Skip to main content

Purpose and types of spillways

Short answer

A spillway safely passes floodwater from the reservoir to prevent overtopping and damage; crests may be ungated or gated. Types include ogee, chute, side channel, shaft, siphon, and tunnel. For example, an ogee spillway has an S-shaped crest shaped like a nappe, providing atmospheric pressure at design head; discharge Q = C L H^(3/2) with C ≈ 2.2 (SI).

What is the purpose of a spillway? Write down the types of spillways based on their key features and describe them (explain one of them with a neat sketch).

Purpose of a spillway

A spillway is a structure that safely passes surplus water (floods) from the reservoir to the downstream river without damaging the dam. It controls the reservoir level, protects the dam (especially an earthfill dam) from overtopping, and dissipates the energy of the released water so that the toe and river bed are not scoured. The crest is either ungated (crest at the full supply level) or gated (crest below the full supply level, with gates that hold the water up to that level).

Types of spillways (by key features)

TypeKey feature
Overflow (ogee) spillwayWater passes over a crest shaped like the lower nappe of a sharp-crested weir; part of a concrete gravity dam
Chute (trough) spillwayOpen channel with a steep slope carrying water down the abutment; common for earth dams
Side channel spillwayCrest is parallel to the channel; flow turns 90° into a trough, used in narrow valleys
Shaft (morning glory) spillwayFunnel-shaped inlet and a vertical shaft joined to a horizontal tunnel
Siphon spillwayClosed conduit with a siphon action; operates automatically at a set level
Tunnel (conduit) spillwaySpillway through a tunnel in the abutment
Stepped / labyrinth spillwaySteps or zig-zag crest to dissipate energy or increase the crest length

Ogee (overflow) spillway

The crest has the shape of the underside of the nappe of a sharp-crested weir (USBR profile), so that the pressure on the crest is nearly atmospheric at the design head. The downstream face is sloping and ends in a curved bucket or stilling basin.

        FSL   ____  crest gates / piers
  ~~~~~~~~~~~/ ~~\
 Reservoir  | ogee  \
            |crest   \  d/s face (slope)
            |          \
            |           \___ bucket / stilling basin
          --+--------------==========--> river
             Dam body (concrete gravity)
  • Discharge: Q=C L H3/2Q = C\,L\,H^{3/2}, with C≈2.2C\approx2.2 (SI) for the design head, LL the effective crest length and HH the head over the crest.
  • At heads below the design head the nappe clings to the crest (pressure above atmospheric); at heads above it, negative pressure and cavitation can occur.
  • Advantages: efficient, cheap in a concrete dam, and gives a large capacity. A stilling basin or bucket dissipates the energy.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Hydropower Engineering chapter 4 questions →