Manning's Equation Calculator – Open Channel Flow for Trapezoidal, Rectangular & Triangular Sections

Manning's Equation Calculator – Open Channel Flow

Compute flow rate (Q), velocity, normal depth for trapezoidal, rectangular & triangular sections · 2D profile · PDF · Batch

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📐 1. What is Manning's Equation?

Manning's equation is the most widely used formula for open channel flow analysis. It relates flow rate (Q) to channel geometry, slope, and roughness coefficient:

Q = (1/n) × A × R^2/3 × S^1/2
Where: n = Manning's roughness, A = cross‑section area, R = hydraulic radius (A / wetted perimeter), S = channel slope.

This calculator supports trapezoidal, rectangular, and triangular channels – the most common shapes in irrigation, drainage, and river engineering.

🚀 2. Live Manning's Equation Calculator

Channel Shape

Bottom Width (B) m

Side Slope (z : 1)

Flow Depth (D) or normal depth m

Manning's n (roughness)

Channel Slope (S) (m/m)

Output Unit

Batch CSV Upload

✅ CSV format: Shape (trapezoid/rectangle/triangle), B, z, D, n, S

Get PDF Report via Email

CSV Template

📊 2D Channel Cross‑Section Profile

📈 Flow Rate Trend & History

📂 3. How to Use the Calculator

Select channel shape: Trapezoidal, rectangular, or triangular.
Enter geometry: Bottom width (B), side slope (z – for trapezoid/triangle), flow depth (D).
Enter Manning's n and channel slope (S). Typical n values: concrete (0.012-0.018), earth (0.022-0.030), natural stream (0.030-0.050).
Select output unit: m³/s or cfs (cubic feet per second).
Compute: Click "Compute Flow" – results show Q, velocity, and 2D profile.
Batch CSV: Upload multiple designs for bulk analysis.
PDF Report: Download a professional report with the cross‑section image.
History: All calculations saved locally; export or clear anytime.

📈 4. Real‑World Example

Irrigation canal, Bangladesh
Trapezoidal channel: B = 4 m, z = 1.5, D = 1.2 m, n = 0.025, S = 0.0008.
Area = B×D + z×D² = 4×1.2 + 1.5×1.44 = 4.8 + 2.16 = 6.96 m²
Wetted perimeter = B + 2×D×√(1+z²) = 4 + 2×1.2×√(1+2.25) = 4 + 2.4×1.803 = 8.327 m
Hydraulic radius R = 6.96 / 8.327 = 0.836 m
Manning: Q = (1/0.025) × 6.96 × (0.836)^0.6667 × (0.0008)^0.5 ≈ 10.8 m³/s.
The calculator will do this instantly and draw the profile.

❓ Frequently Asked Questions

❓ What is Manning's n?

Manning's n is a roughness coefficient that depends on channel material. Higher n means higher resistance, lower flow.

❓ Can I find normal depth using this tool?

This version computes Q from given depth. For normal depth, you would need iterative solving; future version will add that.

❓ Is the tool free?

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

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