Findings & Next Steps

Findings: 

• The findings of the engineering analysis are that “The Lake Lure dam is in overall fair condition considering its age.”  
• Last year’s Hurricane Florence forecasted rainfall of 15 inches within a 24-48 hour period; however, the dam performed well. The Town’s staff and Emergency Preparedness Team (with guidance from Schnabel Engineers) lowered the lake level 7 feet in anticipation of forecasted rain.  Schnabel Engineers were on site for the storm event, and they were impressed with the Town’s readiness during the storm.  
• Based on the results of the engineering assessment, there are several items that warrant repair, monitoring, and /or additional investigation/assessment.  
• It is true that the dam does not meet current NC DEQ Dam Safety requirements. But according to Schnabel, a renowned dam engineering firm, “it is very common not to meet the standards,” particularly for dams built in the Lake Lure era. This is due to the safety requirements changing in the 1970s.  
• Specifically, the standards the dam does not currently meet are: 
  1. Hydraulic capacity:  Lake Lure Dam is designated as a very large, high-hazard dam. NC DEQ Dam Safety requires very large, high hazard dams to be capable of safely passing the Probable Maximum Precipitation (PMP) without overtopping the dam.  The PMP is an extremely low probability event with an expected rainfall total of about 30 inches in 24 hours.  With the spillway gates fully operable, Lake Lure Dam can pass about 67 percent of the PMP prior to overtopping Buffalo Shoals Road.   NC Dam
     Safety also requires that very large, high-hazard dams include a functional reservoir drain that will allow
     the entire lake to be drained, if necessary.  Lake Lure Dam does not have a functional reservoir drain,
     though the lake level can be lowered up to 16 feet below normal levels if needed.  
  2. Structural stability of the arch-buttress dam sections (the majority of the dam) under seismic loading conditions: There is the potential for overstressing the concrete in the arch regions of the dam during an extreme earthquake. This risk exists if the Town were to experience a 10,000-year earthquake during the winter months.  Studies show there is a 1% probability of this event occurring every 100 years.
  3. Global stability of the concrete gravity gated spillway sections:  The concrete gravity gated spillway sections do not meet global stability requirements for the load scenarios analyzed. This means the actual factor of safety is less than the required factor of safety, which is typically 1.5 to 2 times that required for the dam to remain stable.  

Note: The arch-buttress regions of the dam do meet dam safety standards for stability for the following conditions:   

1. During normal pool (water level at normal operating conditions);
2. During a 200 year flood (a flood event that is expected to occur once every 200 years);
3. And in the event of PMP.

Next Steps: The Town is prioritizing our next steps with the following actions:

1. Meeting with NC DEQ Dam Safety: Review findings, evaluate dam rehabilitation options and determine costs.
2. Meeting with NC Department of Transportation (NCDOT): The Buffalo Creek Road Bridge currently passes over/on the dam. NCDOT wants to replace the Bridge as it does not meet current standards.        Based on the assessments from Schnabel Engineering, a recommendation has been made that the best place for the bridge is to remain on the dam.  Therefore, it is critical for dam renovation and bridge rehabilitation work to be coordinated as one project.   
3. Investigations: Geotechnical investigation of the left abutment retaining wall and the dam concrete rock interface, a remote inspection of the tunnel at the tailrace (a water channel below the dam), and inspections of the penstock (intake structure that channels water to the generators) and flood gates.  
4. Analysis: Evaluate rehabilitation alternatives to meet NC Dam Safety requirements and associated budgetary costs. Evaluate left abutment wall stability and abutment scour potential, perform additional downstream scour analysis and structural assessment of the intake tower. Scour is caused by swiftly moving water that can undermine the foundation of a structure.
5. Capital Improvement Projects: We will repair or replace the intake tower drum gate hoist (stops water flow through the penstock) and make interim improvements, including repair of the cover plate welds at the penstock.  To ensure routine monitoring and maintenance, Town staff will monitor the penstock, complete detailed inspections of the spillway gates, monitor the concrete arches and buttresses for cracking, and develop standard operating procedures for lowering the intake tower drum gate.

Conclusion: The Town is focusing on these high-priority recommendations from the engineering analysis that has just been completed. We know that people are concerned about the integrity of the dam.  The dam has performed very well during its lifetime and is even evaluated as an “engineering marvel” by engineers.  The dam is graded to be in fair condition based on its age, and we are confident the dam is safe unless we experience a significant (10,000 year) earthquake or rainfall in excess of 30 inches of rain in a 24 hour period.  That being said, we are doing everything within our power to ensure our dam is safe for generations to come.   

Based on all that we have learned from this exhaustive expert analysis, we are confident the dam will continue to serve us well as we make these priority repairs.   We welcome you to stay tuned to our progress by attending our monthly Town Council Meetings on the second Tuesday of the month at 5:00 pm in the Lake Lure Municipal Hall.