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Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 12/4/2009 Publication Date: 3/1/2010 Citation: Hunt, S., Kadavy, K.C. 2010. Energy dissipation on flat-sloped stepped spillways: Part 1. Upstream of the inception point. Transactions of the ASABE. 53(1):103-109. Interpretive Summary: In recent years, dam safety issues are on the rise due to environmental changes including urbanization. Consequently, many of these dams require increased spillway capacity in order to meet state and federal dam safety regulations. Stepped spillways applied over the top of existing embankment dams has become a popular choice to address dam safety concerns related to inadequate spillway capacities. Typically, the chute slope has the same slope as the downstream embankment face or in some cases have the same slope as the existing auxiliary spillway. Design guidelines and literature in general for these stepped spillways are very limited, so further research on these stepped spillways is warranted. A two-dimensional, physical model was constructed to evaluate flow velocities, energy dissipation, and where along the length of the chute the flow starts becoming an air water mixture. The findings from this research found that relationships provided in previous literature can be used to determine where along the length of the chute the flow starts becoming an air water mixture. This is important because air within the spillway flow can impact the design of the training walls meant to contain the flow in the spillway. Additionally, the velocity profiles showed unique patterns that can be described by mathematical relationships. The results on energy show that energy losses increase in a linear fashion from near zero at the crest to approximately 30% near the point where the flow starts becoming an air water mixture. This research is expected to assist engineers with the design of stepped spillways applied on relatively flat embankment dams. Technical Abstract: In recent years, hazard classifications for many existing embankment dams have changed because hydrologic conditions have been altered. Consequently, many of these dams require increased spillway capacity in order to meet state and federal dam safety regulations. Stepped spillways have become a popular choice for providing increased spillway capacities to existing embankment dams. Stepped spillways in these applications are typically placed over the existing embankment. Consequently, the chute slope has the same slope as the downstream embankment face or in some cases the same slope as the existing auxiliary spillway. Typical slopes for existing embankments range from 2(H):1(V) and flatter. Design guidelines and literature in general for these stepped spillways are very limited, so further research on these stepped spillways is warranted. A two-dimensional, physical model was constructed to evaluate the inception point, velocities, and energy dissipation in a 4(H):1(V) slope spillway chute having 0.04 m high steps. Model unit discharges ranging from 0.10 m2/s (1.1 cfs/ft) to 0.82 m2/s (8.9 cfs/ft) were tested. Water surfaces, bed surfaces, and velocities were collected during the tests. The findings from this research were that relationships provided by Chanson can be used to determine the inception point for flatter slopes. Additionally, the velocity profiles transition from uniform at the crest to approaching a one-sixth power law distribution at the inception point for all tested flows. The results on energy show that energy losses increase in a linear fashion from near zero at the crest to approximately 30% near the inception point for all tested flows. This research is expected to assist engineers with the design of stepped spillways applied on relatively flat embankment dams. |
