understanding Weirs in Fluid Mechanics

Weirs are one of the simplest and most practical tools in open channel flow. In many rural places such as rural irrigation systems, remote streams, small treatment plants; there’s no budget for sensors or flow meters. A weir gives accurate flow data using gravity and geometry.

What is a Weir?

A weir is a barrier across a channel that forces water to flow over a notch. The height of water above the crest (the weir’s edge) determines the flow rate. Weirs are structures in open channels. They precisely measure fluid flow. These hydraulic principles are critical for various applications.

Basic Concepts of Weir

Head (H)
Vertical Distance from water surface to weir crest.

Discharge (Q)
Volume of fluid flowing per unit of time.

Rectangular Weir

• Straight notch with horizontal crest
• Best for large, steady flows

Triangular Weir (V-notch)

• V-shape opening (usually 90 degrees)
• More accurate for low flows

Trapezoidal Weir (Cipolleti)

• Combines advantages of rectangular and v-notch