Overflow channel or spill channel, is a structure used to control the release of water from a dam or reservoir. It is designed to safely divert excess water during periods of heavy rainfall or when the water level in the reservoir exceeds its maximum capacity.

The spillway serves as an emergency outlet for the dam, preventing overtopping and potential failure. It is typically located on or near the dam and consists of a channel or chute that allows water to flow downstream at a controlled rate. The design of the spillway is crucial to ensure its ability to handle high volumes of water without causing damage to the dam or downstream areas.

Spillways can be classified into different types based on their design and operation. Some common types include:

1. Ungated Spillway: This type of spillway allows water to flow freely over its crest without any gates or controls. It relies on the natural flow characteristics to safely discharge excess water.

2. Gated Spillway: These spillways have gates or valves that can be opened or closed to control the flow of water. The gates are operated manually or automatically, depending on the need to regulate the water level.

3. Fuse Plug Spillway: This type of spillway consists of low-cost removable blocks or plugs that are placed in pre-designed holes. When the water level rises, the plugs are dislodged, allowing water to flow through the openings.

4. Side Channel Spillway: These spillways are located on the sides of the dam, diverting excess water to a separate channel. They are often used in combination with other spillway types to increase the overall discharge capacity.

5. Emergency Spillway: This type of spillway is designed to handle extreme events when the primary spillway and other structures are unable to safely release water. Emergency spillways are usually used as a last resort and are often uncontrolled, allowing water to flow freely.

Overall, spillways are essential components of dam infrastructure. They ensure the safety and stability of dams by providing a controlled means of releasing excess water, reducing the risk of flooding and potential damage downstream.