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Research Project: Development of Engineering Tools for the Design and Rehabilitation of Safe, Efficient Embankment Protection Alternatives, Hydraulic Structures, and Channels

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Title: Inception point for stepped chute designs with multiple sections of different step heights

item Hunt, Sherry
item Kadavy, Kem

Submitted to: American Society of Civil Engineers Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/8/2020
Publication Date: 1/1/2021
Citation: Hunt, S., Kadavy, K.C. 2021. Inception point for stepped chute designs with multiple sections of different step heights. American Society of Civil Engineers Journal of Hydraulic Engineering. 147(4): 06021001.

Interpretive Summary: Some aging earthen dams in the US are in need of repairs or upgrades to meet today’s standards. Some engineers upgrade dams by placing stepped spillways over the top of the dam. These dams typically range in height from 30 to 100 feet. These types of spillways can cause a white-water appearance on the water surface. White-water is most commonly recognized from rapids along rivers with rocky stream beds. Equations have been developed to determine the length from the downstream edge of the top of the spillway to point where white-water is first observed. Additional research was completed to determine the effect of multiple step heights in a stepped spillway. Assuming one step height dominates for calculation of the length to the white-water can result in significant error. Accounting for the multiple step heights in the calculations can reduce the error. This research can assist engineers in addressing public safety in stepped spillway design.

Technical Abstract: In the literature, a classical free-surface inception point empirical relationship for smooth chutes serves as a basis for the development of a free-surface inception point relationship for stepped chute applications. This study expands research on the free-surface inception point for stepped chutes with multiple sections with different step heights along the chute bottom surface. Data show that existing free-surface inception point relationships for stepped chutes do not always accurately predict where the free-surface inception point will occur when assuming a single step height in stepped chutes designed with multiple sections with different step heights. Accounting for the different step height sections and assuming a constant rate of boundary layer development for each step height allows the error for predicting the freesurface inception point location to be reduced to 10% for stepped chutes with two different step height sections. The research results contribute to improving knowledge on the developing free-surface inception point in stepped chutes, and it provides additional validation data for modeling these flows in computational fluid dynamics models.