento District)

Modeling is an essential part of the Corps’ engineering and design phase and allows engineers to test the design against extreme storms and improve it before construction begins. Engineers used the best from both physical and computer models to validate the effectiveness of the labyrinth design that will translate better into the full-scale construction, ultimately saving both time and money. (Illustration provided by John Prettyman, Sacramento District)

Learn how extreme storm water flows and 3-D modeling helped develop the best design to modernize Isabella Lake Dam.

  • Take a video tour of Sacramento District’s new Isabella Lake Dam scale model at Utah State University’s Water Research Laboratory: http:/youtube/wXTvvKkSxpo
  • More photos available on Sacramento District’s Flickr page: http://bit.ly/1ms3YiF