Inception Point for Embankment Dam Stepped Spillways

Abstract

Retrofitting embankment dams with stepped spillways has become a common design practice, particularly for those dams that change hazard classification from low to high. For embankment dams retrofitted with stepped spillways, the chute length is often insufficient for developing aerated flow or an inception point. The inception point is a key spillway design parameter used in energy dissipation, flow depth, and air entrainment prediction relationships. Original research for developing an inception-point relationship for stepped spillways was based on primarily gravity (θ≥26.6°) stepped spillways, with the majority having an ogee crest control section. The resulting, inception-point relationship tends to overestimate the inception-point location for broad-crested weir stepped spillways (θ≤26.6°) when the Froude surface roughness (F*) is less than 10. Consequently, research on broad-crested weir stepped spillways retrofitted for embankment dams has been conducted to provide an optimized inception-point relationship. This study provides additional data allowing further refinement (i.e., F*≤28) of the inception-point relationship for broad-crested stepped spillways (θ≤26.6°), and it provides a new relationship for broad-crested weir stepped spillways for F*>28.