Overflow channel or floodway, is a structure designed to provide a controlled release of water from a reservoir or dam during periods of high water flow. It is typically located at the top of the dam and is used to prevent the reservoir from overflowing or causing damage to the dam.

When the water level in the reservoir surpasses a certain predetermined level, the spillway is opened to allow the excess water to flow out. This helps to prevent the dam from being overtopped and potentially being damaged or breached.

Spillways are designed to handle large volumes of water and are often constructed with reinforced concrete or other durable materials. They may consist of various components such as gates, chutes, or tunnels, depending on the specific design and requirements of the dam.

The operation of a spillway is typically controlled by dam operators or engineers who monitor water levels and make decisions on when and how much water should be released through the spillway. This helps to manage the flow of water downstream and mitigate the risk of flooding in surrounding areas.

In addition to their primary function of flood control, spillways can also be used for other purposes such as hydropower generation or fish passage. Some spillways are equipped with turbines that can generate electricity as the water is released, providing an additional source of renewable energy.

Overall, spillways play a crucial role in managing water levels in reservoirs and preventing potential disasters associated with dam overtopping.