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 $0.11{\text{m}}^3/\left(\text{s}\cdot \text{m}\right)$ to $0.82{\text{m}}^3/\left(\text{s}\cdot \text{m}\right)$ were tested. Hubert Chanson developed a relationship for predicting the inception point location for primarily steep $\left(\theta \ge 22°\right)$ 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 $\left({\mathsf{F}}_{\ast }\right)>10$ 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 $\left(\theta \le 22°\right)$ stepped spillways with $1<{\mathsf{F}}_{\ast }<100$ . This relationship is similar to Chanson’s, but it is optimized for flat-sloped stepped spillways with a broad-crested weir.