CANAL FALL/DROP STRUCTURES

A fall or drop is an irrigation structure constructed across a canal to lower down its water surface and destroy the surplus energy liberated from the falling water.

The location of a fall has to be judiciously worked out such that there should be a balance between the quantities of excavation and filling. Further the height of the fall has to be decided, since it is possible to provide larger falls at longer intervals or smaller falls at shorter intervals.

There are various types of fall structures, Many of these structures used boulder masonry as their construction material, whereas now brick masonry or, more commonly, mass concrete is being used commonly in modern irrigation projects.

type of falls are: Ogee falls, rapids Well type fall or syphon well fallnotch falls, vertical falls and sarda type fallglacis type fallsMontague type fallInglis or baffle fall

Ogee fall The Ogee fall was first constructed by Sir Proby Cautley on the Ganga Canal. This type of fall has gradual convex and concave curves, with an aim to provide a smooth transition and to reduce disturbance and impact. This preserved the energy (with out dissipating it).

The rapid-fall was tried in some of the north-Indian canals which were constructed with boulders cemented together by lime concrete

The trapezoidal-notch fall consists of one or more notches in a high crested wall across the channel with a smooth entrance and a flat circular lip projecting downstream from each notch to disperse water

Falls with vertical drop The vertical-drop fall uses a raised crest to head up water on the upstream of the canal section and allows it to fall with an impact in a pool of water on a depressed floor which acts like a cushion to dissipate the excess energy of the fall. This type of fall was tried in the Sarda canal of Uttar Pradesh, which came to be commonly called as the Sarda-type fall.

Falls with drop along inclined glacis These are falls with inclined glacis along which the water glides down and the energy is dissipated by the action of a hydraulic jump at the toe of the structure. Inclined drops are often designed to function as flume measuring devices. These may be with and without baffles and supplemented by friction blocks and other energy dissipating devices

Classification of Falls

Meter and non-meter falls:

Meter falls are those which also measure the discharge of the canal. The non-meter falls do not measure the discharge. Generally glacis type fall is suitable as a meter. The vertical drop fall is not suitable as a meter due to the formation of partial vacuum under the nappe.

Flumed and Unflumed falls:

A fall may either be constructed of the full channel width or it may be contracted. The contracted falls are known as the flumed falls while full channel width falls as the unflumed falls.

Comparison of different types of fall