Introduction:
In open channel flow, subcritical flow refers to a situation where the flow velocity is less than the wave velocity. Subcritical flow is characterized by a gradually varied flow profile, which means that the water surface slope gradually changes along the channel.

Control Section:
The control section in a gradually varied flow profile is the location where the water surface slope is maximum. It is the point where the flow conditions change, such as a change in channel slope, channel width, or a sudden contraction or expansion of the channel.

Downstream End:
The control section for subcritical flow is located at the downstream end of the channel. This is because the flow velocity at the downstream end is less than the wave velocity, which leads to the formation of a hydraulic jump. A hydraulic jump is a sudden change in flow conditions where the flow transitions from supercritical to subcritical. At the hydraulic jump, the flow energy is converted into turbulent kinetic energy, resulting in a rise in water surface elevation. Therefore, the downstream end acts as the control section for subcritical flow.

Upstream End:
The control section is not located at the upstream end because the flow conditions at this location are not changing. The flow is gradually varied and subcritical throughout the channel, and there is no significant change in the water surface slope at the upstream end.

Intermediate Sections:
The control section is also not located at any intermediate section of the channel. Although the water surface slope may vary along the channel, it is the maximum at the downstream end, where the hydraulic jump occurs. The water surface slope decreases gradually towards the upstream end, but there is no specific point where the slope is maximum other than the downstream end.

Conclusion:
In conclusion, the control section for a gradually varied flow profile in subcritical flow is located at the downstream end of the channel. This is because the flow conditions change at this point, leading to the formation of a hydraulic jump. The upstream end and intermediate sections of the channel do not act as control sections as there are no significant changes in the flow conditions.