Courtesy Photo | The Pfaffensprung sediment bypass tunnel is visible from above the dam and the outlet....... read more read more
JOINT BASE ELMENDORF-RICHARDSON – Although the project development team was thousands of miles from home, the scenery felt strangely familiar. The peaks and glaciers of their daily lives in Alaska had been replaced with similar features in the central European nation of Switzerland, where staff from the U.S. Army Corps of Engineers – Alaska District toured dam infrastructure in the heart of the rugged Swiss Alps as part of a temporary duty trip. After cataloguing their findings abroad, the team would then return to Alaska with a renewed perspective for the improvement of the Lowell Creek Flood Diversion project set in similar mountains near Seward.
Improvements in Alaska
The project at Lowell is a critical piece of infrastructure for the community of Seward on the shores below. As the first flood control project constructed by USACE in Alaska, the existing dam and attached tunnel reside in the mountains above Seward. In the event of an inundation of water, the dam diverts the surging Lowell Creek away from the community through a tunnel in the mountain and into Resurrection Bay. Without this important piece of engineering, residents and structures would suffer from the floodwaters. While the tunnel and dam have served the area well, improvements are needed to ensure safety and efficiency.
The existing tunnel requires care and regular attention to ensure it is operating at its necessary capacity. To alleviate some of the burden on this tunnel and increase protection capabilities, the district will construct an additional 18-foot-wide tunnel and refurbish the existing tunnel. This augmentation of the Lowell project will greatly enhance the protection provided to the community of Seward.
As the project takes shape, the team can look back on their tour of Swiss dam infrastructure as a worthwhile endeavor. District staff hosted a group of contractors for an industry day at the site in August, which served as an important venue for feedback regarding the design of the upcoming tunnel.
Digging deeper
Sediment bypass tunnels are an important component of some dams that encounter debris such as glacial silt, which is a common material in many Alaskan waters. These tunnels allow sediment and other debris such as trees to pass freely through the infrastructure. In the event of a large flood at a protective dam, tunnels at the site prevent a buildup of material that can threaten the dam and the community of Seward below. These elements act as a “safety valve” for some dams to discharge buildup of material behind them and maintain their storage capacity. In the case of a dam such as Lowell that primarily exists to divert water, a buildup of materials could be catastrophic.
When constructing such a tunnel, engineers use pavers to line their tunnels. Typically composed of a high-strength concrete or stone, these large blocks perform very differently based upon their material. The use of granite pavers in the sediment bypass tunnels was a particular area of interest for the visiting USACE team who are considering them for the Lowell project. Compared to other materials, granite is more resilient as the lining material of a sediment bypass tunnel.
“There are not many places in the world that employ sediment bypass tunnels in their dam infrastructure,” said Coleman Chalup, geotechnical engineer and technical lead for the Lowell Creek Flood Diversion project. “It’s a method that occupies an important role where it is used, but they are not common.”
After researching the best place to learn about these infrastructural pieces, the group settled on Switzerland as a key area to focus their investigation. The nation is renowned for its breathtaking mountainscapes, glacial rivers and state-of-the-art dam infrastructure, which includes multiple sediment bypass tunnels. For these reasons, the team felt a strong need to pursue more knowledge abroad.
“It was a natural choice for us to delve into this subject further,” said Carly Lynch, project manager for the Lowell Creek project. “Switzerland’s dam infrastructure makes extensive use of sediment bypass tunnels, and we definitely wanted to see that up close.”
Changing scenery
In the mountainous canton of Uri, staff from the Alaska District, Seattle District and Omaha District toured the inner workings of the Pfaffensprung sediment bypass tunnel, which is part of a dam that impounds the River Reuss. This system has been in use for over 100 years and has employed granite pavers in the lining of its tunnel for the entirety of its existence. Over the decades, Swiss engineers at the site have replaced and repaired the pavers at this site, which have also included steel liner and basalt as the material of choice.
The team also visited the sediment bypass tunnel at Solis Dam in the nearby canton of Grisons. Like the dam at Pfaffensprung, Solis is a hydropower-producing dam, providing electricity to the surrounding area. Its tunnel is very similar to the design for the Lowell Creek project, spanning around 18 feet by 18 feet in diameter. According to Chalup, the Solis tunnel normally remains closed until an accumulation of sediment and other material reaches a level where the dam requires a discharge. At this point, the tunnel is engaged, and most of the buildup is cleared from the dam’s pool.
“Maintaining the holding capacity of dams is the number-one priority of these sediment bypass tunnels,” Chalup said. “Too much sediment behind a dam can limit the amount of water that the dam can impound, and that can be a real threat to the infrastructure.”
ETH Zurich, a Swiss university with a significant history studying the nation’s dam infrastructure, is currently conducting a study of abrasion upon different kinds of pavers in the Pfaffensprung and Solis tunnels. Their researchers imparted valuable insight to the USACE team about their studies and observation of materials in the nation’s sediment bypass tunnels. The researchers placed test sections of granite, basalt, high strength concrete and others to observe their performance in a real-world environment.
“Their research on the test sections was invaluable for our own observations and conclusions,” Lynch said. “The ability to observe the wear in different materials side-by-side will have a large impact on our own design considerations at Lowell.”
Paving the way
The process of saltation has a significant impact on the lining of a tunnel over time. This geological term describes the movement of hard particles such as sand or miniscule rock over a surface through air or water. In the case of the dam infrastructure at Pfaffensprung, the movement of glacial silt over the pavers caused uneven wearing and eroded edges in the tunnel. To keep a tunnel running efficiently, careful maintenance and repairs on the worn surfaces is vital, and each material performs differently in practice. For this reason, engineers must choose their lining material very carefully when designing and constructing sediment bypass tunnels.
Granite pavers stood out as the “winning formula” for the dams in Switzerland, and the USACE group learned extensively about their characteristics and life cycle on their trip. While granite is more expensive upfront than high-strength concrete, the former’s longevity emphasizes its value for the Lowell project.
“The granite pavers were more cost-effective for them in the long run because of their comparative durability,” Chalup said. “Because of that fact, we’re definitely leaning in that direction back home in Alaska.”
Overall, their experience in Switzerland provided these USACE professionals with a renewed perspective toward their own project and justification for their choices in its design. Moving forward, their observations will have important benefits for the community of Seward beneath the infrastructure at Lowell.
“Our learning at Pfaffensprung and Solis will be invaluable going forward,” Lynch said. “We are now better equipped to design the project at Lowell Creek with that experience in our bag.”
| Date Taken: | 08.23.2024 |
| Date Posted: | 08.23.2024 13:49 |
| Story ID: | 479322 |
| Location: | JOINT BASE ELMENDORF-RICHARDSON, ALASKA, US |
| Web Views: | 83 |
| Downloads: | 0 |
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