Table of Contents
A Canal Fall/Drop may be defined as a solid masonry structure that is made on the canal if the natural ground slope is more vertical than the prepared channel bed slope.
If the difference in slope is less, a single fall can be made. If it is of higher then falls are built at standard appropriate intervals.
1. Necessity of Canal Fall
If the slope of the field suddenly turns to a steeper slope, the allowed slope of the bed cant be controlled. In order to maintain the slope, it requires severe earthwork in the filling.
In this type of case, falls are made to control undue earthwork during filling Where the slope of the land is somewhat uniform as well as the slope is more elevated than that of the allowed slope of the canal bed.
2. Location of Canal Falls
The location of the canal fall relies upon the following factors:
a. Topography of canal
b. Economy of excavation or filling
The two factors will choose the location of the canal fall across the canal. By understanding topographic situations we can deliver the needed type of fall which will provide good outcomes. At the same period, the supplied falls are economical and more applicable.
So, economic calculation is also significant. Unbalanced earthwork on the upstream and downstream outcomes in the project is cheaper.
3. Types of Canal Falls
The different types of canal falls are as follows.
a. Ogee Fall
b. Rapids Fall
c. Stepped Fall
d. Well Type Fall
e. Trapezoidal Notch Fall
f. Simple Vertical Drop Fall
g. Straight Glacis fall
h. Montague Type Fall
i. English Falls or Baffle Fall
a. Ogee Fall
The ogee curve may be defined as a mixture of convex curves as well as concave curves. Thus, Ogee Fall includes convex as well as concave curves.
Such an accumulative process seeks to ensure smooth flow transfer and then also reduces effects.
Unless the natural layer of the canal is suddenly moved to a steeper slope, ogee fall is recommended with this canal. Stone pitching is provided upstream as well as downstream of a break.
b. Rapids Fall
Rapid fall consists of a long sloping glacis. It is constructed if the available natural ground surface is plane and long.
For this, a bed of rubble masonry is provided and it is finished with cement mortar of a 1:3 ratio.
To maintain the slope of the bed curtain walls are provided both upstream and downstream. Rapid falls are high-priced constructions.
c. Stepped Fall
Stepped fall includes vertical steps at gradual breaks. Stepped fall is the adaptation of rapid fall.
It is appropriate for the canal which includes it upstream at a very high level as compared to downstream.
These two levels are attached by delivering vertical steps or drops.
d. Well Type Fall
Well-type falls are called siphon drop falls. In this case, an inlet well with the pipe at its base is made upstream.
The pipe holds the water to a downstream well or reservoir. If the discharge ability is better than 0.29 cumecs then the downstream well is selected otherwise reservoir is proper.
e. Trapezoidal Notch Fall
In the case of trapezoidal notch falls, a lofty crested wall is made across the channel and trapezoidal notches are delivered in that wall.
Trapezoidal falls are very cheap and appropriate for fewer discharges.
Nowadays, this type of fall is employed commonly because of its clarity and popularity.
Simple vertical drop fall consists single vertical drop which permits the upstream water to drop with a sudden hit on the downstream.
The downstream works as a cushion for the upstream water and removes excess energy.
This type of fall is used in Sarda Canal UP (India) and therefore, it is also known as a Sarda Fall.
g. Straight Glacis fall
In straight glacis, fall consists of a raised crest constructed over the canal and now a delicate rectangular surface is provided from the elevated crest to either the downstream.
The water that reaches upstream gets the elevated crest and falls to the inclined surface with generous energy dissipation.
h. Montague Type Fall
Montage fall is similar to straight glacis fall but in this case, the glacis is not straight. It is delivered in a parabolic form to present the vertical component of velocity which enhances the energy dissipation to more years.
i. English Falls or Baffle Fall
In this case, a straight glacis fall is developed as a baffle platform attached to a baffle wall. This is appropriate for any discharge.
The baffle wall is made close to the toe of the straight glacis at the needed distance in the planned height.
The primary objective of the baffle wall is to make a hydraulic jump from the straight glacis to the baffle platform.
3. Advantages of Canal Fall
The advantages of canal fall are mentioned below:
a. A canal fall is constructed over the canal to decrease the bed level in order to maintain the planned slope if there is a grade in the ground level to maintain the designed slope since there is a rise in the ground level.
b. Canal collapse is planned to reduce the slope of the canal bed. If the angle of the canal bed is not decreased, the amplitude of the flowing water increases to an extraordinary level.
c. If, on a sloppy site, the depth of the canal bed is not decreased, the building of the canal at a level more elevated than that of the ground becomes hard.
d. Construction of the canal fall would be employed to create steam.
e. Some of the canal falls are caused by the slope lowering the speed of the water falling in the canal. When the canal sinks sideways, the free-falling water damages the bottom of the canal.
4. Disadvantages of Canal Fall
The disadvantages of canal falls are as follows:
a. Higher structure will lead to higher repair and maintenance costs.
b. Higher seal as well as percolation loss.
c. The adjacent region is flooded owing to some probable failure in the embankment.
d. Problems of irrigation works.
e. As a result, the irrigation canal could not be set on high banks. Falls are then involved at appropriate regions to decrease the level of demand for the irrigation canal.
f. The canal water instantly downstream of the collapsed structure contains enormous kinetic energy that, although not demolished can scour the canal bed but also banks downstream of the fall. It will also risk the stability of the fall system. As a result, a fall in the canal is often followed by steps to disperse excess energy that, in effect, is a result of the construction of the fall.
g. The location of the fall is largely determined by the topography of the region but the need to combine the fall with many other structural elements, including the bridge, and the regulator, respectively. Throughout the context of the main canals, the construction costs must be regarded.
h. In addition, the relative economy of increasing the number of smaller falls (attaining balanced earthwork but the ease of installation) compared to the smaller number of major falls (resulting in lower construction costs and improved power generation) has also been figured.
i. Throughout the situation of channels that irrigate the command area immediately, a reduction should be made until the channel bed is drained. The full supply level of the river should be held just below the ground surface for a length of up to around 500 meters downstream of the fall, as the command area in this range can be irrigated by channels upstream of the fall.
5. References1. Content Filter & Authenticity Checking Team, Dream Civil International (Our team checks every content & detail to maintain quality.) |
Read Also: Groynes
