Spillways and their types

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What are spillways?

Spillways also known as the “overflow channels” are the most essential part of dames. The spillways are the structures constructed on the dams which allow the controlled flow of water from the dam to the downstream. The downstream level is equal to the level of the river-bed before the construction of the dam. The construction of the dam makes the water level rise, and as a result, lowering the level downstream. Spillways protect the dam water to overflow from the dam. The overflowing water damages the dam, because of erosion caused by free-falling water. A safe discharge of water out of the water body is made possible by means of spillways. Understanding the technology of spillways, we first, need to understand hydrology. We should know the tremendous ability of water that can affect the constructed structures of the dam. The consistent rain and snowfall make the water flow downstream. We need to store water, in order to meet the need throughout the year. Dams need to be ready all through the year, to optimize the level of water in the reservoir. Storing a large amount of water becomes risky due to hydrological effects of water. An overflow of water from a dam may be catastrophic. A sudden rapture in the dam may cause deadly flooding. We need spillways to safely release the water from out of the dam.

Types of Spillways:

Keeping in mind the topography and other civil engineering parameters, spillways can be classified into the following types:

  1. Ogee Spillways:

It is an improved form of a vertical flow of water. As the vertical flow of water may cause damage to the base of the downstream, therefore the ogee spillways resolved this problem. In an ogee spillway water smoothly flows corresponding to an ogee-shaped channel.

  1. Drop Spillways:

A drop spillway is designed at small dams. The waterfalls freely and vertically from the dam to the downstream. The crest of the spillway is projected ahead, in order to protect water from damaging the base downstream. The basement erosion may cause rapture in the dam.

  1. Siphon Spillways:

The surplus water of the reservoir is ejected through a U-shaped conduit called siphon spillway. By using the difference in the heights of intake and outlet a siphon creates a pressure difference, which removes excess water from the reservoir. Removal of air (priming) from the U-shaped bend of the conduit makes it work. The flow of water through the conduit at a level lower than the level of the reservoir creates pressure in the upper part of the siphon, but the water injects from the reservoir fills the plausible space.  

  1. Trough/Chute Spillways:

It is a smooth declining pathway to the downstream. The declining pathway is like an ogee spillway. The purpose of the chute spillway is to reduce the vertical fall of the water. The vertical fall of the water may cause damage to the base of the downstream, resulting in a rupture in the dam. But chute spillways are designed like an ogee curve. They are paved and lined with concrete in the bottom and sides to protect the base topography and dam.

  1. Shaft Spillways:

In these spillways, an L shaped shaft with a funnel mouth is submerged in the water body. It has a specific height, regarding the required level of water. The rising water, which may overflow the dam, is poured into the mouth of the shaft. The shaft transmits the water out of the dam, thus keeping an optimum level and safe discharge of the water.

  1. Stepped Spillways:

Stepped spillways are designed to reduce the speed of flowing water. These types of spillways have been used for 300 years. In spite of modern spillways, the use of stepped spillways is still in practice. The flowing water when passed several steps of an inclined pathway reduces its speed.

7. Side Channel Spillways:

It is a contributory spillway. It is approximately parallel to the level of the water body. It lies above the main spillway. Its purpose is to reduce the level of water during heavy rainfalls. When much water accumulates in the dam, the water tends to overflow. It protects the overflow of the dam. 



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