Lake Johnson Dam Emergency Repairs and Overtopping Protection

Client: City of Raleigh Public Utilities Department
Location: Raleigh, NC

Lake Johnson Dam, originally constructed in 1923, is a “medium” size, “high” hazard structure which impounds a recreational lake and also hosts one of Raleigh’s most popular pedestrian trails. Spillway overtopping during Tropical Storm Alberto in 2006 resulted in undermining of the right spillway training wall as the lake level nearly rose to the crest of the 45-foot high earthen dam. The City of Raleigh retained Hazen and Sawyer to design and manage construction of emergency repairs and conduct a comprehensive dam safety evaluation and determine how the spillway could be improved to adequately pass the design flood event.

Overtopping of the spillway training wall during Tropical Storm Alberto on June 14, 2006.

Damage and erosion observed behind the right spillway training wall.

Permanent repairs to the right spillway training wall were designed, permitted, and constructed within two months of the storm event.

Site grading and preparation for installation of articulated concrete block (ACB) revetment system.

Placement of articulated concrete block (ACB) revetment system.

Aerial showing construction of articulated concrete block (ACB) revetment system.

Downstream swale repair and stream bank stabilization.

Residences and infrastructure located immediately downstream of Lake Johnson Dam.

Lake Johnson Park is one of Raleigh’s most popular recreational areas.

Project Outcomes and Benefits

  • Hazen and Sawyer staff responded immediately following the storm event to perform a damage assessment in the field and the initial stabilization project was completed within 48 hours.
  • We designed permanent modifications to the right training wall to prevent future overtopping and also designed repairs to the spillway slab. These repairs were designed, permitted, and constructed within two months of the storm event.
  • We overcame design and construction challenges associated with armoring the earthen dam embankment, including construction phasing and maintaining public access to the lake’s popular trails.
  • Hazen and Sawyer prepared construction plans and technical specifications, provided construction monitoring and record documentation, and was responsible for state dam safety certification.
  • Downstream from the spillway, we designed a step pool channel to dissipate flow from a stormwater outfall and imbricated riprap wall to stabilize the eroding streambank near a pedestrian bridge.

Hydraulic analyses indicated that the spillway had less than twenty percent of the required capacity and that, under full build-out conditions in the lake’s watershed, the dam could be overtopped by a storm event equivalent to Tropical Storm Alberto. Such conditions would result in significant risk to the dam and downstream roads, structures, and residents.

Hazen and Sawyer evaluated several alternatives to bring Lake Johnson Dam into compliance with state dam safety regulations, where it was determined that armoring the earthen dam embankment with articulated concrete block (ACB) to withstand overtopping flows was the optimal remedial approach.

For more information on this project, or to discuss a similar project in your area, contact Jeffery Powers at jpowers@hazenandsawyer.com