Why Is Cross Drainage Works Important?

Cross Drainage is an essential and inseparable part of irrigation. If you want to understand more about this process, go through this article. 

Introduction to Cross Drainage

Cross Drainage Works directly to systems made for crossing canal water over or another drainage water. Here in this article, you will learn all the details about the cross drainage works, which is an inseparable part of irrigation.

What is Cross Drainage?

Whenever there is an irrigation project, there will be a proper network of main canals, branch canals, and distributors. When there is a network like this, the Construction of the canals may have to cross the natural Drainage like any river or stream. This Construction is built in such a way that helps to prevent the drain water from mixing with the canal water. It maintains the purity of water in the irrigation system. It can be done in different locations within the irrigation area of the project.

By mixing, a few streams are to be combined into one and just only one cross drainage. This will be a completely economical solution.

Also, all crossing canals with such a barrier or obstacles can not be avoided under any circumstances. So, to get an easy flow of can and drainage water, a proper and suitable structure has to be constructed at every crossing point in the respective direction of rivers or streams. These structures are known as the cross drainage works. The artificial canal is usually combined with a ridgeline called a watershed. 

The most important thing is that the cross-drainage system is too expensive to do. The whole cross-drainage work can be avoided. It can be avoided in a few ways mentioned below.

• The artificial canal is usually combined with a ridgeline called a watershed.
• Mixing two or three streams into one and only one cross drainage work to be constructed, and this process makes the structure economical.
• By diverting one stream to another or changing the alignment of the canal.

When crossing the canals with obstacles, it is necessary to construct suitable and strong structures at the points of the canal and those natural obstacles like rivers or streams. Now, one of the most essential things to keep in mind is that the nature of cross-drainage works may differ at different locations. Sometimes, the bed level of the canal may be below the bed level of Drainage. On the other hand, it may be higher than that of the Drainage. The bed levels of the canal and Drainage may be nearly the same. So the structures are different at different places, and the designations are also different.

Needs of cross drainage works

There is a wide array of the necessity of cross-drainage works. Here are those necessities.

• The cross drainage work helps to maintain the smooth running of water and prevent intermixing of water for both canal and Drainage.
• At the site locations of the crossing point, the natural flow direction of the canal and Drainage must be redirected. Thus, to support their flow in respective directions, cross-drainage designs are furnished.
• The watershed canals do not intersect natural drainages. But in the actual direction of the canal network, this perfect situation may not be available, and the obstacles like natural drainages may be situated across the canal. So, the cross-drainage works must be equipped to handle the whole irrigation system with ease and perfection.

Types of Cross drainage works

There are a total of three kinds of cross-drainage works. Those are mentioned below.

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

Type – 1: Cross drainage work carrying the canal over the drain is further divided into two types based on structures: 

• Aqueduct
• Syphon Aqueduct

Type – 2: Cross Drainage work carrying Drainage over the canal structures is also divided into two types:

• Super passage
• Canal Syphon

Type –3: Cross drainage works admitting canal water into the canal are divided into two different categories:

• Level Crossing
• Canal inlets

Type – 1: Cross drainage work carrying the canal over the drain 

• Aqueduct 

This structure’s canal bed level is always above the drainage bed level. In this way, a canal will be constructed above the Drainage. The canal trough will be rested on a few numbers of piers. The drained water will flow through all of those piers from the upper stream to the downside of the stream. The canal water level is referred to as the whole supply level (FSL), and the drainage water level is referred to as the high flood level (HFL). 

• Syphon Aqueduct 

Syphonic aqueducts are more often constructed and better preferred than simple Aqueducts, but this Construction is relatively costlier. In this structure, canal water is the carrier above the Drainage. But in this case, the high flood level of Drainage is always above the canal trough. Also, the drainage water flows under symphonic action. There will be no such atmospheric pressure.

Type – 2: Cross Drainage work carrying Drainage over the canal

• Super passage

The super passage structure carries Drainage above the canal as the canal bed level is below the drainage bed level. The whole water supply level will be below the drainage trough in this structure. The water in the canal flows under gravity. Also, atmospheric pressure will be there in this structure. 

• Canal Syphon

In a canal siphon, Drainage is carried over a canal similar to a super passage. But there are a few changes. The whole supply level of the canal is above the drainage trough. So, the result of this structure is the canal water flows under siphonic action, and there is no atmospheric pressure in the canal. However, the super passage is preferred to the canal siphon. In canal siphon, the flooring of the canal is down, and a ramp-like structure is delivered upstream and downstream to construct syphonic motion.

Type –3: Cross drainage works admitting canal water into the canal

• Level Crossing

When the bed level of the canal is equal to the drainage bed level, then level crossing is to be constructed. This structure will demand a few steps, the Construction of a weir to stop drainage water behind it, the Construction of a canal regulator across a canal, Construction of a head regulator across a Drainage.

• Canal inlets

In this structure, the drainage water to be admitted into the canal is relatively less. The whole drainage water is taken through the side of the canal. When the Drainage is mixed with the canal, then this travels a certain length of the entire canal. This structure is not so much preferred because of its disadvantages. It may pollute canal water. Also, if there is any bank erosion, it may affect the canal structure slowly. So, it is a rarely constructed structure.

Which factors influence the selection of cross-drainage works in the case of irrigation?

There are a few factors that can influence the selection of cross-drainage work in the case of irrigation.

Here we will write down those factors so that you can keep them in mind and apply them whenever required.

• Relative bed level

 The bed levels of the canal and Drainage will influence the type of cross-drainage system selection. 

• Foundation

To implement cross-drainage works, a suitable foundation is required to support the whole system properly without any issues. 

• Relative water level

The water level at the canal and Drainage also impacts the choice.

• Cost

A canal method outweighs the expenditure of establishing a hydraulic system. 

• Discharge

The flow of the Drainage may be more at times; thus, the hydraulic system established should resist weathering prerequisites. 

• Canal alignment

The canal alignment is sometimes modified to accomplish more suitable cross-drainage outcomes. 

• Provision of road

An aqueduct is favored over a super passage cross drainage works because an aqueduct permits the Construction of a road bridge along with it. 

• Availability

The essential construction materials should be available in the locality so that there will be no extra cost of delivery of materials from outside. 

Selection of sites for cross-drainage works

When selecting a site for cross-drainage works, there should be a few points that need to be considered. Here are those factors mentioned below.

• At the site, the Drainage should cross the canal at the right angles. This will keep the expenditure of the structure at a minimum. 
• The stream at the chosen site should be sound. 
• A firm sub-stratum should exist below the drainage level. 
• The site should be such that the high and long approaches of the canals are not needed. 
• The water table should not be very high.
• The site should be specified downstream at the Drainage and canal to eliminate the Construction of numerous structures. 
• This work should be merged with a bridge if attainable so that the bridge can be moved as cross drainage works when needed. 

Determination of maximum flood discharge

There is a term end high flood discharge for smaller drains. It can be done by implementing empirical formulas. On the other hand, for large drains, methods like Hydrograph analysis, Rational formula, etc. 

Usually, the methods that are used in the estimation of the flood flow can be differentiated as

• Physical indications of past floods
• Empirical formulae and curves
• Overland flow Hydrograph and unit Hydrograph

Here you can get detailed information about these. All these are essential in cross drainage and the whole irrigation system.

Physical indications of past floods

The rate of maximum flood discharge can be estimated by questioning the residents of nearby areas. It can be dependent on flood marks left on the river banks. After checking all the high water marks of the cross section area and wetted perimeter of various flow sections, the suitable assumed value of flood discharge can be estimated. 

Also, the maximum flood discharge from the rating curve can be done by a few methods like

• Simple judgment 
• Logarithmic method

Empirical formulae and curves

A few empirical formulas have been developed to estimate the maximum value of flood discharge. In this formula, any river or stream whole flood discharge Q is mentioned as a function of the catchment area A. The procedure is Q=cAn.

Here, c is the coefficient, and n is the index. Both c and n depend on size, shape, location of the catchment, and other topographical factors.

Overland flow Hydrograph and unit Hydrograph

It is a graphical plot of the total discharge of a natural stream or river. It can show the variation of release with time. It can also show the total runoff at the point of measurement.

Summing up

So, here we reached the end of this context, where you have all the detailed knowledge about the cross drainage system. I hope this article helped you with all kinds of questions regarding cross drainage. 

Frequently Asked Questions (FAQs)

1. What are the types of cross-drainage works?

Types of Cross Drainage works are Type – 1: Cross drainage work carrying the canal over the drain. Type – 2: Cross Drainage work carrying Drainage over the canal. Type –3: Cross drainage works admitting canal water into the canal.

2. What are the methods of Drainage?

The conventional engineering-based techniques most commonly used to drain excess water from land are surface drainage, horizontal subsurface drainage, and vertical subsurface drainage.

3. What is the minimum gradient for Drainage?

The minimum fall for a waste pipe is 1 in 40. The maximum fall is 1 in 110. So pipes with a gradient between 1 in 40 and 1 in 110 should have adequate flow to prevent blockages.

4. What are the four types of Drainage?

Four types of Drainage are dendritic, trellis, rectangular, and radial.

5. Mention 3 benefits of Drainage.

• Drainage assures that the soil is properly aerated.
• Drainage reduces soil and nutrient loss from runoff and can help avoid soil erosion.
• Drainage on hill slopes enables to decrease the risk of soil slippage.