Buckhorn Dam and Reservoir Expansion

Client:: City of Wilson

Location:: Wilson, NC

A Hazen and Sawyer study confirmed that the City of Wilson faced an immediate need for a substantial increase in their water supply capacity, and recommended expansion of the existing Buckhorn reservoir as the most viable option. The City then hired Hazen and Sawyer to design one of the first roller compacted concrete dams in the United States constructed on a weathered rock foundation.

Project Outcomes and Benefits

Hazen and Sawyer’s study confirmed that the City of Wilson was facing an immediate need for a substantial increase in their water supply capacity.
Innovative roller compacted concrete dam design featuring a series of individual monoliths has proved to be both more economical and more effective for stream diversion purposes than customary design.
A few months after work had been completed, eastern North Carolina was hit by Hurricanes Dennis and Floyd; the Buckhorn dam performed flawlessly and incurred no damage during the extreme flows.

A Hazen and Sawyer study confirmed that the City of Wilson faced an immediate need for a substantial increase in their water supply capacity, and recommended expansion of the existing Buckhorn reservoir. The City then hired Hazen and Sawyer to design one of the first roller compacted concrete dams in the United States constructed on a weathered rock foundation.
The Roller Compacted Concrete (RCC) dam was constructed in independent monoliths – completed monolith shown here.

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RCC Monolith sections under construction.

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View of RCC conveyor placement system - dam foundation preparation is shown at right. Conveyor capacity of >400 cubic yards/hour allowed for all RCC to be placed in 8 months.

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Alternate view of RCC placement conveyor.

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Stepped Spillway Construction

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Pouring casting beds for on-site casting of upstream facing panels.

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Initial RCC placement was done at night to meet rigorous RCC placement temperature requirements.

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A few months after work had been completed, eastern North Carolina was hit by Hurricanes Dennis and Floyd; the Buckhorn dam performed flawlessly and incurred no damage during the extreme flows.

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Peak flood conditions that resulted from Hurricane Floyd. Peak inflow of >22,000 cfs or about 0.4 of the Probable Maximum Flood. Dam is designed to safely pass the full PMF.

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View from the right abutment of the spillway during post-Hurricane Floyd inspection.

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Conditions immediately downstream of the dam during post Hurricane Floyd inspection. The new dam survived extreme flows due to the hurricane without any damage.

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The expanded Buckhorn Reservoir

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A Hazen and Sawyer study confirmed that the City of Wilson faced an immediate need for a substantial increase in their water supply capacity, and recommended expansion of the existing Buckhorn reservoir. The City then hired Hazen and Sawyer to design one of the first roller compacted concrete dams in the United States constructed on a weathered rock foundation.
The Roller Compacted Concrete (RCC) dam was constructed in independent monoliths – completed monolith shown here.
RCC Monolith sections under construction.
View of RCC conveyor placement system - dam foundation preparation is shown at right. Conveyor capacity of >400 cubic yards/hour allowed for all RCC to be placed in 8 months.
Alternate view of RCC placement conveyor.
Stepped Spillway Construction
Pouring casting beds for on-site casting of upstream facing panels.
Initial RCC placement was done at night to meet rigorous RCC placement temperature requirements.
A few months after work had been completed, eastern North Carolina was hit by Hurricanes Dennis and Floyd; the Buckhorn dam performed flawlessly and incurred no damage during the extreme flows.
Peak flood conditions that resulted from Hurricane Floyd. Peak inflow of >22,000 cfs or about 0.4 of the Probable Maximum Flood. Dam is designed to safely pass the full PMF.
View from the right abutment of the spillway during post-Hurricane Floyd inspection.
Conditions immediately downstream of the dam during post Hurricane Floyd inspection. The new dam survived extreme flows due to the hurricane without any damage.
The expanded Buckhorn Reservoir

This project resulted from a feasibility study, which resulted in a comprehensive review of the City of Wilson and Wilson County’s long-term water supply needs.

Roller compacted concrete (RCC) was selected for the Buckhorn site. Because bedrock at the dam site was more than forty feet below grade, it is also one of the first RCC dams in the United States to be constructed on a weathered rock foundation. Completed in May 1999, the new dam was constructed a few hundred feet downstream of the existing dam and raised the water level in the reservoir by 12 feet.

To allow for differential settlement along its almost half-mile length, the dam was constructed in a series of fifteen individual monoliths, fourteen of which are RCC. Hazen and Sawyer selected an innovative approach to RCC dam design; a stand-alone tower had been customarily used due to concerns about interfering with RCC placement and other constraints. It was made possible by the choice of RCC placement techniques, and has proved to be both more economical and more effective for stream diversion purposes, especially in this particular case.

A few months after work had been completed; eastern North Carolina was hit by Hurricanes Dennis and Floyd. The new reservoir responded as expected, rising sharply over a period of several days. Extensive flooding and damage occurred both upstream and downstream of the Buckhorn dam.

Such conditions were not ideal for breaking in a new dam. However, the Buckhorn dam performed flawlessly and incurred no damage during the extreme flows. The dam proved that it is capable of performing as expected, even under the most extreme conditions.

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