Design of Chastain Meadows Regional Stormwater Management Facility, Cobb County, Georgia

The proposed 28-foot high Chastain Meadows Dam, to be constructed and owned by the Cobb County Water System (CCWS), is the first of nine dams planned by CCW5 to control flooding and improve water quality an a regional basis within Cobb County, Georgia. Cobb County, located northwest of Atlanta, Georgia, is one of the fourteen Metropolitan Atlanta counties. This county has experienced tremendous development and growth in the last 20 years.

The dam is to be constructed across die rather wide floodplain of Noonday Creek in a part of the county where property values have increased significantly. The increase in property values is a result of the close proximity of the dam site to a regional shopping mail (Town Center) at the junction of two interstate highways (I-75 and I-575). Land acquisition costs for an earthen fill embankment with a traditional earthen emergency spillway in an abutment would have been prohibitively expensive, as well as politically difficult, thus eliminating the project for consideration. Elimination of this project could have dealt the regional flood control and water quality improvement program a serious setback. For the project to be viable, the dam would have to be an overflow structure. One of the, challenges was the presence of alluvial soils to depths of 25 feet and the significant costs for removal, if the dam was constricted on competent material (either rock or residual soils). Another challenge was the lack of readily available and economically obtainable soil backfill for construction. McGill- Schnabel evaluated a wide-range of embankment configurations to include a mass concrete gravity structure constructed on rock, and buttress dams on either a rockfill pad or a stabilized soil block. The presence of one of the largest granite quarries in the world within 2 1/2 miles from the site made a rockfill dam with concrete facing a potential option, also. The final design concept involved leaving the extensive amount of alluvial material within the foundation and constructing an earthen embankment with 4H: 1V side slopes across the floodplain. A 570-foot long section of the embankment will be given overtopping protection with a toiler compacted concrete (RCC) overlay. The RCC overlay permitted the use of a soil embankment, the least expensive alternative, even with the costs associated with importing borrow material. Normal flows and all flows up to and including the 2-year event win be discharged through a 30-fbot wide chute. The dam will be a high-hazard Category I structure in that dam failure could result in the probable loss of human life. Therefore, the dam has been designed to the standards specified in the Georgia Safe Dams Act and the Rules for Dam Safety.

As the design concept was being finalized, the owner decided to shift the dam location downstream in order to "partner" with a local developer. The "partnering" would give the owner an economical source of borrow material, reducing the cost of the project.

This project is noteworthy in that an RCC overlay is being used on a now earthen embankment constructed on a very soft foundation, something that has probably never been attempted before. Also. RCC has been proven to be a cost-effective alternative when compared to other more traditional methods of dam construction, even considering the many constraints placed on the project. The partnering of the county with a local developer to further reduce costs created a "win-win" situation and added another dimension to the project. 11 pp., various maps.