What Is A Dam Spillway?

Understanding a Dam Spillway

The Purpose and Importance of Dam Spillways

Dams play a crucial role in water storage, often utilizing stored water for electricity generation, irrigation, and flood control. But what exactly is a dam spillway, and why is it essential?

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Even though modern dams are designed with advanced technology, they may still experience water overflow. Such overflow, if not managed properly, can compromise the structure and safety of the dam. To prevent potential disasters, it is critical to discharge excess water efficiently.

What Exactly Is a Dam Spillway?

A spillway serves as a controlled pathway for releasing excess water stored in a dam. Nearly every dam is equipped with at least one spillway to safely manage water discharge, whether for downstream needs or to prevent the reservoir from becoming overly full.

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A Closer Look at Different Types of Spillways

Designing dams would be simpler if weather conditions like rainfall and snowmelt were consistent. However, since hydrologic variability necessitates the construction of dams, we must account for unexpected water flows.

Dams store water and regulate variable inflows to protect against flooding. During significant storm events, watersheds can generate large volumes of water, which may rush to the reservoir with little warning. Therefore, dams must be able to handle these sudden inflows.

Due to unpredictable seasonal weather trends, engineers find it nearly impossible to design dams with specific discharge capacities. Spillways are incorporated into most dams to manage unexpected increases in upstream water, discharging it safely to prevent overflowing.

Typically, spillways are equipped with a penstock and a large cross-sectional tube, creating a connection between the upstream inlet and downstream outlet.

Categories of Spillways

Spillways are classified into four primary types:

• Overflow spillway
• Chute spillway
• Side-channel spillway
• Limited service spillway

Each type serves specific functions under varying conditions, but they all aim to discharge water to prevent flooding.

Overflow Spillway

Typically used with concrete gravity dams, overflow spillways can be either gated or ungated. They allow water to overflow when the reservoir behind the dam becomes too full.

Chute Spillway

Chute spillways are usually constructed for earth or rock-filled dams. However, they are also found in some concrete dams, especially those located in narrow canyons, where there is insufficient space for an overflow spillway. A chute spillway can be found through the dam’s abutment or in a saddle located slightly away from the main structure.

Side-Channel Spillway

Side-channel spillways are similar in function to chute spillways but are characterized by a unique shape that fits narrow dam abutments. These spillways are generally ungated but can have gates if needed.

Limited Service Spillway

A limited service spillway is used infrequently and can sustain some damage without causing a catastrophic release of water from the reservoir.

Further types of spillways include:

• Siphon spillway
• Shaft spillway
• Auxiliary spillway
• Ogee spillway

Siphon Spillway

Functioning like a siphon, these spillways are built with a hood over the standard spillway. A rising water level triggers siphon action, allowing a larger discharge capacity.

Shaft Spillway

This spillway features a circular crest on the reservoir surface. Water flows through a tunnel to the downstream side. During dam construction, diversion tunnels can be repurposed as the horizontal conduit for this type of spillway.

Auxiliary Spillway

Also known as emergency spillways, these are operated during excessive upstream water flow. Most large dams feature both service and auxiliary spillways.

Ogee Spillway

Typically found in rigid dams, ogee spillways form part of the main dam structure if enough length is available. The downstream profile matches the shape of the water jet from the crested weir.

This is a modified version of a drop spillway. In this case, the spillway’s downstream profile coincides with the lower nappe’s shape of the flowing, falling water jet from the crested weir. Learn more about codes for hydraulic design of barrages Is there a code for hydraulic design of barrages?.

Dam Spillway & Gracon Services

A spillway is designed to control the overflow of water stored upstream of a dam, maintaining the reservoir's water level. They function automatically without mechanical parts; water flows through the spillway when it exceeds a certain level.

However, regular maintenance is necessary to ensure the effectiveness and safety of spillways.

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Spillway: Definition, Importance, Types, Design, & Facts

A spillway is a vital feature for surplus water passage over or around a dam when the reservoir is full. Spillways follow different routes, sometimes over the dam, around it, or through it, and dissipate erosive energy away from the foundation.

An inadequately designed spillway can lead to catastrophic failures, especially in earthen dams not meant for uncontrolled water flow. Older earthen dams often suffer from inadequate spillway capacity due to outdated flood data. For concrete gravity dams, overflow can erode foundations, while arch dams offer more resistance.

Four key aspects of spillways warrant attention:

• Automatic discharge of surplus water without human intervention
• A wide intake to handle large floods without excessively raising reservoir levels
• Minimal increase in downstream floodwater discharge rates
• Controlled energy dissipation to prevent structural damage

Spillways need not be open to the atmosphere; shaft and tunnel spillways offer alternative methods of water conveyance. Self-priming siphons or drop shafts, also called morning-glory spillways, serve as upstream intakes.

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