Last reviewed 16 Sept 2026 · 9 min read
Spillways
A spillway passes surplus flood water from the reservoir to the river downstream safely, so that the reservoir never rises above the maximum water level and the dam is never overtopped.
- Spillway capacity is fixed by the inflow design flood routed through the reservoir (surcharge storage reduces the peak outflow) — see Floods — Estimation & Flood Routing.
- Spillway failure (inadequate capacity) is a major cause of dam failures, especially of earth dams.
Components
- Entrance (approach) channel.
- Control structure — crest (weir, orifice or gates) that regulates outflow.
- Discharge channel (conveyance) — the face of the dam, a chute, tunnel or shaft.
- Terminal structure — energy dissipator.
- Outlet channel — returns water to the river.
Types of spillways
| Type | Description | Suitable where |
|---|---|---|
| Free overfall (straight drop) | Water falls freely from the crest of a thin section; an apron/cushion pool downstream | Low arch dams, small bunds with rock foundation |
| Ogee (overflow) spillway | Crest shaped to the lower nappe of a sharp-crested weir; water glides over the downstream face; energy dissipator at the toe | Concrete and masonry gravity dams — most common |
| Chute (trough) spillway | Crest followed by a steep open channel (chute) conveying water to the river | Earth and rockfill dams; spillway in a saddle away from the dam |
| Side channel spillway | Crest parallel to the channel; water spills sideways into a trough and then turns 90° down a chute or tunnel | Narrow, steep valleys where a long frontal crest cannot fit |
| Shaft (morning glory / bell-mouth) spillway | Circular funnel-shaped crest leading to a vertical shaft and horizontal tunnel | Narrow valleys where a diversion tunnel exists; earth dams; small reservoirs with little debris |
| Siphon spillway | Closed conduit shaped as an inverted U; primes when water rises above the crest and passes large discharges with a small rise in level | Sites needing close control of reservoir level with limited space |
| Tunnel (conduit) spillway | Closed tunnel carries flow from an inlet structure to downstream | Narrow gorges, arch dams |
| Labyrinth spillway | Zigzag crest in plan — large crest length within a small width | Increasing capacity of existing spillways |
| Emergency spillway / fuse plug | Erodible embankment section designed to wash out when a rare flood exceeds service spillway capacity | Supplementary protection for extreme floods |
Notes on special types
- Shaft spillway — at low heads the flow is crest controlled (); at higher heads the flow becomes orifice/tube controlled and discharge increases very little with head. Needs debris protection and air vents; vortices must be prevented.
- Siphon spillway — discharge ; the crown (summit) pressure is negative, so the height of the crown above the downstream water is limited to avoid cavitation; an air vent (deprimer) at FRL stops siphon action when the water level falls; problems include clogging by debris, vibration, sudden surges and ice.
Ogee spillway
Crest profile
The downstream profile follows the lower surface of the nappe from a fully ventilated sharp-crested weir, so that pressures on the surface stay close to atmospheric.
= design head (excluding velocity of approach head); origin at the crest apex; measured downward. The upstream crest is formed by compound circular curves; the profile joins the downstream face (slope of the dam) tangentially.
Discharge
= coefficient of discharge (about 2.2 in SI units at the design head for a high overflow spillway, varying with head ratio and approach depth); = total head including approach velocity head.
Effective crest length with piers and abutments:
= net clear length; = number of piers; = pier contraction coefficient (square-nosed ≈ 0.02, round-nosed ≈ 0.01, pointed ≈ 0); = abutment contraction coefficient (square abutment ≈ 0.20, rounded ≈ 0.10).
- Head greater than design head — the nappe tends to leave the surface → negative pressures on the crest, higher discharge coefficient, but risk of cavitation and vibration.
- Head less than design head — the nappe clings to the surface → positive pressures, lower discharge coefficient.
- Design heads are therefore often chosen a little below the maximum head so that negative pressures at maximum flood remain within safe limits.