What is Canal Fall?

Canal fall is a solid masonry structure which is constructed on the canal if the natural ground slope is steeper than the designed channel bed slope. If the difference in slope is smaller, a single fall can be constructed. If it is of higher then falls are constructed at regular suitable intervals.

Location of Canal Falls

Location of Canal Fall depends upon the following factors:

1. Topography of canal.

2. Economy of excavation or filling.

The above two will decide the location of Canal Fall across canal by understanding topographic condition we can prove the required type of fall which will give good results. At the same time, the provided falls is economical and more useful. So, economical calculation is also important. Unbalanced earth work on upstream and downstream result the project more uneconomical.

Types of Canal Falls and their Importance.

The important types of falls which were used in olden days and those which are being used in modern days are described below:

• Ogee falls

• Rapids

• Stepped falls

• Trapezoidal notch falls

• Well type falls

• Simple vertical drop falls

• Straight glacis falls

• Montague type falls

• English falls or baffle falls.

Ogee Canal Falls

Ogee curve is the combination of convex and concave curves. So, Ogee fall consists of both convex and concave curves gradually. This gradual combination helps to provide smooth transition of flow and also reduce the impact. If the canal natural ground surface is suddenly changed to steeper slope, ogee fall is recommended for that canal. Stone pitching is provided in the upstream and downstream of the fall.

Rapid Canal Falls

Rapid Falls consist 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 1:3 ratio. To maintain the slope of bed curtain walls are provided at both upstream and downstream. Rapid falls are high priced constructions.

Stepped Rapid Canal Falls

As in the name itself, Stepped fall consist vertical steps at gradual intervals. Stepped fall is the modification of rapid fall. It is suitable for the canal which has it upstream at very high level as compared to downstream. These two levels are connected by providing vertical steps or drops as shown in figure.

Trapezoidal Notch Canal Falls.

In case of trapezoidal notch falls, a high crested wall is built across the channel and trapezoidal notches are provided in that wall. Trapezoidal falls are very economical and suitable for low discharges. Now a days this type of falls are using widely because of their simplicity and popularity.

Well Type Canal Falls.

Well type falls are also called as syphon drop falls. In this case, an inlet well with pipe at its bottom is constructed in upstream. The pipe carries the water to downstream well or reservoir. If the discharge capacity is more than 0.29 cumecs then downstream well is preferred. If the discharge capacity is more than 0.29 cumecs then downstream well is preferred otherwise reservoir is suitable.

Simple Vertical Drop Falls (Sarda Type Fall)

Simple Vertical Drop Fall or Sarda Fall consists, single vertical drop which allows the upstream water of fall with sudden impact on downstream. The downstream acts like cushion for the upstream water and dissipate extra energy. This type of all is tried in Sarda Canal UP (India) and therefore, it is also called Sarda Fall.

Straight Glacis Canal Falls.

This is the modern type of construction, in which a raised crest is constructed across the canal and a gentle straight inclined surface is provided from raised crest to the downstream. The water coming from upstream crosses the raised crest and falls on inclined surface with energy dissipation.

Montague Type Canal Falls.

Montague Fall is similar to straight glacis fall but in this case the glacis is not straight. It is provided in parabolic shape to introduce the vertical component of velocity which improves the energy dissipation to more extent.

In this case, straight glacis fall is extended as baffle platform with baffle wall. This is suitable for any discharge. The baffle wall is constructed near the toe of the straight glacis at required distance in designed height. The main purpose of the baffle wall is to create hydraulic jump from straight glacis to baffle platform.

Canal Head Regulator.

Structure at the head of canal taking off from a reservoir may consist of number of spans separated by piers and operated by gates. Regulators are normally aligned at 90° to the weir. Up to 10” are considered preferable for smooth entry into canal. The functions of canal head regulator are:

1. To admit water into the off taking canal.

2. To regulate the supplies into the canal.

3. To indicate the discharge passed into the canal from design discharge formula and observed head of water on the crest.

4. To control the slit entry into the canal. During heavy floods, it should be closed otherwise high slit quantity will leave to the canal.

Types of Canal Head Regulator.

Following are the common types of Canal Head Regulator:

1. Still pond regulation.

2. Open flow regulation.

3. Slit control devices.

1. Still pond regulation.

• Canal draws water from still pond.

• Water in excess of canal requirements is not allowed to escape under the sluice gates.

• Velocity of water in the pocket is very much reduced; slit is deposited in the pocket.

• When the slit has a level about ½ to 1 m below the crest level of head Regulator, supply in the canal is shutoff and sluice gates are opened to scour the deposited slit.

Open flow regulation.

• Sluice gates are opened and allow excess of the canal requirement.

• Top water passes into the canal.

• Bottom water maintains certain velocity in the pocket to keep the slit to remain in suspension.

• Canal is not closed for scouring the slit.

Slit Control Devices.

Another type of Canal Head Regulator is the slit control device.

• Slit control at head works can be controlled by providing a divide all to create a trap or pocket.

• Create scouring capacity of under sluices by concentrating the currents towards them.

• Paving the bottom the approach channel to reduce disturbance because due to disturbance sediment remains in suspension.

Cross Head Regulator.

Definition

A cross regulator is a structure constructed across a canal to regulate the water level in the canal upstream of itself and the discharge passing downstream of it for one or more of the following purpose:

1. To feed off taking canals located upstream of the cross regulator.

2. To help water escape from canals in conjunction with escapes.

3. To control water surface slopes in conjunction with falls for bringing the canal to regime slope and section.

4. To control discharge at an outfall of a canal into another canal or lake.

It is also called a canal head regulator. A cross regulator is generally provided downstream of an off taking channel so that the water level upstream of the regulator can be raised. Whenever necessary, to enable the off taking channel draw its required supply even if the main channel is carrying low supply. The need of a cross regulator is essential for all irrigation systems which supply water to distributaries and field channels by rotation and therefore, require to provide full supplies to the distributaries even if the parent channel is carrying low supplies. Cross regulators may be combined with bridges and falls for economic and other special considerations.

Necessity/Importance of Canal Falls:

When the slope of the ground suddenly changes to steeper slope, the permissible bed slope cannot be maintained. It requires excessive earthwork in filling to maintain the slope. In such a case falls are provided to avoid excessive earth work in filling when the slope of the ground is more or less uniform and the slope and the slope is greater than the permissible bed slope of canal.

In that case also the canal falls are necessary. In cross drainage works, when the difference between bed level of canal and that of drainage is small or when the F.S.L of the canal is above the bed level of drainage then the canal fall is necessary to carry the canal water below the stream or drainage.