The correct answer is: D. Under negative pressure.
A canal syphon is a type of hydraulic structure that is used to carry water across a valley or other low-lying area. It consists of a pipe that is laid across the valley and supported by piers or aqueducts. The water flows through the pipe under negative pressure, which is created by the difference in elevation between the two ends of the pipe.
The negative pressure in a canal syphon causes the water to flow at a higher velocity than it would if it were flowing under atmospheric pressure. This higher velocity helps to reduce the amount of friction that occurs between the water and the pipe, which in turn reduces the risk of the pipe collapsing.
The negative pressure in a canal syphon also causes the water to rise up inside the pipe. This rise in water level is known as the “siphon effect.” The siphon effect can be used to create a waterfall or to power a waterwheel.
Here is a brief explanation of each option:
- Option A: Under atmospheric pressure. This is not the correct answer because the flow in a canal syphon is under negative pressure.
- Option B: Pipe flow. This is not the correct answer because the flow in a canal syphon is not a type of pipe flow.
- Option C: With critical velocity. This is not the correct answer because the flow in a canal syphon is not at critical velocity.
- Option D: Under negative pressure. This is the correct answer because the flow in a canal syphon is under negative pressure.