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Spillway Design Tested by CSU Engineers

Monday, September 30, 2019

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Last week, civil engineers at Colorado State University were reported to be testing an experimental spillway intended for Gross Dam located in southwest Boulder County, Colorado.

The dam-raising project is part of Denver Water’s Gross Reservoir Expansion Project and is being designed by Stantec and primary contractor AECOM, with commissioned help from CSU civil engineering professors Chris Thornton and Rob Ettema.

About the Reservoir Expansion

After over a decade of planning, the expansion was announced in the January issue of Rocky Mountain Water, where Denver Water reported that the Gross Reservoir Expansion Project would nearly triple the water capacity of the existing reservoir that currently provides water to 1.4 million people in Denver and surrounding suburbs.

Following a four-hour public hearing by the Boulder County Board of County Commissioners in March, by April the project was reported by The Mountain Ear to be moving forward with Boulder County’s review process.

In June, reports claimed that Denver Water had secured the bulk of its required regulatory approvals for the project but was still waiting on a final decision from the Federal Energy Regulatory Commission.

The following month, Denver Water’s Board of Water Commissioners approved a two-year, $4.5 million contract with joint venture Kiewit Barnard for the planning and preconstruction work during the final design phase of the $464 million expansion project.

“This is a major milestone in our 16-year effort to expand Gross Reservoir, as its original designers intended decades ago, to ensure a more reliable water supply in a future marked by greater uncertainty in weather patterns,” said Denver Water Chief Executive Officer and Manager Jim Lochhead.

In total, the project calls for 900,000 cubic feet of concrete to be added to the existing structure and pending final approvals, is slated to be complete by 2025.

About the Spillway

Requiring close to 500 custom-built pieces, Taylor Hogan, a civil engineering master’s student and Hydraulics Laboratory manager, helped CSU civil engineers design and build a 1:24 working scale model of the heightened dam’s new spillway. The team is currently using its hydraulic expertise to confirm key design and functionality aspects of the infrastructure.

“Computers have come a long way, but they’re not even close to being able to resolve what’s happening in terms of interaction of forces,” Thornton said. “Turbulence and air entrainment are very hard to model accurately.”

Colorado State University

Requiring close to 500 custom-built pieces, Taylor Hogan, a civil engineering master’s student and Hydraulics Laboratory manger helped CSU civil engineers design and build a 1:24 working scale model of the heightened dam’s new spillway. 

Made up of a series of steps, the stepped spillway dissipates energy from the water as it flows over the dam by trapping air bubbles in its stairs, allowing the water to safely descend. In mimicking the dam’s current profile, CSU civil engineers also added a slight arch to the spillway’s design.

Because the dam is slated to be raised 131 feet over its current height of 340 feet—increasing the reservoir’s capacity by about 25 billion gallons—the stepped design is reported to necessitate that increase.

“The expansion will allow Denver Water to add balance and resiliency to its water collection system, which today is at risk of damage from natural disasters such as wildfires and floods,” said Jeff Martin, Denver Water’s Gross Reservoir Expansion Project program manager. “It will also help to manage the greater uncertainty that comes with a changing climate.”

To ensure constant proper performance, CSU engineers are also testing different variations of capacity, flow rate and ability to handle a major influx (like experiencing a storm or natural disaster) and are documenting the model’s performance.

What’s Happening Now

According to CSU researchers, in order to complete the design of the spillway they will need to complete optimizing the layout of the energy-dissipation basin at the bottom of the spillway and ensure the design meets all safety requirements.

Design engineering on the overall dam project is expected to extend through the end of 2020.

In an anticipated project timeline provided by Denver Water, starting in 2020, public access to the southern shore will be closed and quarrying operations are expected to take place through 2025. Also beginning in 2020, the construction team will be installing temporary facilities and developing the site.

From 2020-23, the dam is projected to undergo surface preparation, grouting and plant set ups. By mid-2022, the agency expects that dam raising activities will begin, with forestry clearing and first fills taking place from 2024-26.


Tagged categories: Colleges and Universities; Engineers; Infrastructure; Infrastructure; Locks and dams; NA; North America; Performance testing; Program/Project Management; Testing + Evaluation

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