V Notch Weir

ENGINEERING – HYDRAULIC & OPEN CHANNEL FLOW CALCULATOR V Notch Weir A precise tool.
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What is the V Notch Weir & How does it work?
The V‑notch (Thomson) weir is a widely used device for measuring low‑flow discharges in open channels. By allowing water to flow through a V‑shaped opening, the weir creates a relationship between the upstream head and the volumetric flow rate that can be expressed analytically. The discharge through a V‑notch is governed by the energy equation and the geometry of the notch. Assuming ideal flow, the theoretical discharge is derived as
Q = \frac{8}{15} C \sqrt{2g} \tan\left(\frac{\theta}{2}\right) h^{5/2}
Q = discharge (mΒ³/s), C = discharge coefficient, g = acceleration due to gravity (m/sΒ²), ΞΈ = notch angle (radians), h = head above the notch (m)
In practice, the coefficient C (typically around 0.58–0.62) corrects for viscosity, surface tension, and non‑ideal velocity profiles. Accurate measurement of the head and the notch angle is essential for reliable flow estimation.
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Frequently Asked Questions
What is a V-notch weir?
A V-notch weir is a device used to measure low-flow discharges in open channels by allowing water to flow through a V-shaped opening.
How does the discharge through a V-notch weir relate to the upstream head?
The discharge through a V-notch weir is governed by the energy equation and the geometry of the notch, creating a relationship between the upstream head and the volumetric flow rate.
What is the theoretical discharge formula for a V-notch weir?
The theoretical discharge formula for a V-notch weir is Q = frac{8}{15} C sqrt{2g} tanleft(frac{theta}{2}right), where C is the discharge coefficient, g is the acceleration due to gravity, and ΞΈ is the notch angle.
What factors affect the accuracy of a V-notch weir measurement?
The accuracy of a V-notch weir measurement can be affected by factors such as water temperature, flow uniformity, and sediment in the channel.
How is the discharge coefficient (C) determined for a V-notch weir?
The discharge coefficient (C) for a V-notch weir is typically determined through calibration experiments or using empirical data specific to the weir design and flow conditions.
What are some common applications of V-notch weirs?
V-notch weirs are commonly used in irrigation systems, small streams, and other low-flow discharge measurement applications where accuracy is important.
How does the angle of the V-notch affect the discharge calculation?
The angle of the V-notch (ΞΈ) directly affects the discharge calculation as it influences the flow characteristics through the weir. A change in ΞΈ will alter the relationship between the upstream head and the discharge.

Results are for informational purposes only and do not constitute professional advice.