When a canal crosses a natural drain, a structure called cross drainage works is constructed to prevent water from the drain from getting into the canal. Such structures are, however, avoided as much as possible as they are very expensive and lead to making the irrigation project uneconomical. Notably, there are different types of Cross Drainage Works depending on whether the canal passes over, below, or at some level of the drain. 

This descriptive blog encompasses all the relevant details pertaining to Cross drainage works that will be of invaluable significance when it comes to keeping yourself updated for your GATE CE, SSC JE CE, and RRB JE Civil preparation.

What is Cross Drainage Works?

Cross drainage works are structures constructed to manage the intersection of a canal and a natural drainage channel, such as a river, stream, or nallah (seasonal watercourse). These works ensure that the flow of the natural drain and the canal remains uninterrupted and neither body of water interferes with the other.

Purpose of Cross Drainage Works:

• To allow natural drainage (rainwater runoff, streams, rivers) to cross a man-made canal without disrupting the canal’s water supply.
• To protect the canal from flooding or damage due to heavy flows in the natural watercourse.
• To maintain the functionality of both the irrigation system and the natural drainage system.

Cross-drainage works refer to the infrastructure and techniques used to facilitate the smooth passage of water across natural or artificial obstacles, such as roads, railways, canals, or embankments. These works are crucial in ensuring the efficient management of water resources and preventing flooding or waterlogging in the surrounding areas.

Types of Cross Drainage (CD) works:

The three types of cross drainage works are:

• Type 1: CD work carrying canal over the drain
• Type 2: CD work carrying Drainage over the canal
• Type 3: CD works admitting canal water into the canal

Type 1: Canal Passing Over Drainage [HFL below FSL]

Aqueduct

• The canal bed level in an aqueduct is higher than the drainage bed level, necessitating the construction of the canal above the drainage. It is planned to build a canal trough where water will flow from upstream to downstream. This canal trough is supposed to be supported by many piers. The piers allow the water to drain from upstream to downstream. Full supply level (FSL) refers to the water level in the canal, and high flood level (HFL) refers to the water level in the drainage system (HFL). The canal bed level is lower than the HFL. Aqueducts are similar to bridges in that they carry canal water in a trough, below which drainage water flows under the influence of gravity and air pressure.

Siphon Aqueduct

In a siphon aqueduct, the canal trough is above the drainage, while the drainage has a high flood level (HFL) above the canal. The natural drainage water moves under siphonic action since there is no air pressure there. The siphon aqueduct structure is built in such a way that the drainage flooring is depressed downward in order to discharge high-flow drainage water through the depressed concrete floor. Even though they are more expensive, syphonic aqueducts are more frequently built and are chosen over plain aqueducts.

Fig 1: Siphon Aqueduct

Type 2: Drainage Over Canal (HFL above FSL)

When the High Flood Level (HFL) of a drainage channel is above the Full Supply Level (FSL) of a canal, it means that the natural drainage (like a river or stream) flows at a higher elevation than the canal. In such cases, cross drainage works are designed to allow the drainage to pass over the canal without disrupting either flow.

Super Passage

A Super passage is constructed when the drainage bed level lies below the canal bed level. It facilitates for transporting the drainage over the canal. Drainage water runs via the drainage trough, which is built at road level, from upstream to downstream. On the other hand, canal water flows through the piers that are built below the drainage trough as supports. In this arrangement, the drainage trough lies below the entire supply level of the canal. The water in the canal is under the pressure of the atmosphere and flows by gravity. This can be considered as Aqueduct construction in reverse.

Fig 2: Super passage

Canal Syphon

Similar to a super passage, drainage is transported over a canal in a canal siphon. However, in the case of Canal Syphon, the full supply level is above the drainage trough. As a result, there is no air pressure in the canal, and the water flows as a result of the siphonic activity. When contrasted, the super passage is frequently favored over canal Syphon since the water in canal Syphon is under a drainage trough, making it difficult to remove any silt that may have been deposited easily. To create siphonic activity, the canal floor is lowered, and ramp-like structures are set up on upstream and downstream. 

Fig 3: Canal Syphon

Type 3: Drainage Merged With Canal (HFL = FSL)

When drainage water is blended with canal water, building costs are lower, but silt removal and canal water maintenance is significantly more challenging. As a result, the cross drainage works that belong to this category are built with utmost care.

Level Crossing

A level crossing must be built when the canal's bed level is equal to the bed level of the drainage system. They include the following actions:

1. Construction of a weir so as to prevent drainage water
2. Construction of canal regulator across the canal
3. Construction of head regulator across the Drainage

Fig 4: Level Crossing

Functioning of a level crossing: Both regulators are opened to separate the drainage water from the canal water at periods of peak canal water supply and drainage. The head regulator is shut off after the drainage has been cleaned. Nonetheless, the cross regulator remains continually operational throughout the year to provide the canal with water.

Canal Inlets

The drainage water that is allowed to merge with a canal is very less. At the canal inlets, the drainage is carried along the canal banks. After traveling a set distance in the canal mixed with drainage, the solids from the drainage are sucked up by a suction device and dumped nearby in a watershed region. Using canal entrance construction has a number of drawbacks, including the possibility of canal water pollution from drainage and high maintenance costs due to canal structural deterioration from bank erosion. As a result, this kind of construction is not often conformed to.

Fig 5: Canal Inlets

Selection of Types of Cross Drainage 

The selection of the type of cross-drainage works depends on various factors, including the nature of the obstacle, the volume and velocity of water flow, the topography of the area, and the intended purpose of the structure.

Considerations for Cross Drainage Works

• Water flow characteristics: Analyzing volume, velocity, and frequency to design suitable structures.
• Topography: Assessing terrain for proper location, alignment, and elevation.
• Hydraulic analysis: Evaluating dimensions to ensure efficient water passage.
• Environmental impact: Considering ecological effects and mitigation measures.
• Structural integrity: Designing robust structures to withstand forces and prevent damage.
• Longevity and maintenance: Planning for durability and ease of upkeep.
• Cost-effectiveness: Optimizing functionality within budget constraints.

Necessity of Cross Drainage Works

Cross Drainage Works play an important role in irrigation engineering due to the following reasons: 

• They maintain water flow smoothly and prevent water from mixing in the canal and drainage systems.
• The natural flow direction of the canal and drainage cannot be changed at the crossing point sites. Hence, cross drainage works are offered to keep their flow in the appropriate directions.
• Natural drainages are not crossed by the watershed canals. Nevertheless, due to the real canal network's direction, this perfect situation could not exist, and natural drainage systems might block the canal. To run the irrigation system, cross drainage works must be provided.

Factors Influencing the Selection of Cross Drainage Structure 

The selection of the site for cross drainage structure depends on

• Relative Bed Level: The decision will be affected by the relative bed levels of the canal and mainline.
• Foundation: A strong foundation is necessary for installing the advised drainage system.
• Cost: The advantages of the canal system should surpass the price of building the proposed hydraulic construction.
• Discharge: The hydraulic structure put in place must be able to endure the seasonal condition since there are occasions when the mainstream flow will be higher than expected.
• Accessibility: At that specific location, the proposed structure should have access to the necessary construction and communication supplies.

Thats all about the different types of Cross drainage works and their functions. To stay updated with all the topics related to the upcoming examinatios, including SSC JE and GATE, you should get enrolled in the AE/JE Civil Coaching and GATE Civil Coaching available on the Testbook App.

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