- Pennsylvania American Water
- Lackawanna County, Pennsylvania
- Our Role
- Engineering, Design, Permitting, Geotechnical Investigation, Drilling, Laboratory Analysis.
- 101-foot-high, 538-foot-long
- Construction Cost
- $27 million
- 16 months
Spillway now passes 100 percent of the probable maximum flood
Massive regrading of right hillside arrests long-term hillside instability
Rock anchors and RCC reinforced masonry spillway improve stability.
Originally built in 1901, the Nesbitt Dam is a composite, earth-fill, embankment, and stone masonry structure that is built on a vertical ledge of bedrock on one side and founded on soil on the other. This high-hazard dam ranks fifth in Pennsylvania for downstream population at-risk impacts with 67,700 individuals located in the flood inundation zone. Its safety issues included inadequate spillway capacity, deficient structural stability, seepage, and insufficient embankment and abutment stability. Holding nearly 1.28 billion gallons of water, it is a primary source of water supply for 75,000 customers, which required the rehabilitation to be completed with a nearly full reservoir and the spillway to be in operation throughout the project.
Despite the challenging geology and extremely limited site conditions, the project meets PADEP standard, and provides the safe operation of this essential water supply dam.
To prepare for the complex reconstruction, the project 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 safely collecting and conveying seepage from the earthen portion of the dam. This was accomplished with the installation of drainage features, including four relief wells, 1,300 linear-feet of toe drain pipe, and 7,000 cubic-yards of blanket drainage aggregate.
Additional spillway capacity was created by armoring the earth embankment with roller-compacted concrete (RCC). 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. 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 cubic-yards of the right hillside stabilized the hillside and protected the rehabilitated structure.
- Dam listed on National Register of Historic Dams
- Geology was challenging - left abutment is essentially a vertical rock ledge, but the bedrock on the right abutment is several hundred feet below the ground surface
- Spillway capacity increased for extreme flood events with RCC, a modified crest, and increased side channel
- Dam stability improved with RCC armoring and 20 rock anchors
- Hillside stability improved with massive regrading
- Rehabilitation completed 15 months ahead of schedule.
Environmental considerations were taken for construction completed in the Spring Brook stream channel, classified as a high-quality watershed by PADEP and as a wild trout stream by Pennsylvania Fish and Boat Commission
During construction, the site was subject to stringent erosion and sediment controls; the contractor opted to include additional erosion and sediment control best management practices to protect the watershed from pollution from the beginning
To improve the aesthetics and reduce runoff, the RCC armor on the dam’s embankment was covered with soil and seeded with grass.
- Honor Award, 2014, American Council of Engineering Companies, Engineering Excellence Awards Competition.