Canal FallsCanal Falls

Canal Falls

The longitudinal slopes of canals (Particularlyunlined channels) are fixed as per sedimenttransport capacity i.e. for non-silting and non-scouring velocity. It may happen that thescouring velocity. It may happen that thetopography of the land on which the canal is tobe laid is steeper than the designed bed slopes ofthe canal, resulting in excessive filling and highembankments on which the canal is to be laid.

Canal Falls

This result in an increase in the cost ofconstruction and also the differential head i.e.the difference between the ground water tableand the water level in the canal may causeand the water level in the canal may causeexcessive seepage which results in water logging.

Canal Falls

Canal Falls

To avoid this, the bed of the canal is given asudden drop or fall at a suitable place, so that itmay run partially in excavation (cutting) andpartly in filling, depending upon the commandpartly in filling, depending upon the commandareas. The structure built to safeguard the dropis called a canal fall structure.

Requirements of Canal Fall Structures

The basic requirements for the canal falls are:

Suitable Energy Dissipation Arrangement:

In the case of vertical falls there should be a proper In the case of vertical falls there should be a propercistern to take the impact of the falling water.

In case of glacis and modern weir-type falls, energydissipation is achieved by the formation of thehydraulic jump at the toe of the glacis plus frictionblocks.

REQUIREMENTS OF CANAL FALLS

Fluming

If there has to be a bridge, then a suitable amountof fluming (constriction) is to be provided foreconomy and for this a suitable u/s and d/seconomy and for this a suitable u/s and d/stransition has to be given.

Crest Level

The crest must be designed to maintain normalsupply depths in the canal u/s.

REQUIREMENTS OF CANAL FALLS

Seepage Path

To increase the seepage path a pucca floor isprovided on both sides (i.e. u/s and d/s side)and then the stone pitching is also done due toand then the stone pitching is also done due toscouring nature of water.

Low Cost

The initial cost of construction and the cost ofmaintenance should be as low as possible.

Location of Canal Falls

The following factors are considered for thelocation of falls

For main and branch canals which do not For main and branch canals which do notdirectly irrigate the site is determined on thebasis of economy in earth works.

All the excavated earth is utilized in making upbanks.

The depth of excavation is known as thebalancing depth of excavation. A suitable sitewill be where, the depth of excavation is less.

Location of Canal Falls

For the distributaries and minors, falls may belocated d/s of the outlets as this helps in increasingthe command area, and in improving the efficiencyof outlets.

The site should be selected keeping in mind the The site should be selected keeping in mind therequirements for a road crossing, as a bridgecombined with a fall offers an economical structure.

A regulator may be combined with a fall d/s of theoff-taking channels as has been done inBombanwala headworks on Upper Chenabcanal where BRBD canal and Rayya branch canaltake off.

Historical

Ogee Type Fall

Historical

Rapids

Sloping glacis were 1:10 to 1:20

Were successful

Phenomenon of hydraulic jump was introduced Phenomenon of hydraulic jump was introducedto dissipate energy

Very expensive

TYPES OF FALLS

Canal falls can be divided into two general typesfrom the construction and energy dissipationpoint of view:

1. Vertical drop falls

2. Sloping glacis type

Vertical Drop Falls This consists of

(1) a vertical retaining wall, (2) a cistern to take the impact of the falling jet and (3) friction blocks to destroy the remaining energy.

They may or may not be flumed. In the case offfluming, proper curves have to be given to u/sbanks.

The cistern element is that portion of the d/s floorwhich is depressed below the normal bed level, toprovide a cushion of water to take the impact offalling water.

Vertical Drop Falls

Sloping Glacis Type

Such types of falls are similar to modern weirand depends on the formation of a hydraulicjump at the toe of the glacis for destroying thekinetic energy. For further reduction in velocitykinetic energy. For further reduction in velocityfriction blocks may be used, as for vertical dropfalls. Fluming may also be done in this case.

Sloping Glacis Type

Friction Blocks

To dissipate the energy more, after the cisternelement, the staggered friction blocks are provided.The height of blocks is 1/4 of total depth of water onThe height of blocks is 1/4 of total depth of water onthe d/s side. Depending upon design and shape, theupper portion of the blocks may also be pointed.

Fluming

Normally a canal fall offers a good opportunity toprovide a bridge or a canal regulator. Economy canbe achieved by constricting the canal width.