Weir Flow Calculator – Rectangular & V-Notch Weir Discharge with 2D Nappe Profile

Weir Flow Calculator – Rectangular & V-Notch

Francis formula · 2D nappe profile · PDF report · Batch CSV · Trend chart

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📐 1. Weir Flow – Why It Matters

Weirs are structures used to measure flow rate in open channels. A rectangular weir is the most common type, while V-notch weirs are used for low flows. The discharge depends on the weir geometry and the head (H) – the height of water above the crest. This calculator uses the Francis formula for rectangular weirs and the Kindsvater-Shen formulation for V-notch weirs.

Rectangular weir: Q = C_d × L × H^1.5 (C_d ≈ 1.84)
V-notch weir (90°): Q = 1.34 × H^2.5
General V-notch: Q = (8/15) × C_d × tan(θ/2) × √(2g) × H^2.5

🚀 2. Live Weir Flow Calculator

Weir Type

Crest Length (L) m

Head (H) m

Discharge Coefficient (C_d) – leave 0 for auto

Notch Angle (θ) degrees

Unit

Batch CSV Upload

✅ CSV: length, head, cd, angle (optional)

Get PDF Report via Email

CSV Template

📊 2D Nappe Profile (Water Sheet)

📊 Discharge vs Head

📈 Flow History & Trend

📂 3. How to Use the Calculator

Select weir type: Rectangular or V-Notch.
Enter crest length (L) and head (H).
For rectangular weir: Enter Cd (leave 0 for auto, 1.84).
For V-notch: Enter notch angle (θ) – standard 90°.
Compute: Click "Compute Flow" – discharge appears, nappe profile updates.
2D Nappe: Shows water sheet over the weir crest.
Discharge chart: Shows how Q varies with H.
Batch CSV: Upload multiple designs for bulk analysis.
PDF Report: Download a detailed report with profile image.
History: All calculations saved locally; export or clear anytime.

📈 4. Real‑World Example

Irrigation canal in Bangladesh
Rectangular weir: L = 2.0 m, H = 0.30 m, Cd = 1.84.
Q = 1.84 × 2.0 × 0.30^1.5 = 1.84 × 2.0 × 0.164 = 0.604 m³/s.
The calculator will display this and draw the nappe profile.

❓ Frequently Asked Questions

❓ What is the discharge coefficient?

Cd accounts for friction and contraction. Typical values: 1.84 for sharp-crested rectangular weirs, 0.6 for V-notch.

❓ Which weir type is more accurate?

V-notch weirs are more accurate for low flows; rectangular weirs are better for higher flows.

❓ Is the tool free?

Yes, completely free. Batch processing supports up to 100 rows per CSV.

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