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Lake Johnson Dam Overtopping Protection

Raleigh, North Carolina

Structure Rehabilitation


City of Raleigh, North Carolina


Hazen and Sawyer Environmental Engineers and Scientists


Faulconer Construction Company


Summer 2011

Technical Description:

  • ArmorFlex® 88,826-SF tapered block mat

When Tropical Storm Alberto dropped 7.2-inches of rain, the spillway structure of Lake Johnson Dam in Raleigh, North Carolina, suffered damage. After completing initial emergency repairs within six weeks following the storm, the City of Raleigh pursued additional improvements to the dam and spillway to create adequate capacity to comply with North Carolina dam safety regulations.

Built in 1923, this medium-sized, high hazard dam had a 50-foot spillway that was less than 20% of the current required 1/2 PMP (probable maximum precipitation) level requirements — approximately 20 inches of rainfall for Raleigh. Building a new or auxiliary spillway was considered. Ultimately, the City chose to rehabilitate the dam by raising it one foot and installing hard armor for overtopping protection. This solution was the most cost effective, and saved an estimated $1 million.

To increase its capacity to better handle severe rain events and meet current regulations, Lake Johnson Dam was raised one foot, downstream slopes were flattened to 3:1, and the dam’s surface was hard armored for overtopping. Additionally, manually-operated sluice gates at the intake tower were replaced with motorized gates.

The City of Raleigh hired Hazen and Sawyer, a firm of environmental engineers and scientists, to facilitate the project. Hazen and Sawyer chose the Armortec® ArmorFlex® hard armor articulated concrete blocks from Contech Engineered Solutions to protect the dam from overtopping.

ArmorFlex is a flexible, interlocking matrix of cellular blocks, laced together longitudinally with revetment cables to form hard armor mattresses. The open-cell Class 40t block includes a tapered one-half-inch difference in thickness from one side to the other to ensure water cascades from block to block in the mattress configuration of the installation. This product is a solid solution for high hazard, high velocity and hydraulic jump applications. Plus, the tapered design eliminates projection height, resulting in a zero-inch impact zone.

Further, since the open-cell design allows for vegetation growth, the aesthetics for this popular recreational area were not compromised.

Product material was made locally, and proved beneficial since there was limited storage room for materials on-site. Even the challenging curved design of the dam wasn’t an issue since the ArmorFlex mats could be made in varying sizes to accommodate the curves, which provided ease-of-handling and rapid installation.

General contractor Faulconer Construction secured subcontractor Vertical Walls to install the 88,826-SF of ArmorFlex articulated concrete block mats. They started installation at the smaller south end of the dam, and then moved to the larger north section of the dam.

ArmorFlex from Contech Engineered Solutions proved to be the best solution for logistical, functional, aesthetic and economic reasons.

Technical Description:

  • ArmorFlex® 88,826-SF tapered block mat

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