Carbonton Dam removal: A 6th Anniversary Look Back

Hard Hat News, 2006. Demolition of the Carbonton Dam, by Gwen Laird Pernie

In the Central Piedmont Region of North Carolina an effort is underway to restore 10-miles of the Deep River, a tributary to the Cape Fear River Basin. The river has been environmentally damaged for the past 200 years, when the first dam was built at Carbonton to power a mill on the riverbank. This $8.2 million dam-removal project, the largest ever of its kind in North Carolina, is also the first dam removal in the state to be done primarily for mitigation purposes. This project is the culmination of a five-year planning effort by Restoration Systems, LLC, of Raleigh, NC (owner of the dam) to re-establish the native aquatic ecology of the Cape Fear River Basin to gain “mitigation credits” under the North Carolina Ecosystem Enhancement Program. It was planned and is being implemented in close coordination with multiple county, state, and federal regulatory agencies. The dam is a former hydropower generating facility that is licensed under the Federal Regulatory Energy Commission. Site contractor for the project is Backwater Environmental of Pittsboro, NC, a subsidiary of Osborne Company General Contractors of Eden, NC. Osborne will handle the physical demolition and heavy work of the project, while Backwater will handle the earthmoving, grading, site restoration, and softer work. Restoration Systems retained Milone & MacBroom Inc. (MMI), of Cheshire, Connecticut and Greenville, South Carolina to investigate the existing dam and assist with agency permitting, and to design the removal strategy, prepare construction documents, and handle inspection during demolition. MMI is also designing a future public recreation park that will occupy the south riverbank at the dam site. According to Ken Kloeber, branch office manager of MMI’s southeastern regional office in Greenville, the most challenging aspect of the project was to design a dam removal sequence and methodology that will get heavy equipment in and out of the river quickly. “The challenge is to minimize the exposure of the removal operations on the Deep River system and the aquatic critters that this entire project will benefit,” Kloeber explained. “We wanted to avoid short term effects while accomplishing the greater good in the long term.” “An additional challenge was that much of the planning involved working with and getting approval from many county, state, and federal agencies–sometimes with overlapping authorities–on a fast-track schedule,” Kloeber continued. “Hydropower dam demolitions involving FERC approval typically take years in the planning and design stage. With the owner, contractor, and agencies cooperating, this one took a matter of months.” “Every dam removal is unique because no two are constructed the same,” Kloeber said. “We had to first evaluate and understand the structural components of the spillway and the powerhouse before designing the demolition methodology and sequence. It’s typically easiest to demolish dams in reverse order of their construction sequence.” “The planning of the particular project has been especially interesting because we were able to locate original 1921 construction photos of the powerhouse and spillway in their de-watered state, which gave us insight into how the facility was constructed,” Kloeber stated. “It literally painted a picture of what was hidden under the water, and allowed us to anticipate in order to save time.” “Typically, dam removals involve starts and stops as structures are uncovered and new situations need to be assessed, Kloeber said. “Having the right information and evaluations of the structures up front has allowed this demolition to proceed very smoothly. To the credit of Restoration Systems, the owners recognized the need for adequate investigations right away, and they provided us the resources to make that happen.” “Because of its age and type of construction, the Carbonton Dam is eligible for the National Register of Historic Places,” stated Kloeber. “So, Restoration Systems has taken extra care to preserve the history of the structures.”

“Before starting demolition, detailed drawings of both the dam and powerhouse were created and the structures photo-documented in order to record the information that the site holds,” Kloeber said. “In the future an interpretive center at the park that Restoration Systems is creating will lay out the dam’s construction, contribution through history, and its demolition and the resulting restoration of the Deep River back to its indigenous state.” For nearly 80 years the dam was a functioning hydroelectric power plant. The 17-feet high and 270-feet long reinforced concrete dam and powerhouse were completed in 1926 as the first electrical power plant in the Sandhills. More recently, it supplied excess power to Progress Energy until 2004, when it was shut down due to operation and maintenance cost. The spillway was built using the “flat slab-buttress” type of construction. “This type of dam is named after Nils Ambursen, the Norwegian-born engineer who patented his design in 1903. The first of this type was built a year later, and by 1930 more than 200 Ambursen Dams were constructed, mostly in the eastern US,” Kloeber stated. “Its advantage was the efficiency of construction, because it reduced the concrete needed by about 40 percent compared to a mass concrete dam. The inclined face of the flat reinforced concrete spillway slab is supported by concrete pillars or buttresses every 17-feet across the river. Half are pinned into bedrock–while the others rest on a concrete foundation. The force of the water behind the dam is what holds the system in place, rather than the weight of concrete as in a mass-concrete dam. The system creates a wall of 17-foot x 17-foot hollow rooms across the Deep River.” Removal of the first of these sections began in November of 2005. According to Kloeber, the first phase of the demolition was the lowering of the impoundment behind the spillway in a step-wise fashion designed to minimize the impact on the river and aquatic species downstream. The lowering was scheduled for October, when releasing colder water had less impact on the dissolved oxygen in the river. Continuous water quality monitoring occurred during the critical release, and the results show much less impact than what happens during a typical rainstorm. “Initially for two weeks water was released through the turbine shaft and then through the mud gates in the powerhouse while flow was carefully evaluated. Once we knew it was safe to proceed, in sequence one of the 17 foot wide concrete gates in the spillway was slid sideways using an excavator–then the second gate–and finally the third gate,” Kloeber explained.” The final step was to remove 70-feet, or five of the hollow rooms of the spillway, which lowered the water to nearly to its pre-dam level.” “In order to gain access to and demolish the concrete spillway, Backwater is proceeding with removing approximately 2000-cubic yards of sand and gravel that is trapped in a sediment wedge behind the spillway and about 1600-cubic yards of woody debris and silt deposited as a 1/4-acre island of twisted tree limbs and trunks in the riverbed just above the dam,” Kloeber said. Kloeber explained, “The sand and gravel will be relocated to reconfigure the island so the material can be slowly reintroduced into the river system. The Deep River is starved of sediment downstream of the dam, and this will help restore that. This type material is critical to mussels and bottom organism in the aquatic environment.” Standard equipment was utilized at the job site. “When possible track hoe excavators were operated from dry land,” Kloeber said. “However when we needed to get in the water we specified diversion dams constructed of available material–the concrete from the spillway, and the sand and gravel in the sediment wedge.” Other equipment, which is all owned or rented by Backwater Environmental, included track dump trucks for hauling woody debris from the island and concrete, excavator mounted hydraulic hammers, and dozers. “The dam will be demolished by conventional methods without explosives,” Kloeber said. “Because the spillway is thin–18-inches–each of the rooms are easily removed by cracking the spillway slab half way up with the hydraulic hammer, then grabbing it with an excavator to peel it off the supporting buttresses. The free-standing buttresses are then rocked side to side with an excavator and dislodged from their bedrock foundation.” Every effort will be made to recycle and reuse debris from the demolition. “We have specified that Backwater recycle the 840-cubic yards of concrete rubble acquired from the spillway, “Kloeber stated. “The concrete can be used for toe protection to stabilize and control erosion of the newly exposed south riverbank, as fill material to establish rough grades for a parking lot at the new public park, or can be made available as fill to local contractors.” The Carbonton Dam removal is a 10-year project–with 5 of those years dedicated to monitoring how quickly the Deep River ecology improves. The removal of the Carbonton Dam is part of a project by Restoration Systems to fulfill a mitigation contract with the North Carolina Ecosystem Enhancement Program, an innovative state program charged with improving the state wetlands and watercourses using funds from the Department of Transportation and developers. Restoration Systems will monitor the Deep River ecological and water quality improvements for five years at 53 testing locations. Once restored, the Deep River will be an average of 200-feet wide and 3 to 5-feet deep, and will be used for canoeing, kayaking, and general river recreation. The Carbonton Powerhouse will be restored and refurbished to use it as a corporate retreat and conference center. The Carbonton project is one of two dam removal and river restoration efforts currently underway by Restoration Systems. The Lowell Dam site on the Little River in Kenly, NC, is in its final stages of site restoration after the dam was removed in December 2005. Complete demolition of the Carbonton Dam is expected to be complete by the end of January 2006, and the beginning of the restoration of the 10-miles of river can begin. Construction of the park is set to begin in the spring of 2006, with Restoration Systems eventually dedicating the property to The Triangle Land Conservancy of Lee County for public use, and will provide an endowment to operate and maintain the future five-acre River Park.