V Notch Weir Calculator Spreadsheet

What is a V‑Notch Weir

A V‑notch weir is a triangular‑shaped flow‑measuring device used to determine the discharge of water in open channels. Water flows over the sharp‑crested V‑shaped opening, and the height of the water above the crest (the head) is used to calculate flow rate using established hydraulic equations.

The most common angle is 90 degrees, but 60° and 30° notches are also used depending on the required sensitivity. Angles up to 100 degrees can also be used in some cases. V‑notch weirs are especially effective for low‑flow measurement, because the triangular shape provides greater accuracy at small discharges compared to rectangular weirs.

Typical applications include:

• - Monitoring baseflow in streams
• - Measuring discharge from treatment systems
• - Flow monitoring in SuDS features
• - Industrial and agricultural flow measurement
• - Calibration of small‑scale hydraulic systems

Key advantages include high accuracy, simple construction, predictable hydraulics, and suitability for a wide range of low‑flow conditions.

Common V‑Notch Weir Design Methods

Designing a V‑notch weir involves ensuring that the weir geometry, approach conditions, and hydraulic assumptions are correct so that flow measurements remain accurate and repeatable.

Common design methods include:

• - Sharp‑Crested Weir Theory — The standard discharge equation for a V‑notch weir is: Q = Cd * (8/15) * sqrt(2g) * tan(θ/2) * H^(5/2)
• - Discharge Coefficient Selection — Engineers select a coefficient based on notch angle, crest sharpness, and laboratory‑derived calibration data. Typical values range from 0.58 to 0.62 for sharp‑crested 90°
• - Approach Flow Conditions — The upstream channel must provide smooth, uniform flow. Designers ensure adequate straight‑channel length and avoid turbulence, sediment buildup, or vegetation interference.
• - Free‑Flow (Unsubmerged) Conditions — The downstream water level must be low enough to avoid submergence. Designers check the submergence ratio to ensure accurate measurement.
• - Head Measurement Setup — The head is measured at a specified distance upstream (typically 3–4 times the maximum head) to avoid velocity effects.

Each method must consider notch angle, crest geometry, channel conditions, sediment load, and maintenance requirements to ensure long‑term accuracy.

How Does the CivilWeb V‑Notch Weir Design Spreadsheet Work?

The spreadsheet uses the above flow rate formulas to calculate the flow rate over the weir. The user simply inputs the weir dimensions and angle, the width of channel and depth of downstream water. A useful diagram is also provided within the spreadsheet to make it clear exactly what the inputs are referring to.

The spreadsheet then calculates whether the flow over the weir is fully contracted or not. Then the Kindsvater-Shen equation is used to determine the flow rate over the weir.

Why Use a V‑Notch Weir Design Spreadsheet?

A V‑notch weir design spreadsheet helps engineers manage the many variables involved in calculating flow, checking submergence, and ensuring the weir meets hydraulic requirements. Because flow depends on head, notch angle, discharge coefficient, and site conditions, a spreadsheet ensures calculations are accurate, consistent, and easy to update.

Benefits include:

• - Automated calculation of flow for any head and notch angle
• - Quick comparison any weir notch angle such as 30°, 60°, 90° or 100° notch performance
• - Built‑in checks for fully contracted flow or not
• - Scenario testing for different flow ranges or site constraints
• - Clear documentation for calibration, monitoring, and regulatory reporting

For hydrologists, drainage engineers, and environmental regulators, a spreadsheet becomes a practical tool that improves accuracy and reduces design time while ensuring reliable long‑term flow measurement.

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