Weir Flow Calculator (Rectangular & V-Notch) – Discharge Measurement Tool

🌊 PROJECT / SITE NAME:

📘 Guide Sections

Weirs are the simplest and most accurate way to measure open channel flow. This calculator uses the Francis formula for rectangular weirs and the Kindsvater-Shen formula for V-notch weirs. Perfect for irrigation engineers, hydrologists, and field technicians.

⚠️ Why Weirs for Flow Measurement?

✅ Low cost – a simple plate or concrete crest
✅ High accuracy – ±2% with proper installation
✅ No moving parts – maintenance-free
✅ Works in sediment-laden water (unlike flumes)

📐 Governing Equations

Rectangular weir (Francis): Q = 1.84 × (L – 0.2H) × H^1.5

V-notch weir (90°): Q = 1.37 × H^2.48

V-notch weir (any angle θ): Q = (8/15) × C_d × tan(θ/2) × √(2g) × H^2.5

🧮 Interactive Weir Calculator

📐 Weir type:

📏 Weir crest length L (m):

📐 V-notch angle θ (degrees):

💧 Head over crest H (m):

📈 Flow Rate

Parameter	Value
Discharge Q (m³/s)	--
Discharge Q (L/s)	--
Recommended head range	--

💡 For accurate measurement, H should be between 0.05 m and 0.6 m.

📏 Installation Guidelines

Weir plate must be vertical and smooth upstream face
Crest should be sharp (0.5–2 mm thick)
Head measurement point: 4×H upstream from weir
Approach channel should be straight for at least 10×H
For rectangular weirs: L > 2H (avoid end contraction issues)

📝 Field Example

Irrigation canal: rectangular weir L=1.0 m, measured head H=0.15 m.
Q = 1.84 × (1.0 – 0.2×0.15) × 0.15^1.5 = 1.84 × (0.97) × 0.0581 = 0.104 m³/s = 104 L/s.

❓ FAQs

Q: What if H is very small?
A: Below 0.03 m, accuracy drops. Use a narrower weir or V-notch.

Q: Submerged weir?
A: This calculator assumes free overfall. If downstream water level is above crest, correction needed.

📘 Complete Hydraulic Field Tools Kit

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"Weir Flow Calculator (Rectangular & V-Notch) – Discharge Measurement Tool"