Canal falls, also known as weirs or barrages, play a vital role in hydraulic engineering by regulating the flow of water in canals and rivers. These engineered structures are designed to control water levels, facilitate irrigation, and prevent erosion, making them essential components of water management systems worldwide. Canal falls come in various forms and are built to suit specific purposes, from diverting water for agricultural needs to generating hydropower.

This article explains Canal Falls and its different types. This topic is important for all the upcoming civil engineering examinations, including SSC JE CE and RRB JE Civil.

What is Canal Fall?

A canal fall is an engineered structure built within a water channel to facilitate the controlled lowering of water levels and the safe dissipation of excess energy from descending water, preventing potential erosion of the channel's bed and banks. When the natural terrain slope exceeds the desired channel bed slope, this variance is rectified by incorporating vertical falls or drops at appropriate intervals along the channel's bed.

Necessity of Canal Fall 

When the field's slope abruptly becomes steeper, regulating the canal bed slope becomes challenging and necessitates extensive earthwork in the filling process. To manage this situation efficiently, falls are constructed to prevent excessive earthwork when the land slope remains relatively consistent and exceeds the designated canal bed slope.

Location of Canal Falls

The location of a canal fall is contingent upon key determinants, including:

• The natural topography of the canal's surroundings.
• The economic considerations related to excavation and filling.

These factors collectively influence the strategic placement of the canal fall. A thorough understanding of the topographical context aids in selecting the most suitable type of fall and optimising its performance and cost-effectiveness. Simultaneously, an economic assessment plays a vital role in decision-making, as achieving a balanced distribution of earthwork between upstream and downstream sections proves to be the more economical choice for the project.

Different types of canal falls are designed based on factors like discharge, bed slope, soil condition, and energy dissipation needs. Each type has a unique shape and hydraulic behaviour suited for specific site conditions. Understanding their design features helps in selecting the right fall for efficient canal regulation and erosion control. The various types of Canal Falls are:

a. Ogee Fall:

Fig 1: Ogee Fall

The ogee fall incorporates a combination of both convex and concave curves. This design ensures a smooth transition of water flow and minimises adverse effects. It is typically recommended for canals unless a sudden change in natural terrain to a steeper slope occurs, in which case stone pitching is added both upstream and downstream to manage the transition effectively.

b. Rapids Fall:

Fig 2: Rapids Fall

The rapid fall features a long sloping glacis and is constructed when the natural ground surface is relatively flat and extensive. A bed of rubble masonry is used, finished with a cement mortar mixture. Curtain walls are provided upstream and downstream to maintain the slope of the bed, although it's worth noting that rapid falls can be more expensive to construct.

c. Stepped Fall:

Fig 3: Stepped Fall

Stepped fall involves vertical steps at gradual breaks and is an adaptation of the rapid fall design. It is suitable for canals where the upstream is significantly higher than the downstream, connecting these two levels with vertical steps or drops to control water flow effectively.

d. Well-type Fall:

Fig 4: Well Type Fall

Well-type falls, also known as siphon drop falls, incorporate an inlet well with a pipe at its base upstream. This pipe conveys water to a downstream well or reservoir, with the choice between the two depending on discharge capacity.

e. Trapezoidal Notch Fall:

Fig 5: Trapezoidal Notch Fall

Trapezoidal notch falls feature a raised crest wall across the channel with trapezoidal notches. They are cost-effective and suitable for lower discharge conditions, making them increasingly popular due to their simplicity and efficiency.

f. Simple Vertical Drop Fall:

Fig 6: Simple Vertical Drop Fall

The simple vertical drop fall consists of a single vertical drop, allowing upstream water to descend with a sudden impact on the downstream section, which acts as a cushion and dissipates excess energy. This design is used in the Sarda Canal UP in India and is often referred to as a Sarda Fall.

g. Straight Glacis Fall:

Fig 7: Straight glacis fall

A straight glacis fall includes a raised crest over the canal and a smooth rectangular surface slopes from the crest to the downstream section. This design efficiently dissipates energy as water from upstream flows over the elevated crest and onto the inclined surface.

h. Montague Type Fall:

Fig 8: Montague Type Fall

Similar to a straight glacis fall, the Montague fall features a non-straight glacis, often in a parabolic shape, enhancing the vertical component of velocity and thus increasing energy dissipation.

i. English Falls or Baffle Fall:

Fig 9: English Falls or Baffle Fall

In this design, a straight glacis fall is combined with a baffle platform and wall. The baffle wall is strategically placed near the toe of the straight glacis to create a hydraulic jump, making it suitable for various discharge scenarios.

Maintenance of Canal Falls

Proper upkeep of canal falls is crucial to ensure smooth water regulation and to prevent damage to the canal structure. Over time, water flow can cause wear and tear, especially at locations where the velocity is high or sediment accumulates. Neglecting maintenance can lead to costly repairs or even structural failure.

Routine inspection and timely action are necessary to keep the fall functioning efficiently. Field engineers and irrigation staff are usually assigned to observe and address any signs of deterioration. Below are the key maintenance tasks followed on-site:

• Desilting of the stilling basin and downstream bed to prevent blockage.
• Replacing loose or damaged stone pitching on upstream and downstream slopes.
• Patching up cracks near the crest, glacis, or side walls with proper repair mortar.
• Removing weeds or vegetation growth that may obstruct flow or trap debris.
• Checking the condition of energy dissipators like baffle platforms, if provided, and repairing them if damaged.

Energy Dissipation in Canal Falls

When water drops from a height in a canal fall, it gains high velocity. If this energy is not controlled, it can damage the canal structure and cause erosion. That’s why energy dissipation is an important part of canal fall design.

Structures like baffle walls, stilling basins, and stepped surfaces help break the flow and reduce velocity. The goal is to make the water safe before it continues downstream. Each type of fall uses different methods to achieve this, depending on how much energy needs to be controlled.

Comparison of Canal Falls Based on Discharge

Different canal falls are suitable for different flow rates. Engineers select a fall type based on how much water passes through the canal, the terrain, and construction cost. The aim is to match the fall with the canal's needs. The table below compares major types of canal falls and where they are most effective:

Advantages of Canal Fall

• Controls water flow and minimises erosion.
• Facilitates energy dissipation.
• Enables water level regulation.

Disadvantages of Canal Fall

• Construction cost.
• Maintenance and repair expenses.
• Potential for sediment accumulation.

This blog enumerated about Canal falls and also discussed the different types of Canal falls. This topic is important for your upcoming exams. If you are preparing for State and Central level Civil examinations and other diploma-level exams, get enrolled in AE/JE Civil Coaching on the Testbook App.

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