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A TALE OF TWO DAMS

Innovative Water Solutions for the Old and the New

If quantity is a global indicator of importance, water wields a superpower all its own. The Blue Planet holds more than 326 million trillion gallons of water. Let that number sink in—18 zeroes.

Access to a clean, safe water source was central to the rehabilitation of the century-old Nesbitt Dam in Pennsylvania and the development of the new Miner Flat Dam and Reservoir for the White Mountain Apache Tribe at Fort Apache Indian Reservation in Arizona. The projects ensure the delivery of clean water for two populations of people and demonstrate how the built environment can protect and enhance the natural environment.

Perched on a Ledge: The Nesbitt Dam Rehabilitation

Originally built in 1901 and listed on the National Register of Historic Places, the Nesbitt Dam is a 101-foot-high, 538-foot-long composite earth-fill embankment and stone masonry structure. Impounding nearly 1.28 billion gallons of water, it is a primary source of water supply for 75,000 customers in Lackawanna County, Pennsylvania. The $27.5 million project marks one of the largest capital improvements that owner Pennsylvania American Water has undertaken in northeast Pennsylvania.

Extreme Conditions

How does one rehabilitate a 113-year-old, high-hazard dam built on a vertical ledge of bedrock on one side and founded on soil on the other? The first step for Pennsylvania American Water was to engage an experienced and skilled team to carefully evaluate the dam’s safety deficiencies and develop innovative solutions for this project’s complexities. Gannett Fleming, as project engineer, and ASI Constructors, Inc. took on the many challenges.

Public safety, continued access to water, and environmental issues were considerations throughout construction. The dam is a water supply for 17 communities, providing raw water to the 12-million-gallon-per-day Nesbitt Water Treatment Plant, and service could not be interrupted. Further, because Nesbitt Dam ranks fifth in Pennsylvania for downstream population risk impacts, the safety of the 67,700 residents living downstream was paramount.

Geological conditions posed additional challenges; three documented glaciations took place within the project’s historic footprint. During the last ice age, a series of glaciers moved through modern Lackawanna County. As the glaciers advanced and retreated with seasonal freezing and thawing, they covered the area surrounding Nesbitt Dam with glacial debris consisting of sand, rounded gravel, and boulders. Acting as incisors, the ice and debris carved the bedrock in the pre-glacial stream valley beneath what is now the foundation and right embankment of the Nesbitt Dam. This glacial abrasion also created a nearly vertical rock ledge beneath the present left dam foundation.

The left masonry portion of the dam is founded on this bedrock ledge that drops off and slopes downstream and to the right across the site, 100 feet below the surface of the right embankment. As a result, the earth embankment portion on the right side of the dam is founded on soil – a foundation that required detailed evaluation and analysis.

Well-Justified Concerns

More recent events further pointed to the need for repairs to stabilize the structure. In 2005, a high-water event eroded the spillway apron, displacing 4,000 cubic-yards (cy) of material. Although water never overtopped the dam, the damage required emergency repairs to return the structure to safe operation.

Also key in the rehabilitation was getting the dam to pass a Probable Maximum Flood (PMF) event—a hypothetical flood event of low probability used for the purpose of project design or evaluation. Pre-rehabilitation calculations indicated that the existing 200-foot spillway could pass only 30 percent of the PMF before overtopping the dam.

Prime for Rehabilitation

To prepare for the complex reconstruction, the team used Gannett Fleming’s proprietary 3-D geometry software to gain a visual understanding of the project’s geology from which they could develop long-term solutions, including the following:

Increase Spillway Capacity

Additional spillway capacity was created by armoring the earth embankment with roller-compacted concrete (RCC) to prevent a dam failure from embankment overtopping during a PMF. The dam’s crest was modified to make it more efficient at passing extreme flood flows, and the side channel was enlarged to provide additional flow capacity during extreme flood events.

Rock-Solid Stability

The project team developed an innovative solution using deep foundations that are rarely used in gravity dam design because these types of dams typically rely on their weight to hold back the force of water. However, the high-hazard Nesbitt Dam needed an extension of the right training wall, requiring the use of deep foundations to support it. 

The masonry spillway section of the dam was supported with RCC, and 20 rock anchors were installed to improve stability of this portion of the masonry structure. To effectively dissipate energy, the downstream face of the structure was constructed with a stepped pattern. The toe of the structure was curved to convey spillway flow into the 220-foot-long armored downstream apron and stream channel.

Massive re-grading of 46,000 cy of the right hillside stabilized this area and protected the rehabilitated structure. In addition, to improve the aesthetics and reduce runoff, the RCC armor on the dam’s embankment was covered with soil and seeded with grass.

Contain Seepage

To safely collect and convey seepage from the earthen portion of the dam, drainage features were installed, including four relief wells, 1,300 linear-feet of toe drain pipe, and 7,000 cy of blanket drainage aggregate.

A Renewed Lease on a Prized Resource

Despite the challenging natural environment, the team was able to complete this rehabilitation project 15 months ahead of schedule. The dam now meets Pennsylvania Department of Environmental Protection standards, readying this essential resource for the next century.

Contact Cari Beenenga, PE, via email for more information.

 

While dam rehabilitation requires continuous monitoring and technological advances, Gannett Fleming also is designing new dams to provide smarter and safer water supplies where INADEQUATE SYSTEMS PREVIOUSLY existed.

Water for Tribal Lands

Since 1871, the serene tribal lands of the White Mountain Apache Tribe have been home to the Fort Apache Indian Reservation. Among the top 10 tribal nations in the U.S., this Apache home covers approximately 1.6 million acres, with a population of more than 15,000 people. Approximately 200 miles northeast of Phoenix, Arizona, the area is rich with agricultural opportunities, outdoor recreation, and wildlife and is brimming with attractions such as a resort, cultural museum, and a casino. Yet, after more than 140 years of occupying the area, one critical resource has been unreliable or completely absent—water.

Currently, a small well field serves the drinking water needs of the reservation, but production from these wells has declined during the last few years. Decades in the making, work is underway for a comprehensive water system that will dramatically improve the quality of life for the reservation’s residents and visitors.

In December 2010, Congress passed the White Mountain Apache Water Rights Quantification Act, authorizing the Tribe, in accordance with the provisions of the Indian Self Determination and Education Assistance Act, to plan, design, and build the White Mountain Apache Rural Water System. The system will divert, store, and distribute water from the North Fork of the White River, bringing long-term water stability and reliability to the Fort Apache Indian Reservation.

The project includes a dam, a storage reservoir, a pumping plant, treatment facilities, and a distribution system. Gannett Fleming is providing design services for the new dam.

The new Miner Flat Dam and Reservoir will provide storage of up to 8,620 acre-feet of water as part of a long-term solution for the Tribe’s drinking water shortages. The RCC dam will be constructed approximately 7.5 miles from the intersection of Highway 260 and Indian Route 73. It will be approximately 160 feet in height with a crest length of about 450 feet for storage of water to be released downstream to the new water treatment plant. The water treatment plant will treat 12.4 million gallons per day, and the 60-mile long water distribution system will deliver domestic, commercial, municipal, and industrial water to the 30,000 residents of 15 communities located along a 113-mile stretch of the White Mountains.

Construction of the Miner Flat Dam, pipelines, and water treatment plant throughout the next five years is expected to create as many as 700 new jobs during the construction phase. In addition, highly skilled tribal members will be needed to operate the water system and provide this vital natural resource well into the future.

Contact Dean B. Durkee, Ph.D., PE, via email for more information.

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