Posted by Spillways are essential safety components of any dam to channel excess floodwater downstream in a controlled way. Proper spillway design prevents overtopping, structural damage, and catastrophic failure under extreme inflow. Engineers select spillway types based on dam height, site topography, foundation conditions, and hydrologic demands. In this guide, we’ll explore the principal spillway categories, their features, advantages, and limitations.
Overflow (Ogee) Spillway
Overflow or “ogee” spillways have a crest shaped like the underside of a jet stream. They sit on the dam crest or abutment and are ideal for concrete gravity or arch dams. Water flows smoothly over the curved rise, minimizing energy loss and flow separation.
- Features
- Parabolic crest profile matching nappe trajectory
- Simple, ungated operation or radial gates
- Advantages
- Very high discharge capacity
- Low maintenance, no moving parts if ungated
- Limitations
- Requires erosion-resistant abutments
- Unsuitable for earthen dams without concrete apron
Chute (Open-Channel) Spillway
Chute spillways convey water down a sloped, open concrete channel from crest to riverbed. They adapt well to earth-and-rockfill dams where topography allows a continuous spill path. Velocity builds along the slope and is usually dissipated at the base by a stilling basin.
- Features
- Rectangular or trapezoidal channel profile
- May include guide walls or side vanes
- Advantages
- Flexible alignment to terrain
- Effective for moderate-to-high heads
- Limitations
- Large excavation footprint
- Requires robust energy dissipation downstream
Shaft (Morning Glory) Spillway
Shaft or bell-mouth spillways draw water into a vertical or inclined circular shaft. They resemble an oversized drain and feed into a horizontal tunnel through the dam or abutment. This design suits narrow valleys or sites with limited crest width.
- Features
- Circular crest opening, typically 3–10 m diameter
- Transition shaft leading to outlet conduit
- Advantages
- Minimal surface footprint
- Efficient in high-head, confined sites
- Limitations
- Complex hydraulic behavior (vortex, air entrainment)
- Expensive to construct and maintain
Side-Channel Spillway
Side-channel spillways divert overflow parallel to the dam face into a short channel before turning downstream. They are used where direct plunge paths aren’t possible and space beside the dam is limited. The channel cross-section manages flow before a downstream chute or tunnel.
- Features
- Entrance crest set parallel to reservoir face
- 90° or curved turn into outlet conduit
- Advantages
- Fits narrow abutments or valley walls
- Can be retrofitted to existing structures
- Limitations
- Lower discharge efficiency than overflow types
- Turbulent turns require careful design
Stepped Spillway
Stepped spillways incorporate a series of transverse steps along the chute slope. Each step dissipates energy by breaking up flow, reducing downstream erosion. They suit new concrete or roller-compacted concrete dams where energy reduction is critical.
- Features
- Uniform or varying step height and length
- Aerates flow to prevent cavitation
- Advantages
- Excellent energy dissipation on slope
- Reduced need for downstream stilling basin
- Limitations
- Complex hydraulic modeling required
- High initial construction cost
Siphon Spillway
Siphon spillways use atmospheric siphon action to draw water once the reservoir rises above the crest. Air vents break the siphon when levels drop, stopping flow automatically. They are often used as self-regulating overflow structures.
- Features
- Inlet crest, siphon legs, and air-trap vent
- Automatic start and stop based on water level
- Advantages
- No mechanical gates or operators
- Smooth regulation of low flows
- Limitations
- Limited to moderate heads (typically < 30 m)
- Prone to air leakage and cavitation
Labyrinth and Ski-Jump Spillways
Labyrinth spillways increase crest length via zigzag walls, boosting capacity without wider valleys. Ski-jump spillways eject flow horizontally, projecting the jet several meters before free‐fall, which minimizes downstream scour.
- Features
- Labyrinth: sawtooth crest planform
- Ski-jump: curved ejector lip
- Advantages
- Labyrinth: high discharge for a given width
- Ski-jump: concentrated energy dissipation in air
- Limitations
- Labyrinth: complex formwork and maintenance
- Ski-jump: requires clear flight path and stilling basin
Understanding the right spillway type is crucial for dam safety, operational reliability, and cost management. Each design balances hydraulic performance, site conditions, and construction complexities. As flood regimes evolve, innovative hybrid spillways and adaptive designs will play an increasingly important role in resilient water infrastructure.
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