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Inception Point Relationship for Flat-Sloped Stepped Spillways

Hunt, Sherry L., Kadavy, Kem C.
Paper 2009 no.096571
hydraulic structures, water flow, physical models, weirs, dams (hydrology)
Many small earthen embankments are faced with hazard classification changes due to urban encroachment. As a result, some embankments have inadequate spillway capacity. To bring the dam into compliance with state and federal dam safety laws, rehabilitation of the dam is often required. RCC stepped spillways are becoming a popular choice for addressing these issues. However, design guidelines for RCC stepped spillways applied to small earthen dams are scarce, especially for spillways with slopes flatter than 2(H):1(V). A two-dimensional, physical model was constructed to evaluate the air entrainment inception point location in a 4(H):1(V) stepped spillway. Step heights of 38 mm, 76 mm, and 152 mm were evaluated. The physical model was constructed with a broad-crested weir, and model unit discharges ranging from 0.11 m3/(s·m) to 0.82 m3/(s·m) were tested. The research findings show that Chanson's relationship effectively predicts the location of the inception point for slopes as flat as 4(H):1(V) for Froude surface roughness values (F*) greater than 10, which in this study corresponds to model step heights of 38 mm and 76 mm. Chanson’s relationship did not adequately predict the location of the inception point for F* less than 10 which for this study corresponds to a model step height of 152 mm. A new relationship for predicting the location of the inception point was developed from this data set, and it is applicable for flat sloped stepped spillways with F* ranging from 1 to 100. This relationship is similar to Chanson’s, but it is optimized for flat-sloped stepped spillways with a broad-crested weir crest section. This research is expected to assist engineers with the design of stepped spillways applied on relatively flat embankment dams.