Harmonizing Dam Rehabilitation with Floodplain Management

Gwinnett County operates and maintains 14 NRCS dams. The structures were constructed by the Soil Conservation Service throughout the 1960's, 70's, 80's, and 90's.as low to medium hazard dams. Each dam is located within the suburbs of metropolitan Atlanta, 30 minutes to 1 hour drive from downtown Atlanta. The surge in population and growth of these areas results in development around these structures. Recently, twelve (12) of these dams have been reclassified as high hazard structures in which a probable loss of life is predicted in the event of a dam breach. Gwinnett Count has undertaken an aggressive capital improvement project to upgrade the NRCS dams to comply with the high hazard structure design criteria.
In the last 4 years Gwinnett County has rehabilitated 3 NRCS watershed structures including the Yellow River Watershed Dam Nos. 14 and 17 and the No Business Creek Watershed Dam No. 1. The latest structure scheduled for rehabilitation is the Yellow River Watershed Dam 15 (Y-15). Y-15 is a project that has been ongoing for Gwinnett County since the year 2000 with sometimes insurmountable political, technical, and financial hurdles. Residents did not support the project and application of the design criteria resulted in massive and costly spillway improvements to bring the structure into compliance with the high hazard design criteria. In mid 2004 Golder Associates Inc. was contracted by Gwinnett County for the design of the conceptual solution proposed by previous consultants. Golder's evaluation of the conceptual design identified several modifications could be made to the preliminary design parameters. During preliminary design, the proposed spillway was significantly modified such that the size of the chute was reduced by 30% (over 200 linear feet) and the proposed ogee weir was replaced by a broad-crested weir. These changes were possible by modifying the spillway crest to permit additional flow during more frequent storm events, but the FEMA mapped 100 year floodplain remains unchanged. Golder's proposed reduction in the size of the spillway structure aided in the final acceptance of the project by local residents.
The proposed spillway layout is also a unique shape in that traditional design methodologies may not be applicable. A model study is planned in the next several months to evaluate the spillway's hydraulic performance.
This presentation will discuss the unique design methodologies and the results of the proposed model study. 9 pp. 2 references.