TYPES OF CANAL FALL
Canal Falls is usually one of the following:
(i) The canal falls largely maintain the normal depth-discharge relationship:
Notch falls — trapezoidal or rectangular shape — are the type. A rectangular notch or low wire is not able to accurately perform the normal depth-discharge relationship but it is more suitable for economies of scale and discharge measurement.
In the fall of the trapezoidal notch, several trapezoidal notches are made in the chest wall.
This system provides an opening for flow up to the bed stage and thus removes the silt in the fall upstream channel.
The shape of the trapezoidal grade is determined based on full supply and half supply conditions (1).
(ii) canal waterfalls that maintain a nearly constant water surface level in the upstream channel:
When a subsidiary channel is taken upstream of a fall, or when a fall is combined with a hydroelectric plant, it is desirable to maintain the surface of the water in the source channel as stable as possible.
Siphon Falls and High-Crested Wire Falls meet this need. The siphon falls, though, are not so effective, very expensive, and, therefore, only used as siphon spillways in dams and not as canal waterfalls.
High-crested weir falls are not usually flammable so q is small, to keep the discharge per meter long in the fall.
If the discharge intensity is small, the head is small and, therefore, the water level on the fall can be maintained at a relatively steady level.
The smallest value of q will facilitate the discharge of energy. such waterfalls are, therefore, relatively inexpensive.
Also, see: Types of canals
In general, the length of the fall is limited to the width of the channel but can be increased by providing an extension after a contraction in the channel. However, this kind of provision raises the cost of fall construction.
Depending on the type of wire crest, these waterfalls can be used as metering devices after proper calibration, whether wide or narrow and the flow condition, whether open or submerged.
The high-crest fall with vertical impact was first used in the Sarda Canal system in UP. The drop rate does not exceed 1.80 m in any collapse in this canal system.
However, This system requires a large number of small waterfalls as there is a layer of pure sand beneath a thin layer of clay sand.
Therefore, the depth of excavation had to be reduced for channel construction so that leakage losses were kept to a minimum.
Watch this for a better understanding of the types of canal fall:
(iii) Canal waterfalls permit a change in water levels over the fall:
The need for such waterfalls arises when the upstream subsidiary channel of the falls has to be fed
a minimum supply level in the original channel. Such falls include trapezoidal or rectangular notes.
However, trapezoidal notches are relatively expensive and characterize the operation of controllers, such as stop logs and vertical stripes.
See also: Alignments of irrigation canal
In general, this category of falls, therefore, consists of rectangular notes composed of one of the following three types of controllers:
(a) Raising or lowering the sluice gate-gate helps regulate the upstream level.
(b) Horizontal stop logs inserted into grooves — their removal or insertion causes the required change in the upstream level.
(c) Vertical strips (or needles) – These change the effective width of the channel (ie, the opening width) and do not lead to silting.
(iv) In addition to the main types of waterfalls above, there may be waterfalls designed to meet specific requirements.
Cylindrical waterfalls (also known as good waterfalls), pipe falls, chutes, etc. are designed for specific needs.
For example, a cylindrical fall (Fig. 10.4) is suitable for low discharge and high drop but canal or rapids may need to be built if the canal is to carry lumber.
However, Canal waterfalls, alternatively, can be classified based on their ability to measure discharge. Similarly, they can be meter falls or non-meter falls.
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Also read: List of canals in India