Spillway function, vertical and radial gates

Function of a spillway

A spillway safely passes surplus flood water from the reservoir to the downstream river. Its functions are:

• to prevent overtopping and failure of the dam during floods;
• to keep the reservoir below the maximum water level;
• to release water without eroding the dam toe or riverbed (with an energy dissipator).

Vertical lift gate

A vertical lift gate is a rectangular steel gate that moves vertically in grooves (slots) in the piers.

     hoist / gantry crane
     ______|______
    | |    |    | |  pier
    | |=========| |  gate (raised to open)
 ~~~| |  skin   | |
    | |  plate  | |
 ___|_|_________|_|___ crest
      wheels in pier slots

• Types: plain sliding gate (slides on steel faces, high friction), fixed-wheel gate (wheels on axles run on rails in the piers), Stoney gate (free roller train between gate and pier).
• The full water thrust is carried by the wheels to the piers, so the lifting force must overcome gate weight + friction of wheels/seals; a high hoist bridge or gantry crane is needed.
• Advantages: simple, compact in the flow direction, can be lifted out completely for maintenance, suits stop-log/emergency use.
• Disadvantages: large hoisting force, tall piers and hoist structure, slots disturb flow and may cause cavitation.

Radial (Tainter) gate

A radial gate has a curved skin plate (a segment of a cylinder) carried by radial arms on trunnion pins anchored in the piers.

            hoist
              |
   ~~~~~~~~~  |\  curved skin plate
   ~~~~~~~~~  | \___ arms ___ o trunnion
              | /         _/
   ___________|/_________/_____
       crest         flow -->

• Water pressure on the curved plate passes through the trunnion, so no moment about the pin; the hoist lifts only the gate weight and friction—very small hoisting force.
• It rotates upward to open; flow passes under the gate.
• Advantages: light hoist, smooth flow (no deep slots), suits large spans, easy automatic control; most used on large spillways.
• Disadvantages: long piers needed for trunnions, heavy concentrated trunnion loads.