Rehabilitation of the Houston Lake Dam, Houston County, Georgia

The Houston Lake Dam, located on Mossy Creek in the central portion of Georgia, failed shortly after midnight on July 6, 1994 due to excessive overtopping flows associated with Tropical Storm Alberto. The dam, originally constructed in the early 1840's, had been used as a mill site during the Civil War. Historical research on the dam indicated the dam had been overtopped several times requiring substantial repairs. The latest major repair took place in 1969 when the spillway structure was damaged due to heavy rains and was subsequently rebuilt and renovated.

Houston Lake Dam, under the Georgia Safe Dams Act, is classified as a Category I Dam based on the lake's storage capacity and is required to pass the flow associated with 50% of the Probable Maximum Precipitation (PMP). The Phase I inspection report prepared in May 1980 indicates that the dam would be overtopped by 14.5 feet during the Probable Maximum Flood and was considered unsafe in its condition at the time of that report.

Damage to the dam was extensive due to the overtopping flows associated with Tropical Storm Alberto. The breach that developed during the failure was adjacent to the principal concrete outlet structure and reached a maximum width of approximately 130 feet. One of the discharge bays of the concrete outlet structure was completely ripped off along with the bridge decking. The dam's earthen embankment was also overtopped and suffered severe erosion on the downstream slope in several locations.

Redesign of the dam began in early 1995 with preliminary hydrologic and site surveys. The dam has a 112 square mile drainage basin consisting of primarily agricultural uses. Due to property constraints and the large flows required to be passed through the dam it was quickly determined that the most feasible method for rebuilding the dam would involve overtopping the dam during the large storm events. Other site constraints included Georgia Highway 127 that parallels a significant portion of the dam and the bridge over Mossy Creek that is approximately 300 feet downstream of the dam.

The selected spillway configuration utilizes a concrete labyrinth principle spillway and a 650 feet long overtopping section on the dam's earthen embankment. The labyrinth spillway consists of six cycles with an effective weir length of 1130 feet. The combined footprint of the labyrinth weir structure and drop stilling basin is approximately 209 feet by 138 feet. The overtopping section is activated by storms associated with rainfall events greater than the 1/3 PMP and is protected with Armorflex Class 60 mats, a cellular concrete block revetment system manufactured by Armortec.

Geotechnical concerns for the dam were addressed by the installation of a clay blanket on the upstream face that tied into a slurry wall located approximately ten feet from the upstream toe. The slurry wall was extended into the coastal plain soils beneath the dam.

The Georgia Safe Dams Program approved the design in April 1997 with the construction contract issued to Cline/RE Joint Venture, LLC for $2.49 million in June 1997. Construction of the renovations to the dam are proceeding and are scheduled to be completed by May 1998. 12 pp., 13 references.