Civil Engineering - Irrigation - Discussion

A canal syphon is constructed where the full supply level of the canal is higher than the bed of the drain. The used barrels in this case also act as inverted syphons and the canal water is flowing under pressure in the syphons.

This comes under the cross drainage work. Basically, cross drainage work are three types

1. Cross drainage work carrying the canal over the drain (HFL<FSL)

Aqueduct: In an aqueduct, the canal bed level is above the drainage bed level so the canal is to be constructed above the drainage.

The canal water level is referred to as full supply level (FSL) and the drainage water level is referred as high flood level (HFL). The HFL is below the canal bed level.

Syphon aqueduct: In a syphon aqueduct, canal water is carried above the drainage but the high flood level (HFL) of drainage is above the canal trough. The drainage water flows under symphonic action and there is no presence of atmospheric pressure in the natural drain.

The construction of the syphon aqueduct structure is such that, the flooring of the drain is depressed downwards by constructing a vertical drop weir to discharge high flow drain water through the depressing concrete floor.

2. Cross drainage work carrying drainage over the canal (HFL> FSL)

Super passage: Super passage structure carries drainage above the canal as the canal bed level is below the drainage bed level. The drainage trough is to be constructed at road level and drainage water flows through this from upstream to downstream and the canal water flows through the piers which are constructed below this drainage trough as supports. The full supply level of canal is below the drainage trough in this structure. The water in the canal flows under gravity and possesses atmospheric pressure. This is simply a reverse of the Aqueduct structure.

Canal syphon: In a canal syphon, drainage is carried over a canal similar to a super passage but the full supply level of the canal is above the drainage trough. so the canal water flows under symphonic action and there is no presence of atmospheric pressure in the canal.

When compared, the super passage is more often preferred than the canal Syphon because, in a canal Syphon, the big disadvantage is that the canal water is under a drainage trough so any defective minerals or sediment deposited cannot be removed with ease like in the case of a Syphon Aqueduct.

3. Cross drainage works admitting canal water into the canal (HFL=FSL)

Level crossing: When the bed level of the canal is equal to the drainage bed level, the level crossing is to be constructed. This consists of the following steps:

1. Construction of weir to stop drainage water behind it

2. Construction of canal regulator across a canal

3. Construction of head regulator across a Drainage

Canal inlet: In a canal inlet structure, the drainage water to be admitted into the canal is very less. The drainage is taken through the banks of a canal at the inlet. And then this drainage mixed with canal travels a certain length of the canal, after which an outlet is provided to create suction pressure and suck all the drainage solids, disposing it to the watershed area nearby.

There are many disadvantages in the use of canal inlet structure, because the drainage may pollute canal water and also the bank erosion may take place causing the canal structure to deteriorate so that maintenance costs are high. Hence this type of structure is rarely constructed.

Where, HFL = high flood level & FSL = full supply level

Agree, Pipe flow is the correct answer.

The Correct answer is option D.

When the canal is flowing under the drainage in the syphonic action, there is no open space between canal water and drainage bed.

Because of having contracted flow, the canal flows under pressure, and we know that fluid flows in pressure in the pipe flow.

Now please can someone explain me, at the time of syphonic action in canal water and getting this much high pressure during flow, how can the pressure be negative?

The correct answer is option (D) under negative pressure.

Siphon action means the flow of water under vacuum without any external energy. Thus this vacuum pressure is below or at absolute pressure. Thus below absolute pressure, it is vacuum pressure which we take negative pressure. Therefore, syphon action occurs under a negative pressure gradient.

I think it's negative pressure.

It's pipe flow. Only water head is involved in transporting water from one side to another.

Atmospheric pressure on both inlet and outlet is equal. Negative pressure only works in inverted you shaped syphon. Critical velocity won't happen as the section of flow is changing direction not it's size.

The correct Answer is B.

Which one is the correct answer? Please explain.