Identify the spillway crest gate, which consists of a hollow steel cylinder or drum spanning between the piers constructed on the crest of the spillway.?

Option 2 : Rolling gates.

Explanation:

Rolling gate:

• It consists of a hollow steel cylinder as large in diameter as the height of the opening and spanning between the piers.
• Each pier has an inclined rack.
• The gate is wound around the periphery of the cylinder.
• The gate is also attached to the incline rack with the help of toothed gears.

Bear-trap gates:

• Bear-trap gates are also known as movable drum gates.
• A bear trap gate consists of two leaves of steel, with one leaf hinged on the upstream side and the other on the downstream side over the crest.
• The bear trap gates are suitable for low navigation dams.
• These gates have a wide base but a deep recess is not required.
• These gates may be designed to operate automatically.

Drum gate:

• Drum gates are useful for longer spans of the order of 40 m or so and medium heights of 10 m.
• The drum gate consists of a segment of a cylinder that may be raised above the spillway crest or may be lowered into the recess made into the top of the spillway.

Tainter gate:

• A tainter gate(or radial gate) has the upstream face cylindrical.
• The axis of the segment of the cylinder forming the u/s face is horizontal.
• The face is formed of a steel skin plate shaped to a segment of a hollow cylinder supported on a steel framework.