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Title: Research and design of Renwick Dam stepped spillway

Author
item Hunt, Sherry
item Kadavy, Kem

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/25/2008
Publication Date: 5/12/2008
Citation: Hunt, S., Kadavy, K.C. 2008. Research and design of Renwick Dam stepped spillway. In: Proceedings of the ASCE EWRI 2008 World Environmental & Water Resources Congress, May 12-15, 2008, Honolulu, HI. 2008 CDROM.

Interpretive Summary: The United States Department of Agricultural (USDA) Natural Resources Conservation Service (NRCS) has financially and technically assisted with the construction of nearly 11,000 small watershed dams in the U.S. Many of these structures will reach the end of their planned service life prematurely due to hazard classification changes directly related to the urbanization of the area surrounding the structure. To meet the dam safety requirements related to the change in hazard classification, the spillway often requires an increase in spillway capacity. NRCS expects 10% of their total structures to involve the design of roller compacted concrete (RCC) stepped spillway to address the needs of an increase in spillway capacity. Typical RCC stepped spillways for NRCS applications will be constructed on existing embankments where the downstream embankment face slope ranges from 2(H):1(V) to 4(H):1(V). Literature in this realm of stepped spillways is very limited. A specific study utilizing a two-dimensional, 1:8 scale physical model was conducted to evaluate the design flow in a 4(H):1(V) stepped spillway chute. Air entrainment was deemed insignificant for the design of the spillway training walls for this structure. Spillway drops ranging between 9.1 m to 12.2 m (30 to 40 ft) were tested with both stilling basin and riprap dimensions evaluated for dissipating the remaining flow energy. A spillway drop of 9.1 m (30 ft) spillway drop revealed undesirable flow conditions as the flow exited the stilling basin. A spillway drop of 9.8 m (32 ft) corrected the stilling basin performance and proved to be a viable design option. A spillway drop of 12.2 m (40 ft) indicated an alternative solution. The differences between the solutions were the dimensions of the stilling basin and the riprap necessary to protect the structure from failure. The ultimate design solution will depend on the feasibility of the design. This paper is intended to increase the knowledge and understanding of stepped spillways applied to relatively flat-sloped dam embankments and dimensioning of the energy dissipating stilling basin and associated riprap in the downstream channel.

Technical Abstract: The United States Department of Agricultural (USDA) Natural Resources Conservation Service (NRCS) has financially and technically assisted with the construction of nearly 11,000 small watershed dams in the U.S. By 2017, half of these structures will reach the end of their planned service life due to age, and others will need rehabilitation prematurely due to hazard classification changes as a result of urbanization and alterations in land use and topography. In many cases, spillways have inadequate spillway capacities. Approximately 10% of the 11,000 NRCS assisted structures are expected to have roller compacted concrete (RCC) stepped spillways to increase the spillway capacity of the existing structure. Typical RCC stepped spillways for NRCS applications will be constructed on existing embankments where the downstream embankment face slope ranges from 2(H):1(V) to 4(H):1(V). Literature in this realm of stepped spillways is very limited. A specific study utilizing a two-dimensional, 1:8 scale physical model was conducted to evaluate the design flow in a 4(H):1(V) stepped spillway chute and the effects air entrainment has on the design of the spillway training walls and stilling basin dimensions. Water surface and bed profiles were collected during testing. Observations show that the air entrainment inception point for relatively short spillway chute drops up to 12 m (40 ft) is expected near the bottom of the spillway chute for prototype flows higher than 7.0 m2/s (75 cfs/ft). For these flows, air entrainment is not expected to influence the design of the training walls significantly especially under high tailwater conditions. Spillway drops from 9.1 m to 12.2 m (30 to 40 ft) were tested with both stilling basin and riprap dimensions evaluated for dissipating the remaining flow energy. For the 9.1 m (30 ft) spillway drop, the stilling basin required to dissipate the remaining energy was approximately 36% longer than the stilling basin required for the 12.2 m (40 ft) spillway drop. Additionally, the end sill for the energy dissipating stilling basin for the 9.1 m (30 ft) spillway drop was 2 times the height required for the 12.2 m (40 ft) spillway drop. Yet, the hydraulic performance of the stilling basin for the 9.1 m (30 ft) spillway drop was undesirable, so the spillway was further tested with a drop height of 9.8 m (32 ft). This paper is intended to increase the knowledge and understanding of stepped spillways applied to relatively flat-sloped dam embankments and dimensioning of the energy dissipating stilling basin and associated riprap in the downstream channel.