Spillway (Fig. 2) - A structure over or through which flood flows are discharged.
In the World Register of Dams spillways are described as either uncontrolled (L) (i.e. overflow or free overflow spillways) or controlled (V) (i.e. gated spillways).

[In French, “ évacuateur de crue ” includes any means of discharging flood water including an appropriately sized bottom outlet, sometimes termed “ orifice spillway ” (Am)]

Surface spillway - Spillway in which the upper surface of the flow is in contact with open air. They can be subdivided into (i) gated or controlled type and (ii) uncontrolled or free overflow type.

Submerged spillway or Orifice spillway (Am.) - Spillway in which water flows in with a head and flows out as a jet. All orifice spillways are equipped with gates.

[In French, “ évacuateur de crue ” includes any means of discharging flood water including an appropriately sized bottom outlet, sometimes termed “ orifice spillway ” (Am)]

Auxiliary spillway (Fig. 2) - A secondary spillway designed to operate only during exceptionally large floods.

Emergency spillway (Fig. 2) - A secondary spillway designed to operate only during exceptionally large floods.

Fuse plug spillway - A form of auxiliary or emergency spillway comprising a low embankment designed to be overtopped and washed away during a very rare and exceptionally large flood.

Side spillway - A spillway the crest of which is roughly parallel to the channel immediately downstream of the spillway.

Saddle spillway - A spillway constructed at a low saddle on the perimeter of a reservoir.

Skijump spillway - A spillway chute or conduit at the bottom of which there is a reverse curve which throws water up into the air to dissipate energy at a safe distance from the dam.

Shaft spillway - A vertical or inclined shaft into which flood water spills.

If the upper part of the shaft is splayed out and terminates in a circular horizontal weir, this is termed a bellmouth or morning glory spillway.

Siphon spillway - A spillway with one or more siphons built at crest level. This type of spillway is sometimes used for providing automatic surface level regulation within narrow limits, or when considerable discharge capacity is necessary within a short period of time.

Spillway tunnel - A tunnel conveying water from the spillway to the river downstream.

Spillway channel - A channel conveying water from the spillway to the river downstream.

Spillway chute - A steeply sloping spillway channel.

Reinforced grass spillway - Spillway in which the chute is covered with grass reinforced to withstand erosion.

Energy dissipator - Any device constructed in a waterway to reduce or destroy the energy of fast flowing water.

Stilling basin (Fig. 2) - A basin constructed so as to dissipate the energy of fast flowing water, e.g. from a spillway or bottom outlet, and to protect the river bed from erosion.

Plunge pool or Plunge basin - A natural or sometimes artificially created pool which dissipates the energy of free falling water. The basin is located at a safe distance downstream of the structure from which water is being released.

Baffle block or Impact block - A block of concrete constructed in a channel or stilling basin to dissipate the energy of water flowing at high velocity and aid in flow stability.

Chute block - A baffle block constructed in a spillway chute.

Nappe interrupter - Protrusion from the crest of a spillway whose purpose is to aerate the underside of the nappe.

Splitter - A wall generally of concrete constructed in a channel or stilling basin or at a spillway crest so as to divide the flow.

Flip bucket - The downstream end of a spillway shaped so that water flowing at high velocity is deflected upwards in a trajectory away from the foundation of the spillway.

Flashboards - Panels of wood, concrete or steel, placed on the crest of a spillway or an overflow dam to raise the retention water level. Some are placed and removed manually; some are hinged on the sill and held up nearly vertical by a hinged prop that has a footing on the sill and can be tripped, dropping the panels on the sill; some are designed to fail after being overtopped.