Spillway and ogee spillway

A spillway is a structure built as part of a dam to pass excess flood water safely from the reservoir to the river downstream, so that the reservoir level never rises above the maximum water level and the dam is not overtopped. It is the safety valve of a dam.

Ogee spillway

The ogee spillway is an overflow spillway whose crest and downstream face follow the shape of the lower surface of the nappe of water falling over a sharp-crested weir at the design head. Its profile is an S-shaped (ogee) curve.

 ~~~~~~~~~~~~~~~~~~__ 
                 _/  \_   crest
                |      \
   dam body     |       \  d/s profile
                |        \
 _______________|_________\____~~~~ TWL
                     bucket

• Upstream portion: compound curves joining the upstream face to the crest.
• Downstream profile: WES shape $x^{1.85} = 2\,H_d^{0.85}\,y$ ($H_d$ = design head), followed by a straight slope and a reverse curve/bucket that delivers water to an energy dissipator.
• Discharge: $Q = C\,L\,H^{3/2}$, with $C \approx 2.2$ (SI units).
• At design head the pressure on the face is nearly atmospheric, giving high efficiency with no cavitation. At higher heads negative pressure (risk of cavitation) and at lower heads positive pressure (lower discharge coefficient) occur.
• Widely used for concrete gravity dams and barrages; may be gated or ungated.