Rehabilitation of Unicoi State Park Lake Dam, White County Georgia, A Case Study

Abstract Only - The Unicoi State Park Dam is an earth embankment dam with a crest length of approximately 740. The crest is approximately 75 feet above the downstream toe from bottom of channel. The dam impounds a 30 acre recreation lake at Unicoi State Park.

In April 1984, the spillway drain outlets discharging from the underdrain system were reported to be flowing under pressure. In 1985, repair work was performed on the spillway, but in 1986 excessive flows form beneath the slab were again observed. The lake was drained in April 1987 to facilitate additional repairs. This construction was performed and completed in May 1987. In April 1988, however, a flow of 90 gallons per minute was observed form the drain trench outlets with minimal discharge over the spillway. Later observations indicated increasing flow from the drain outlets. As a result of these observations, as well as concern about the capacity of the spillway, the structure was deemed unsafe by the Georgia Dam Safety Program and the State Park and Historic Sites Division of the Department of Natural Resources was directed to correct the spillway problem. The dam which carries a state highway (Ga. 356) on its crest is located on a stream about 2.4 miles upstream of the town of Helen, a popular summer resort in the foothills of the Blue Ridge Mountains. The failure of this dam would inundate the entire town of Helen.

In order to meet current safe dam criteria, a new larger capacity spillway was required. A larger capacity spillway could not be placed in the location of the existing spillway. Still the highway needed to be open during construction and be part of the final solution. The solution consisted of a 35-ft cut into the existing embankment and building a new 16-ft high, 3 cycle labyrinth (zigzag) crested spillway. A single 120-ft long bridge spans the spillway with a clearance of 7-ft to accommodate the design flow of 0.65 PMP.

This paper will examine the restraints placed on rehabilitation work that led to the ultimate innovative solution including 35-ft high training walls for the new spillway. It will also examine the significance of the Unicoi State Park Dam, and the hazard the dam presented to the City of Helen.