Spillway classification and ogee spillway

A spillway is a safety structure provided with a dam to pass surplus flood water from the reservoir to the downstream channel safely, so that the dam is not overtopped.

Classification of spillways

Ogee spillway

An ogee spillway is an overflow spillway whose crest and downstream face are shaped to follow the profile of the lower nappe of a fully aerated sheet of water flowing over a sharp-crested weir. The profile looks like an elongated "S" (ogee curve).

        upstream    crest
  ~~~~~~~~~~~~~~~~~~~__  water surface
                  _/  \__
  upstream face  |       \_  downstream
  (vertical or   |         \   profile
   sloping)      |          \  (x^n = K H_d^(n-1) y)
                 |           \
                 |            \___ reverse curve
 ________________|_______________\___~~~~~~ tailwater
                     bucket / stilling basin

Features:

• Upstream crest curve: compound circular curves (radii about $0.5H_d$ and $0.2H_d$) from the upstream face to the crest.
• Downstream profile: WES standard shape, $x^{1.85} = 2.0\,H_d^{0.85}\,y$ for a vertical upstream face, where $H_d$ is design head.
• Straight tangent portion following the dam slope, ending in a reverse curve (bucket) which turns the flow into a stilling basin or ski-jump bucket.

Discharge: $Q = C\,L\,H_e^{3/2}$ with $C \approx 2.2$ (SI) at design head.

Working and merits:

• Since the face fits the nappe, pressures on the surface are near atmospheric at design head, giving high discharge efficiency without damage.
• At heads above design head, negative pressures develop (cavitation risk); below design head, positive pressures reduce efficiency.
• It is simple, reliable, and suitable for concrete gravity and buttress dams; can be gated or ungated.
• Not suitable alone for earth dams, since water would flow over the dam body.