Application of a Mitered Ogee Crest to Steep Stair-Stepped Spillways

Design flow requirements for both new and existing dams have increased over the past 30 years due (in part) to a series of dam failures caused by embankment overtopping. Increased spillway design flow requirements often require the renovation and re-design of the spillway crest for increased discharge without increasing reservoir pool elevation. Mitered ogee crests can be a viable alternative to the use of traditional ogee crests and to replacing broad crested weirs for increasing the discharge capacity of an existing spillway. Although the use of a traditional ogee crest would be the optimum design for increasing discharge, the use of a mitered crest would be simpler and more economical to construct than the profile of a traditional ogee crest. The application of a mitered ogee crest with a stair-stepped spillway is the subject of the following paper. Currently, there is little research available regarding the design of a mitered ogee crest. This paper discusses the discharge capacity of a mitered ogee crest and its application to stair-stepped spillways. A comparison of the performance and capacity of a stair-stepped spillway is made to the design of a traditional ogee crest (Army Corps of Engineers design specifications) and to the design of a broad crested weir. This paper is based on the 1:20 scale sectional and the 1:40 scale spillway physical models of the Gilboa Stair-Stepped Overflow Spillway, conducted at the Utah Water Research Laboratory, Utah State University. The analysis and comparison of the three types (traditional ogee, mitered ogee, and broad crested weir) of crest designs involved measuring discharge capacities and crest pressures. Each crest was tested for a variety of headwater conditions resulting in a series of rating curves for the assessment of three types of crest designs. Results are also presented in the form of discharge coefficient versus total reservoir head. The performance of the stair-steps of the spillway was evaluated for energy dissipation, cavitation, and flow interaction with the spillway steps. Analysis indicates that the mitered ogee crest performed considerably well when compared to the traditional ogee crest design. The mitered ogee crest produces 93% of the flow capacity of the traditional ogee crest while the broad crested weir design passed only 72% as much discharge as that of the traditional ogee crest. Therefore, retrofitting the crest of a stair-stepped spillway with a mitered ogee crest design is a very viable option. However, the spillway transition from the crest to the stair-stepped spillway is also an important flow performance consideration for all three of the crest types.