Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 6/5/2006
Publication Date: 7/9/2006
Citation: Hunt, S.L., Kadavy, K.C. 2006. Physical model study of a proposed converging RCC stepped spillway. In: ASABE Annual International Meeting, July 9-14, 2006, Portland, Oregon. Paper No. 06-2171. 2006 CDROM.
Interpretive Summary: As existing earthen embankments age, hazard reclassification of the structures are often necessary due to the increase urbanization in the vicinity of the dams. The changes in hazard classification often result in a need for dam in order to meet current dam safety requirements. A growing popular choice for dam rehabilitation of these types of structures is roller compacted concrete (RCC) stepped spillways. This model study was a specific request by the Georgia NRCS in order to provide design guidance for a converging RCC stepped spillway for the rehabilitation of site H-3, and this document provides a summary of a small-scale converging stepped spillway tested at the USDA-ARS Hydraulic Engineering Research Unit. The effect of flow convergence on particular design parameters like the spillway training wall height and necessary stilling basin length was the primary point of interest during these tests. Observations and results from this study showed that the flow overtopped the training walls at the maximum tested flow. Additionally, the nature of flow exiting the spillway indicated a need for extending the length of the stilling basin. The design recommendations outlined in this report do not account for the effects of air in the water. Air entrainment may have little significance on the flow depth for this particular design or for spillways having similar design features. This research provides specific design recommendations for the Georgia NRCS site H-3. Researchers hope to gain a better understanding of how the convergence of flow affects some of the design components of the spillway. This research has the potential to provide design aids to field engineers faced with designing these structures.
Technical Abstract: Nearly half of the 10,000 small watershed dams constructed in the U.S. with the assistance of the NRCS will reach the end of their proposed service life in the next 10 years. While many of these structures were originally built to protect agricultural land from floods, urbanization around some of them has; thereby, changed the hazard classification of these dams. To meet current federal and state dam safety standards, rehabilitation of these structures are necessary. If ignored, these dams could place life and property at risk. The Georgia NRCS requested a specific model study for a proposed design of a converging roller compacted concrete (RCC) stepped spillway for the rehabilitation of Big Haynes Creek Watershed Dam Number 3 (H-3) in Gwinnett County, Georgia. The USDA-ARS Hydraulic Engineering Research Unit conducted a three-dimensional, 1:22 scale physical model study to evaluate the hydraulic performance of this structure. The results of the study indicated that the proposed height of the spillway training walls will be overtopped by the maximum expected discharge. Air entrainment was not shown to significantly influence the depth of flow at the highest tested flow rate based on the calculated and observed locations of the air entrainment inception point. However, complete interpretation and/or extrapolation of the data for the prediction of prototype behavior for similarly designed structures may require the addition of an appropriate scaling factor to account for air entrainment depending on the location of where air entrainment starts. Other significant design recommendations resulting from this study include the extension of the stilling basin downstream for added protection of the structure and downstream channel. The design recommendations and overall performance observations resulting from this model study are discussed in this report.