Which of the following can Manning's equation be used to calculate in open channel flow?

• A. Velocity
• B. Flow rate
• C. Critical depth
• D. All of the above

Answer: (A)

Manning's equation is primarily used to determine the velocity of flow in an open channel. It provides a relationship between the flow velocity, roughness of the channel, and the hydraulic radius, incorporating factors such as channel slope and surface roughness. When applying Manning's equation, the specific focus is on calculating how fast water is flowing, which is a fundamental aspect of open channel hydraulics.

Although it is possible to derive flow rate from the velocity calculated using Manning's equation through the product of velocity and cross-sectional area, this is not the primary application of the equation. Similarly, while Manning’s equation does not directly calculate critical depth, other methods exist for determining it based on geometry and flow characteristics.

Thus, the primary and most direct application of Manning's equation is to compute the velocity in open channel flows, making this the most appropriate answer among the options given.