A weir is an obstruction across a channel that can regulate volume flux by creating a hydraulic control. In a shallow water model a sufficiently smooth crest has a critical section, with upstream flow subcritical and downstream flow supercritical under free discharge. Losses, submergence and geometry determine whether this ideal control applies.
For reservoir head above a broad crest, negligible approach velocity, constant reduced gravity, and no head loss, hydraulic control gives and the displayed discharge. Reflecting the vertical coordinate gives the same result for a light layer escaping beneath a lowered roof. An orifice discharge coefficient is a separate closure and should not be silently inserted into this inviscid weir law.
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A "weir" is a small barrier built across a river or creek to manage water flow. Weirs can serve various purposes, including: 1. **Water Level Control**: They help maintain a certain water level upstream, which can be important for irrigation, water supply, or maintaining aquatic habitats. 2. **Hydroelectric Power**: Some weirs can be integrated into hydroelectric systems to generate electricity.