Inception Point Relationship for Flat-Sloped Stepped Spillways

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, 76, 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. Hubert Chanson developed a relationship for predicting the inception point location for primarily steep (θ≥22°) stepped spillways. In this study, Chanson’s relationship effectively predicts the location of the inception point for slopes as flat as 14° for Froude surface roughness values (F∗)>10 which, in this study, corresponds to model step heights of 38 and 76 mm. A new relationship for predicting the location of the inception point was developed, applicable for flatter sloped (θ≤22°) stepped spillways with 1<F∗<100. This relationship is similar to Chanson’s, but it is optimized for flat-sloped stepped spillways with a broad-crested weir.