The City of Greensboro, N.C.’s Lake Townsend Dam suffered from a severely deteriorating spillway and inadequate capacity for the spillway to handle large storm flows. Rapid action was needed to remediate these deficiencies while maintaining full serviceability of the city’s primary water supply. The Lake Townsend Dam project highlights an actively engaged and involved owner who encouraged and supported the engineer, dedicated to delivering the excellence on an expedited basis. The owner and engineer worked with an exceptional contractor focused on a streamlined delivery of a quality product and a cost-effective price for the city.
Lake Townsend is the primary drinking water supply reservoir for the City of Greensboro. Built in 1967, this 45-foot-high, 1,445-foot-long, high-hazard dam consisted of earthen embankments flanking a 276-foot-long, gated concrete spillway. Cracking of the nine spillway piers and two wingwalls was first noted in the late 1970s. Despite repairs, cracking continued unabated along with increasing seepage along horizontal lift lines in the ogee spillway. Investigations in 2003 revealed alkali-silica reactivity (ASR) distress throughout the concrete spillway. Dam safety issues weren’t raised at the time, and repairs consisted primarily of epoxy injection. An underwater inspection in 2006 noted deep separation along horizontal lift lines in the spillway, raising serious stability concerns, especially for high pool conditions during storm events. In addition to these concerns, the following dam safety deficiencies were identified:
- Concrete deterioration was progressing, elevating the risk of a dam failure over time.
- An updated hydrologic and hydraulic analysis indicated the spillway had inadequate capacity to pass the design flood required by the North Carolina Dam Safety Regulations.
- Evaluation of the grass-lined auxiliary spillway found it susceptible to erosion failure due to flood flows.
Based on these deficiencies, loss of the city’s primary drinking water supply and the potential for loss of life and extensive property damage became dominant concerns.
Because of the immediate concerns with the stability of the spillway, an emergency rehabilitation was completed using post-tensioned anchors in the mass concrete section and steel reinforcing plates to secure the piers. This stabilization provided interim protection while an expedited alternatives analysis was completed.
Evaluations indicated that the existing spillway could not be permanently rehabilitated. As such, the stabilized spillway was re-purposed as a temporary upstream cofferdam and diversion control structure during construction of a 300-foot-long, seven-cycle labyrinth spillway just downstream of the existing dam. Because Lake Townsend provides the City of Greensboro’s primary drinking water supply source, all activities would need to be completed with a full reservoir.
Because the city owned two additional dams upstream of Lake Townsend that were also in need of spillway expansion, armoring of the crest and downstream slope of the upgraded and reshaped Lake Townsend embankments with articulating concrete block were designed to allow flood detention storage and provide safe overtopping for extreme floods that include failure of the two upstream water supply dams.
Characteristics of this project that exemplify design creativity and/or mastery of complex technical challenges include conversion of the existing deteriorating spillway into the upstream cofferdam and diversion control structure, a 30-foot-deep excavation for construction of a large labyrinth spillway immediately downstream of the existing embankment and spillway with significant steepening of the downstream slope of the existing embankment, all while maintaining a full reservoir with no disruption of water service to the community.
A tight site bounded by sensitive wetlands, a water treatment plant, and the existing dam necessitated careful planning and construction sequencing, innovative construction techniques, staged diversion of both construction activities and flood flows, and protection and mitigation of environmental impacts. Project engineering and logistics needed to be coordinated with early and frequent outreach and coordination. Regulatory agencies and the city played a key role in successful project completion on schedule and within budget.
The future benefits for the greater community include both mitigation of risk of flood damage and potential loss of life downstream of the project; significantly improved reliability of the primary source of drinking water for the City of Greensboro; and a durable, cost-effective, and operations friendly project upgrade.
This article was provided by Schnabel Engineering, Inc. (www.schnabel-eng.com), an employee-owned company employing 300 in offices nationwide. Schnabel’s specialized services include dam and levee, geotechnical, geostructural, and tunnel engineering; environmental services; geophysical and geosciences services; and construction monitoring and resident engineering.