Hydro Hall of Fame: Celebrating 100 Years at Ninety-Nine Islands

Duke Energy’s 18-MW Ninety-Nine Islands hydroelectric facility has provided a reliable source of clean, renewable power to the Piedmont Carolinas for 100 years and is positioned to continue operating well into the future. Ninety-Nine Islands is one of the 2010 inductees into the Hydro Hall of Fame.

The 18-MW Ninety-Nine Islands hydro station, near Blacksburg, S.C., was commissioned in 1910 and thus celebrated 100 years of service on May 27, 2010. At the time it was built, this project was part of Southern Power Company’s rapidly growing generation portfolio, joining three other hydro stations on the Catawba River – 6.6-MW Catawba Hydro (completed in 1904), 24-MW Great Falls Hydro (completed in 1907), and 28-MW Rocky Creek Hydro (completed in 1909).

The roots of Duke Energy Corporation can be traced to this early interconnected network of hydro stations that brought a newfound economical and reliable source of electricity to the Carolinas. This fledgling collection of hydro stations would provide the “spark” that would drive economic growth and spur a transformation in this sleepy rural region of the South.

Duke Energy and Ninety-Nine Islands early history

In 1905, James B. Duke, Benjamin N. Duke, Dr. Gill Wylie, and W.S. Lee launched Southern Power, the primary predecessor that eventually grew to become Duke Energy. Their initial focus was to meet the growing need for electricity in mills and factories, thus spurring industrial growth in the Carolinas’ textile industries. Prior to this time, most generating plants were rather isolated and served a single nearby mill, factory, or city.

However, this group of men had a larger vision for a series of generating plants, interconnected by high-voltage transmission lines that would form a single combined system. This vision would include the nation’s first comprehensive development of an entire river, the Catawba, which ultimately would provide power from 13 hydro stations on 11 reservoirs. Not only would this be a means to provide more reliable electric service, but it would allow a more integrated and efficient use of multiple hydropower resources.

Construction of the 18-MW Ninety-Nine Islands hydroelectric project was completed in 1910. The resulting reservoir covered about 885 acres, with about 14 miles of shoreline.

By 1909, the vision of an interconnected system also was taking shape as Southern Power placed into service the industry’s first double-circuit, 100-kilovolt line stretching 190 miles from Great Falls, S.C., to Durham, N.C. Additionally, the company soon established linkages with other utility companies and gained national recognition for creating a pioneering “system of systems.”1

The 18-MW Ninety-Nine Islands powerhouse has undergone significant rehabilitation but still retains much of its historical integrity.

Among the earliest chosen hydroelectric sites was a location known as Ninety-Nine Islands on the Broad River. This site had been associated with industrial developments long before the hydro project was planned. King’s Creek, which drains into the Broad River immediately downstream of the facility, was the location of an iron manufacturing plant from about 1815 until the late 1860s. Having already thoroughly scoured the Carolinas to find the best hydroelectric sites, Southern Power engineers considered the Ninety-Nine Islands site to be one of the best prospects.

So, in early 1906, bankrolled by the Duke brothers’ tobacco fortune, the company set out to acquire the land and riparian rights necessary for construction of the project. A year later, Southern Power had title to all the necessary land and began construction of a railroad track to the site, as well as offices, warehouses, and housing for the construction crews.

By April 1907, Southern Power was aggressively developing and building multiple projects simultaneously. Even before the plant at Great Falls was completed, Southern Power was in the midst of building another hydro plant at Rocky Creek on the Catawba River, as well as beginning the Ninety-Nine Islands project. An economic downturn in late 1907 forced work at Ninety-Nine Islands to cease temporarily as Southern Power focused its resources on completing the construction of Rocky Creek Hydro. In 1908, the company resumed its efforts at Ninety-Nine Islands, contracting with B.H. Hardaway Company to built this new facility.

Construction of the facility began with the installation of a cofferdam upstream of the dam site between Stroup Island and the eastern bank of the Broad River. A second cofferdam was constructed downstream of the dam site. Granite for the Ninety-Nine Islands Dam was quarried from the Broad River and forms the core of the 891-foot-long, 86-foot-high cyclopean concrete dam. In the early 1900s, cyclopean concrete was a common construction technique in which large stones, many weighing more than several hundred pounds, are placed and embedded in the concrete as it is deposited.

When the project became operational in 1910, the reservoir covered about 885 acres with about 14 miles of shoreline. The six generating units contained Allis-Chalmers horizontal back-to-back Francis turbines with a total generating capacity of 18 MW. The powerhouse itself contained two sections: the generator bay and a transformer house. The generator operating floor rests on a masonry base with six arches above the tailrace and features many decorative elements, including multi-light and butterfly windows, a narrow clerestory, and a slate roof. The transformer house is a two-story steel frame structure clad in red brick that houses the transformers, control room, choke coils, and other electrical equipment.

Preparing for the next 100 years

By the late 1990s, the turbine-generating units and many other auxiliary components at Ninety-Nine Islands had deteriorated with age and were in need of a major rehabilitation effort. In addition, heavy transport of silt and debris, common to the Broad River basin, increasingly was creating intake blockage problems for the generating units after high water situations.

As part of Duke Energy’s overall system-wide upgrade program, known as HydroVision, a comprehensive upgrade plan was initiated in 1996. The goal was to extend the life of the Ninety-Nine Islands facility for at least another 40 years. Extensive reviews were conducted to assess all civil, electrical, and mechanical rehabilitation needs. As results were received, appropriate adjustments had to be made to the scope of the work in order to maintain alignment with the economic business case for the project.

First, analysis for extreme high water conditions showed that dam remediation would be required to increase the factor of safety for preventing dam sliding during a Probable Maximum Flood (PMF) event. To meet these modern Federal Energy Regulatory Commission (FERC) guidelines, a series of 48 post-tensioned anchors were installed to tie down the spillway and bulkheads during work in 2004 and 2005.

Next, to facilitate extensive civil rehabilitation work in the forebay at Ninety-Nine Islands, an eight-month station outage was scheduled in 2006. This complete station shutdown provided additional safety for dive teams and personnel working from barges while also maximizing productivity of construction teams and equipment. Work performed during this outage included:

– Replacing the old timber head gates with new steel head gates containing integral pad gates;

– Installing a drag rake trash removal system to work in concert with new trashracks and a new log boom to help keep the forebay clear of trash and debris accumulation;

– Installing a new 25-ton powerhouse crane and realigning the crane rails in preparation for unit upgrades;

– Upgrading Units 1 and 3 through installation of new turbine-generator equipment, replacement of wearing rings, and complete overhauling of the turbine operating mechanisms;

– Rehabilitating Units 2 and 4, including overhauls of the gate cases; and

– Upgrading station auxiliary systems, including compressed air, service water, bearing oil, and governor oil systems.

As part of the electrical scope review, protective relaying and hardwired control systems installed during a 1989 electrical upgrade were deemed viable for continued long-term service. However, significant upgrades to generator excitation, direct current power systems, and controls for supporting remote operation were needed to remain consistent with Duke Energy’s HydroVision program objectives and equipment standards. All of these upgrades were completed in 2006 and early 2007.

Granite quarried from the Broad River formed the core of the 891-foot-long Ninety-Nine Islands Dam.

In spite of the installation of numerous new components, electronic controls, and auxiliary equipment, Ninety-Nine Islands retains much of its historic integrity. Operationally required upgrades throughout the history of the station have resulted in numerous changes to the architectural features, but these changes have not diminished the integrity of the facility’s historic setting or the role that Ninety-Nine Islands played in the development of the region. As a result, the South Carolina State Historic Preservation Office determined in 2001 that the Ninety-Nine Islands Project was eligible for inclusion in the National Register of Historic Places.

Finally, as a result of the upgrade efforts, in October 2007, FERC certified the Ninety-Nine Islands incremental generation as being eligible for production tax credits. Duke Energy requested the certification based on increased efficiency from upgrades to the turbine-generators that were completed in March 2007. FERC’s order certified a historical average annual generation baseline of 68.8 gigawatt-hours (GWh), anticipated generation with improvements of 74.668 GWh, and incremental generation of 5.868 GWh. FERC certified the percentage of generation due to improvements as 8.53 percent.

Back to the future

A look ahead shows that the Ninety-Nine Islands project also is going to be extremely important as the station begins its second century of service. In addition to continuing as a clean, renewable source of generation, the reservoir is expected to serve as a principal cooling water source in support of the proposed 2,234-MW Lee Nuclear Station just upstream from the dam.

For more than 100 years, Ninety-Nine Islands has played an important role in the development of the Carolinas. With recent upgrades and a planned nuclear station on the horizon, Ninety-Nine Islands is clearly positioned to remain an important, viable renewable energy and water resource well into the 21st century.

Jennifer Huff is senior environmental resource manager and Greg Lewis, PE, is technical manager with Duke Energy. Huff supports hydro project relicensing and historic preservation, and Lewis provides technical direction and upgrade support across Duke’s fleet of 30 hydro stations.



  1. Durden, Robert F., Electrifying the Piedmont Carolinas, Carolina Academic Press, Durham, N.C., 2001.


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