Types of Canal Outlets | Civil Engineering Optional Notes for UPSC PDF Download

Canal Outlets: Types and Characteristics

• Non-Modular Outlets
• Semi-Modular Outlets
• Modular Outlets

Non-Modular Outlets

• Non-modular canal outlets have discharge capacities influenced by water level discrepancies between the distributary and the watercourse. The flow through these outlets varies significantly with changes in water levels in either channel.
• The control of non-modular outlets is managed by a shutter located at their upstream end. These outlets experience less head loss compared to modular outlets, making them suitable for low-head conditions.
• However, maintaining equitable water distribution from non-modular outlets during high demand periods is challenging due to potential discharge variations even with constant distributary water levels.
• Typically, a non-modular canal outlet is constructed as a submerged pipe or a masonry sluice fixed perpendicular to the distributary's flow direction. The pipe diameter ranges from 10 to 30 cm and is placed on a light concrete foundation to prevent uneven settling and leakage issues.

Pipe Outlets and Water Distribution

• Pipe Inlet Placement

  The pipe inlet is usually positioned approximately 25 cm below the water level in the distributary. In cases where significant fluctuations in the distributary water level are expected, the inlet is placed below the minimum water level in the distributary to ensure functionality.

• Variability in Discharge

  Discharge through non-modular outlets fluctuates based on water levels in both the distributary and the watercourse. For fields at higher elevations, where the watercourse level is elevated, the discharge is relatively low. Conversely, fields at lower elevations experience larger discharges due to lower watercourse levels.

• Challenges and Solutions

  Managing water distribution proves challenging due to factors such as varying water withdrawal in head reaches, leading to either dry or flooded tail reaches. Increasing discharge through pipe outlets can be achieved by deepening the watercourse to lower water levels.

• Drawbacks and Benefits

  Ensuring proper and equal water distribution is complex, highlighting a major limitation of pipe outlets. However, non-modular outlets are effective for low heads and are commonly utilized in initial distribution stages or for additional irrigation when surplus water is available.

• Semi-Modular Outlets

  Semi-modular canal outlets, also known as semi-modules or flexible outlets, rely solely on the water level in the distributary for discharge. They are unaffected by water levels in the watercourse as long as a minimum required working head is maintained.

  

Semi-Modular Canal Outlets

• A semi-module is a preferred choice for ensuring equal water distribution across all distributary outlets. However, it comes with the drawback of higher head loss.
• The most basic semi-modular outlet is a pipe outlet that discharges openly into the atmosphere. When the pipe outlet discharges into the watercourse with its exit end positioned above the water level, it functions as a semi-module.
• The working head 'H' is the variance between the distributary water level and the pipe outlet's center. Unlike other types, the discharge through a pipe outlet remains unaffected by the cultivator lowering the water level in the watercourse.
• Additional flexible outlet options include Kennedy's gauge outlet, open flume outlet, and orifice semi-modules.

Kennedy's Gauge Outlet

• R.G. Kennedy introduced this outlet in 1906, comprising an orifice with bellmouth entry, an elongated delivery pipe, and a vertical air column above the throat to allow air circulation around the jet.
• The design ensures that the outlet's discharge is independent of the water level in the watercourse. The water jet flows into a cast iron expanding pipe, typically around 3 meters long, with a cement concrete extension at its end, facilitating water discharge into the watercourse.

Types of Canal Outlets

• Blocked Orifice Outlet:

  An outlet design where the cultivator can manipulate the air vent pipe to increase discharge. However, this design is vulnerable to tampering and is costly, hence not commonly used.

• Open Flume Outlet:

  An open flume outlet is essentially a weir with a narrow throat to ensure supercritical flow. It is constructed with brick masonry, featuring a gradually expanding section downstream of the throat. The outlet structure includes cast iron or steel components for durability. This design facilitates the formation of hydraulic jumps, ensuring that the outlet discharge remains unaffected by the water level in the watercourse. For instance, the open flume outlet commonly found in Punjab demonstrates a discharge proportionate to H^3/2.

  
• Orifice Semi-Modules:

  An orifice semi-module comprises an orifice followed by an expanding flume downstream. The supercritical flow through the orifice leads to the formation of a hydraulic jump in the expanding flume, ensuring that the outlet discharge remains independent of the water level in the watercourse. To maintain consistency in discharge coefficient, the roof block is structured to converge streamlines. It is secured in place using bolts embedded in a masonry key, allowing for adjustments when necessary.

Modular Canal Outlets

Modular Outlets :

• In modular canal outlets, the discharge remains constant irrespective of water levels in the distributary and the watercourse within reasonable limits.
• These outlets can be either with moving parts or without them, known as rigid modules. Outlets with moving parts are complex to design and construct, making them more expensive.
• Modular canal outlets provide a fixed discharge, allowing farmers to plan their irrigation activities effectively. However, during surplus or deficient supplies in the distributary, the tail-end might face issues such as flooding or water deprivation due to the fixed discharge nature of the outlet.
• If there's a need for an outlet in a branch canal experiencing significant discharge fluctuations, a modular outlet is a suitable choice. It is placed at a lower level to ensure proper water distribution during low-supply periods.
• During periods when the branch canal must carry excess water to meet distributary demands, the discharge via the modular outlet remains unchanged, facilitating the smooth transfer of surplus water to the required distributaries.

Modular Outlets in Irrigation

• Gibb's Rigid Module:

  This type of modular outlet consists of an inlet pipe situated under the distributary bank. The inlet pipe transports water from the distributary to a rising spiral pipe that connects to the eddy chamber. This design promotes free vortex motion, causing the water to rise near the outer wall of the rising pipe, thereby sloping the water surface towards the inner wall.
  

  A series of baffle plates of appropriate size are suspended from the roof of the eddy chamber, with their lower ends sloping against the flow direction. As the head increases, the water rises at the outer wall of the eddy chamber, striking the baffles and swirling in the space between consecutive baffle plates. This action dissipates excess energy, ensuring a consistent discharge. However, this type of outlet tends to be more expensive, and its sediment removal efficiency is suboptimal.

• Khanna's Rigid Orifice Module:

  This type of modular outlet, devised by Khanna, operates differently. It is crucial to understand that...

• Explanation of Canal Outlet Functionality
• Design and Purpose of Sloping Shoots
• Maintenance of Constant Discharge
• Flexibility in Adjusting Shoots for Local Needs

Assessment of Charges for Irrigation Water

• Methods for Calculating Irrigation Water Charges
• Factors Influencing Cost Assessment

Delivery of Water to Farms by Rotation Delivery System

• Operational Mechanisms of Rotation Delivery System
• Advantages of Farm Water Delivery via Rotation