Research and Design of Renwick Dam Stepped Spillway

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.