Hydraulic Structures: Siphon Spillway Mechanics and Priming Dynamics
A Siphon Spillway is a closed conduit bent over a dam crest that uses atmospheric pressure differentials to discharge high flows under low operating heads. Flow transitions through three distinct operational phases: Weir Flow: Initial rising water level overflows the lower lip as a simple weir. Priming Phase: Flow seals the downstream leg outlet, entraining and evacuating internal air to form a partial vacuum within the siphon crown. Full Siphonic Flow: Continuous liquid column flow established under total differential head (H). The ultimate siphonic discharge (Q) is evaluated using pipe flow hydraulics: $Q = C_d \cdot A \cdot \sqrt{2 \cdot g \cdot H}$ Where $C_d$ is discharge coefficient $(\approx 0.6\text{ to }0.8)$ and $A$ is throat cross-sectional area. The maximum operating suction head at the crown is limited by water vapor pressure to prevent air pocket formation and cavitation. Siphon spillways installed on medium storage dams across India provide rapid automatic dis...
