Trapezoidal Weir Calculator Spreadsheet

What is a Trapezoidal Weir?

A trapezoidal weir—often called a Cipolletti weir when it has a specific side‑slope ratio—is a flow‑measuring structure with a trapezoid‑shaped opening. Water flows over a sharp‑crested trapezoidal notch, and the height of water above the crest (the head) is used to calculate discharge.

The defining feature is the sloped sides, which compensate for side‑contraction effects and provide more accurate flow measurement across a wider range of discharges than a simple rectangular weir.

Trapezoidal weirs are commonly used in:

• Irrigation channels
• Treatment works and flow‑monitoring chambers
• Industrial discharge systems
• SuDS flow‑control structures
• Natural streams where debris or sediment may be present

Key advantages include:

• Better accuracy across varying flows
• Reduced sensitivity to side contraction
• Improved performance in debris‑prone channels
• Simple construction and calibration

The most common form—the Cipolletti weir—uses side slopes of 1 horizontal to 4 vertical, which simplifies the discharge equation and improves measurement consistency.

Common Trapezoidal Weir Design Methods

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

Widely used design methods include:

• Cipolletti Weir Equation — For a standard trapezoidal weir with 1:4 side slopes, the discharge is calculated using: Q = Cd * L * H^(3/2)
• General Trapezoidal Weir Formula — For non‑standard side slopes, the discharge equation includes additional terms to account for the increased flow area created by the sloping sides.
• Discharge Coefficient Selection — Coefficients depend on crest sharpness, side‑slope angle, and laboratory calibration. Typical values range from 0.60 to 0.65 for sharp‑crested trapezoidal weirs.
• Approach Flow Requirements — Designers ensure smooth, uniform upstream flow, avoiding 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 maintain accuracy.
• Head Measurement Location — The head is measured upstream at a distance of 3–4 times the maximum head to avoid velocity effects.

Each method must consider channel width, sediment load, debris potential, flow variability, and maintenance access to ensure long‑term reliability.

How Does the CivilWeb Suppressed Rectangular Weir Calculator Spreadsheet Work?

The spreadsheet uses the above flow rate formulas to calculate the flow rate over the weir. The user simply inputs the weir crest base length and the head of water over the weir. A useful diagram is also provided within the spreadsheet to make it clear exactly what the inputs are referring to.

The spreadsheet then use the Cipoletti formula to calculate the flow rate over the weir.

Why Use a Trapezoidal Weir Design Spreadsheet?

A trapezoidal weir design spreadsheet helps engineers manage the many variables involved in calculating flow, checking submergence, and ensuring the weir meets hydraulic requirements.

Key benefits include:

• Automated calculation of flow for any head of water
• Scenario testing for different flow ranges or channel geometries
• Clear documentation for calibration, monitoring, and regulatory reporting

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

Get Started Today

Save hours of repetitive calculation time. Whether you’re working on a major project or a small renovation, this tool will give you clarity, accuracy, and confidence.

Buy the CivilWeb Trapezoidal Weir Calculator Spreadsheet now for only £19.99.

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