Mechanical Engineering - Hydraulics and Fluid Mechanics - Discussion

Weirs:
Weirs are sharp-crested, overflow structures that are built across open canals. They are easy to construct and can measure the discharge accurately when correctly installed. It is important that the water level downstream is always below the weir crest, otherwise, the discharge reading will be incorrect.

According to the discharge conditions the weirs are classified as follows:

Freely discharging weir:

If the water level on downstream of a weir is well below the weir crest, then the nappe emerges freely in the atmosphere and thus it is called a freely discharging weir.

Submerged weir:

If the water level downstream of a weir is above the crest level of the weir, then the weir is known as a submerged weir.

So, the answer is option D.

Please explain.