Spillway purposes and ogee advantages

Purposes of a spillway

• Safe passage of floods: it passes surplus flood water from the reservoir to the d/s river and so prevents overtopping of the dam (a major cause of failure, especially for earth dams).
• Reservoir level control: keeps the reservoir below the maximum water level and protects the dam, intake and powerhouse.
• Energy dissipation: together with a stilling basin or bucket, it reduces the energy of the falling water, preventing scour at the dam toe.
• Flood routing and sluicing: gives a controlled release, helps pass debris, ice and silt, and allows lowering of the reservoir in an emergency.

Advantages of an ogee-shaped spillway

The ogee crest has an S-shaped profile; the upper curve follows the lower surface of a free-falling nappe over a sharp-crested weir, and the lower curve is tangent to the downstream face.

        ~~~~~~~~ H
   ___  ~~~~~   .
  /   \ ~  nappe ' .
 |  crest \          '.
 |      ogee curve      '.   d/s face 1:0.8
 |                          '.

• Highest discharge efficiency: $Q=C\,L\,H^{3/2}$ with $C\approx2.2$ ($C_d\approx0.74$), larger than for broad-crested or round-crested weirs, so a shorter crest can pass the same flood.
• Pressure at crest close to atmospheric at design head, so there is no cavitation or vibration; cost of the spillway is low.
• Smooth, stable flow with little energy loss and no separation; the nappe is supported by the crest.
• It fits well as the overflow section of a concrete dam, can be fitted with gates, and links smoothly to a stilling basin, bucket or ski-jump.
• Ability to pass debris and ice over the crest without clogging.