In a standing wave flume, the depth of flow in the throat region should be equal to the critical depth. Let's understand why this is the case.

Definition of Critical Depth:
Critical depth is the depth of flow at which the specific energy of the flow is minimum for a given discharge. It is a critical condition where the flow transitions from subcritical to supercritical or vice versa.

Understanding Standing Waves:
In a standing wave flume, standing waves are generated by superimposing two waves traveling in opposite directions. These waves have equal amplitudes and frequencies but travel in opposite directions. The superimposition of these waves creates nodes and antinodes, resulting in a stationary wave pattern.

Depth of Flow in the Throat Region:
The throat region is the narrowest part of the flume where the standing wave is formed. In this region, the flow is highly turbulent and experiences significant changes in depth.

Importance of Critical Depth:
The critical depth is a significant parameter in open channel flow as it determines the type of flow (subcritical or supercritical) and the behavior of the flow. When the depth of flow is equal to the critical depth, the flow is at a critical condition, and the specific energy is minimum.

Explanation of Options:
a) The depth of flow should always be larger than the critical depth: This is incorrect because if the depth is larger than the critical depth, the flow will be subcritical, and a standing wave cannot be formed.
b) The depth of flow can be less than the critical depth: This is incorrect because if the depth is less than the critical depth, the flow will be supercritical, and a standing wave cannot be formed.
c) The depth of flow should be equal to the critical depth: This is the correct answer. When the depth of flow is equal to the critical depth, the flow is at a critical condition, and a standing wave can be formed.
d) The depth of flow is affected by the downstream depth when a jump is formed: This statement is not directly related to the depth of flow in the throat region of a standing wave flume.

Conclusion:
In a standing wave flume, the depth of flow in the throat region should be equal to the critical depth to create a standing wave. This condition ensures that the flow is at a critical condition, and the specific energy is minimum.

If the flumes are designed so as to pass the flow from subcritical to the supercritical state while passing through the flume, a single measurement at the throat (which in this case becomes a critical section) is sufficient for the computation of discharge. To ensure the occurrence of critical depth at the throat, the flumes are usually designed in such way as to form a hydraulic jump on the downstream side of the structure. These flumes are called 'standing wave flumes'.