Short Answer:

Spillways are safety structures in dams that allow excess water to flow out when the reservoir reaches its full capacity. They protect the dam from overflowing and possible damage by releasing water in a controlled manner into the downstream area.

When water rises above a set level in the reservoir, it flows over or through the spillway. The design of spillways ensures smooth and safe passage of water without causing erosion or structural harm. Spillways help maintain the dam’s safety during heavy rains or floods.

Detailed Explanation:

Working of spillways in dams

Spillways are critical components of a dam used for the safe disposal of surplus water. They are designed to prevent water from overtopping the dam, which could cause erosion or structural failure. Spillways redirect extra water from the reservoir to the downstream river or channel during floods or heavy rainfall events.

Each dam includes a spillway system based on the dam height, expected flood flow, and reservoir capacity. These structures must be carefully designed and maintained to ensure they function effectively during emergency and high-flow conditions.

How Spillways Work

1. Water Level Monitoring
   Every reservoir has a predefined maximum water level. This is called the full reservoir level (FRL). When water reaches or exceeds this level due to inflow from rainfall or upstream sources, the spillway becomes active.
2. Overflow or Gate Operation
   Depending on the spillway type, water may either:
   • Overflow automatically over the crest (in the case of an uncontrolled spillway), or
   • Be released by opening gates (in controlled spillways).

This discharge mechanism ensures excess water does not rise above safe levels.

1. Water Flow Through Spillway Channel
   The released water travels through a specially designed channel or chute, often lined with concrete to prevent erosion. The channel leads the water safely to the river or a stilling basin downstream.
2. Energy Dissipation
   As the water falls from a great height or flows at high speed, it contains a lot of energy. To prevent erosion or scouring at the base, energy dissipation systems like stilling basins, flip buckets, or baffle blocks are used to reduce water speed.
3. Safe Disposal
   Finally, the water is safely discharged into the river or channel, away from the dam, ensuring there’s no damage to the structure or surrounding areas.

Types of Spillways (used based on dam design)

• Ogee Spillway: Curved shape, commonly used in concrete dams.
• Chute Spillway: Straight or sloped channel to direct flow.
• Shaft Spillway (Morning Glory): Funnel-shaped, used in narrow spaces.
• Side Channel Spillway: Located parallel to the dam axis.
• Siphon Spillway: Uses siphon action to discharge water automatically.

Conclusion:

The spillway in a dam works as a safety valve, releasing excess water from the reservoir to prevent overflow and damage. It becomes active when water levels rise above the safe limit and guides the flow safely to downstream areas. With the help of gates, chutes, and energy dissipation systems, spillways ensure the dam remains stable and functions properly during floods or heavy inflow.