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Title: PHYSICAL MODEL STUDY OF A PROPOSED CONVERGING RCC STEPPED SPILLWAY FOR BIG HAYNES CREEK WATERSHED SITE H-3 IN GWINNETT COUNTY, GEORGIA

Author
item Hunt, Sherry
item Kadavy, Kem
item Temple, Darrel

Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/15/2006
Publication Date: 9/10/2006
Citation: Hunt, S.L., Kadavy, K.C., Temple, D.M. 2006. Physical model study of a proposed converging RCC stepped spillway for Big Haynes Creek Watershed Site H-3 in Gwinnett County, Georgia. In: Dam Safety 2006. Proceedings of the Association of State Dam Safety Officials Annual Conference, September 10-14, 2006, Boston, Massachusetts. 2006 CDROM.

Interpretive Summary: It is necessary to increase the flood storage and/or spillway discharge capacity of many older dams because of changes that have taken place in the watershed. Urbanization upstream of the dams increases the rate of runoff into the dams, and urbanization downstream increases the consequences of dam overtopping and potential failure. Development around the structure itself increases land values and limits the options available for rehabilitating or modifying the dam and reservoir. These constraints have resulted in the use of converging Roller Compacted Concrete (RCC) chute spillways constructed over the top of existing dams to increase spillway capacity and dam safety. A study was undertaken by ARS to develop needed criteria for the economical engineering design of this type of spillway. A physical model study of a proposed spillway design was used to determine the performance of training walls designed to contain the converging flow in the chute spillway. Use of sloping sidewalls with steps typical of RCC construction practice was found to perform poorly for the tested conditions and alternate designs were investigated. Removing the construction steps from the sloped sidewalls was found to be an alternative that yielded acceptable performance. The results of this study may be used to develop designs for safe RCC converging chute spillways that minimize construction costs for conditions similar to those studied. Because of the large number of dams constructed with the assistance of the USDA that are now reaching the end of their planned service life and the expansion of urban communities into previously rural areas, potential savings from development of improved designs are substantial.

Technical Abstract: The USDA-NRCS has assisted with the design and construction of nearly 11,000 small watershed dams in the U.S., and by 2016, nearly one-third of these structures will reach the end of their planned service life. Time takes a toll on these structures. Some face structural integrity problems while others have sediment filled sediment pools and sedimentation of the flood pools. In some cases, development surrounding the dams has caused a change in hazard classification. Since many of these structures no longer meet federal and state dam safety requirements, the dam safety community has become increasingly aware of their failure potential. For these structures to continue to function properly and safely, rehabilitation is required. If ignored, these dams could place life and property at risk. Big Haynes Creek Watershed Site H-3 in Georgia is one of the dams that the USDA-NRCS assisted with the design and construction. Since its construction in 1963, the dam has experienced a hazard classification change that requires an increase in spillway capacity. Because of extensive development in the area, a converging roller compacted concrete (RCC) stepped spillway was proposed over the existing dam as a means of providing the additional spillway capacity. The Agricultural Research Service (ARS) performed a physical model study of the proposed design using a 1:22-scale model. The study indicated that the use of sloping stepped sidewalls in the converging section of the spillway did not perform well and that smoothing and extending the walls improved overall performance. Lengthening the stilling basin and extending the stilling basin end sill up the sloping basin walls also improved the performance. This report documents these findings.