Date of Award:
Master of Science (MS)
Civil and Environmental Engineering
J. Paul Tullis
The purpose of this research was to investigate the hydraulic performance of stepped spillways. A thorough investigation was made of all printed material on stepped spillways, and a summary of this material is presented in the text. Data from experiments in the laboratory were used to develop a design procedure for stepped spillways and hydraulic jump stilling basins. The experimental study was conducted at Utah State University - Utah Water Research Laboratory in Logan, Utah. Four models were built and tested in the laboratory under various flowrates.
The crest of the model spillway was constructed in the shape of a standard USBR nappe-shaped crest. Small steps were fitted to the crest so that the envelope of their tips just intersected the crest profile. These small steps allowed a smooth transition of the flow from the nappe-shaped crest to the constant slope region. Two slopes were tested in the research: 0.7H:1.0V and 0.5H:1.0V. At each of these slopes two sizes of steps were tested. Steps did not very in size down the face of the model, although step sizes varied on the different models. The model height and steps tested were for a dam with a prototype height of over 67 feet and steps in the 1-2 foot range. Diagrams and pictures of the four models tested are included in the text and appendix.
Findings from the research showed that given a ratio of step height over critical depth (0.1201.897, the energy dissipation will remain in the 90% region, although there might be problems with the flow leaping away from the structure. Data on the models were slightly scattered, but all of the data from the four models agree to one energy loss graph. Support for these data showed a nearly perfect correlation in the downstream conjugate depths on all four models at similar flowrates.
By using data obtained from the models and literature, a design process with guidelines for designing a stepped spillway is presented. This process includes crest design and step displacement in the transition region, approximate step size, and approximate slope necessary for adequate operation of the stepped spillway.
By taking data collected from the models, and data from USBR design manuals for smooth spillways, a spreadsheet design process was created that compared the size of stilling basins required using either a smooth spillway or a stepped one. Results showed that given a unit flowrate range of 15 cfs/ft to 140 cfs/ft and spillway height of approximately 100 ft, the stilling basin volume was reduced by 62% to 43% respectively. This size reduction can translate to a considerable cost savings in prototype construction.
This study, along with data from other researchers, has proven that a stepped spillway can greatly increase the amount of energy dissipation over that achieved on a standard smooth face spillway. The stepped concept can be used as an excellent energy dissipator and in some cases can totally remove the need for any type of dissipator at the toe of the spillway.
Rau, Jeffrey Scott, "Design and Evaluation of Stepped Spillways for High Dams" (1994). All Graduate Theses and Dissertations. 4045.