Buckhorn Dam and Reservoir Expansion

Client: City of Wilson
Location: Wilson, NC

Hazen and Sawyer provided comprehensive engineering services to the City of Wilson for the expansion of Buckhorn Reservoir, the city’s primary water source, increasing impounding capacity from 0.7 to 6.7 billion gallons and safe yield to 29 mgd. Services included planning studies, environmental and regulatory permitting preliminary design, final design, construction and post-construction services.

Project Outcomes and Benefits

  • Comprehensive engineering services from project planning through construction completion.
  • Selection of RCC instead of conventional earthfill saved $3 million in construction costs and reduced construction time by more than a year.
  • Innovative RCC monolith design provided stable construction on a weathered rock foundation and major saving in intake construction.
  • During the initial impounding sequence in September 1999, Buckhorn Dam came through with flying colors the ultimate test of passing a flood equal to almost ½ the of the probable maximum flood resulting from the one-two punch of Hurricanes Dennis and Floyd.
  • Ongoing services include regular inspections and regulatory compliance services.

The roller compacted concrete (RCC) dam was constructed in independent monoliths.

View of completed monolith section.

The use of an on-site batch plant and high-speed concrete conveyor placement system resulted in rapid placement of the roller compacted concrete (RCC).

Construction of the stepped ogee spillway.

Pouring casting beds for on-site casting of the concrete panels for the upstream face of the dam.

The upstream vertical face of the dam is lined with precast concrete panels, which also served as forms during placement of the RCC. The precast concrete panels are separated from the RCC mass by an impermeable PVC membrane to reduce seepage through the RCC dam.

Initial filling of Buckhorn Dam.

Peak flow conditions resulting from Hurricane Floyd, estimated as approximately 0.4 of the Probable Maximum Flood (PMF). The dam is designed to safely pass the full PMF.

Extensive flooding resulting from Hurricane Floyd immediately downstream as viewed from the dam.

Flooding observed on NC-581 downstream of the dam resulting from Hurricane Floyd.

More recently, peak flow conditions observed resulting from Hurricane Matthew in October 2016. The dam performed flawlessly and incurred no damage during the extreme flows produced as a result of Hurricanes Floyd and Matthew.

The expanded Buckhorn Reservoir.

Following a detailed construction alternatives analysis, Hazen and Sawyer recommended roller-compacted concrete (RCC) as the most economic and overall favorable alternative for the new Buckhorn Dam. Completed in May 1999, the new dam is the first all-RCC dam constructed in North Carolina and one of the first in the U.S. constructed on a weathered rock foundation. It has a structural height of 44 feet and a total length of approximately 2,560 feet. Total project cost was approximately $45 million (2000 dollars), including design, land acquisition, environmental mitigation, and construction.

Keys to constructing a stable dam on weathered rock foundation: (1) broaden the dam base to bring foundation compressive and dam overturning forces to within national safety standards, and (2) design the dam as a series of monoliths to allow for differential settlement along its almost half-mile length. Water tightness is provided by a continuous impermeable membrane along the dam face integral with precast concrete facing panels, which served as stay-in-place forms. The intake structure is constructed as a separate conventional concrete monolith, in-line with the RCC monoliths. This innovative approach avoided the cost of a conventional stand-alone tower without interfering with RCC placement. It further reduced project costs by integrating stream diversion with phasing of the intake monolith construction.

For more information on this project, or to discuss a similar project in your area, contact Jeffery Powers at jpowers@hazenandsawyer.com