Flow control is any feature that imposes a relationship between the flow depth and discharge in a channel.
• A critical flow section, for instance, is a flow control, since at this section Fr=1.0.
• Likewise, various hydraulic structures such as weirs and gates will control the flow.
• Normal flow may be viewed as a flow control also because a normal flow equation, like Manning Equation, describes a depth-discharge relationship.
• However, sometimes the other controls will pull the flow away from the normal flow conditions.