Purposes of spillway; types of gates

Purposes of a spillway

A spillway is the structure that passes surplus flood water from the reservoir to the downstream river safely.

• Releases floods beyond the storage capacity, so the dam is not overtopped (essential for earth and rockfill dams).
• Controls the reservoir level (maximum level) and protects the dam, intake and powerhouse.
• Dissipates the energy of the released water so the d/s river bed and the dam toe are not scoured.
• Passes floating debris, ice and silt in some arrangements; allows controlled releases for flood routing.

Types of spillway gates

Gates on the crest allow the reservoir to be stored above the crest level and give control of the discharge.

1. Vertical lift (fixed-wheel / Stoney) gate: a steel leaf lifted vertically in grooves, with rollers to reduce friction. Simple and reliable, but needs a tall hoist structure.
2. Radial (Tainter) gate: a curved skin plate on radial arms pivoted at a trunnion on the pier. The water thrust passes through the trunnion, so lifting force is small; the most common crest gate.
3. Drum gate: a hollow, hinged drum that rises when water is let into its chamber and sinks into a recess when drained. No piers or hoists; suitable for low heads and long crests.
4. Roller gate: a cylinder with gear teeth that rolls along inclined racks on the piers; used for wide spans.
5. Flap gate / bear-trap / stop-logs: for low heads, small control, or emergency closure.

 Radial gate               Vertical lift gate
      trunnion o               hoist
      /|  arm                    |
 ~~~~/ |_____                    | rope
 ~~~~|=======) skin plate    ~~~~|#|
 ~~~~\_ crest                ~~~~|#| leaf in groove
 ____________                ________