When natural rainfall falls, crops are cultivated. But what happens when there is no natural source of water? Irrigation is nothing but artificial application of water to the land or soil. Irrigation is helpful in the growing of agricultural crops,  sustenance of landscapes, re-vegetation of disturbed soils in dry areas and during periods of inadequate rainfall.

old_irrigation_system_in_india

But, how did this irrigation system develop in India?

The History of Irrigation practice in India can be tracked back to prehistoric times. The earliest reference to Irrigation Practices in India were noted by the ancient sage, Narada. He came to see the Emperor Yudhishthhira around 3150 B.C  and asked some straight-forward questions :

“Are the farmers tough and prosperous?” “Are the dams full of water?” “Are the dams big enough?’ “Is the water distributed to different parts of the kingdom?”

history_of_irrigation_system_in_india

By the above questions we can understand how Rulers and learned people are interested in the Irrigation system of India. Ancient Hindu architects were keen to construct viable projects for irrigation works. This would help the hard-working farmers with necessary water supply.

Around 300 B.C, Megasthenese also left a note on the Irrigation Systems in India during the rule of Chandra Gupta Maurya. The Greek Ambassador of Sleukos Nikator wrote that the “whole country was under irrigation“. To the credit of Chandra Gupta, rock inscriptions were found in the State of Kathiawar which say that he constructed the Sudarsan Lake between 300 B.C and 457 A.D.

The inscriptions also make a note of two major disasters that befell it. Due to one of the disasters the lake became so ugly that it looked like a sandy desert. Chola rulers of South India are also great architects in civil engineering and constructed necessary irrigation works.

[Source : Irrigation and Water Power Engineering, B.C.Punmia]

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What are Spillways for Dams ? Why are they Required ?

Spillways are generally provided to allow the surplus discharge, during floods and other unexpected conditions,  from the upstream side of the reservoir to the downstream. This is nothing but the overflow portion of the dam. It is also called by the name “surplussing work“.

According to Wikipedia,

spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels.

Purposes of Spillways

Spillways carry the flood or excess water that is not to be stored in the reservoir. This surplus water is generally drawn from top of the reservoir created by the dam. Spillway acts as a “safety valve” for the dam. Spillways are constantly provided for all storage and detention dams. It is predominantly important that spillways are provided with sufficient capacity.

spillways_for_dams

Why Spillways are Important?

Spillways become very important in earth and rockfill dams. This is due to the fact that overtopping is a dangerous scenario in the above said dams. The main purpose of the spillway is to have the required capacity to discharge the major floods without causing damage to the dam.

How Spillway should be designed?

Spillways must be also designed in such a way that the reservoir level in the dam is kept below some predetermined maximum level. The amount of design flood discharge that is to be passed over the spillway is determined using the concepts of flood routing.

spillways_example

Conclusion

Spillway must be hydraulically and structurally adequate. It must be located in a position so not to cause any erosion or undermining of the downstream side of the dam. Spillways also should not cause any destruction to appurtenant structures.

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