Manning's equation calculator

Calculate open channel flow rates

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The Manning's Equation Calculator helps civil engineers determine flow rates in open channels using Manning's equation. Calculate flow velocity and discharge for channels, ditches, swales, and culverts.

Manning's equation is the standard method for open channel flow calculations, relating flow velocity to channel geometry, slope, and surface roughness. Proper flow calculations ensure adequate channel capacity for drainage and stormwater management.

Enter channel dimensions, slope, and surface type to calculate flow velocity and discharge. The calculator uses Manning's roughness coefficients for different channel materials and helps design channels with adequate capacity for design storm events.

Features

Everything you need

Manning's equation calculations
Flow velocity determination
Discharge calculations
Multiple surface type support
Roughness coefficient database
Rectangular channel support
Imperial and metric units

How it works

Simple workflow

1
Enter channel width and depth
2
Specify channel slope
3
Select surface type/material
4
Review calculated flow velocity
5
See discharge rate

Use cases

Built for real-world scenarios

Drainage design

Size channels and ditches for adequate flow capacity in drainage systems.

Swale design

Calculate flow capacity for vegetated swales and drainage channels.

Culvert sizing

Determine required culvert sizes based on calculated flow rates.

Flood analysis

Evaluate channel capacity during flood events and design improvements.

FAQ

Frequently asked questions

V = (1.49/n) × R^(2/3) × S^(1/2) for imperial units, where V is velocity (ft/s), n is roughness coefficient, R is hydraulic radius (ft), S is slope. Q = V × A for discharge.

Roughness coefficients: Concrete 0.013, Earth 0.025, Gravel 0.030, Grass 0.035, Natural streams 0.03-0.05. Higher n means rougher surface and lower velocity.

Hydraulic radius R = Area ÷ Wetted Perimeter. For rectangular channel: R = (width × depth) ÷ (width + 2×depth). Larger R means more efficient flow.

Flow velocity increases with slope. Doubling slope increases velocity by √2 (1.41×). Steeper slopes create higher velocities but may require erosion protection.

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We're always looking to improve our calculators. If you have ideas for new features, improvements, or found something that could work better, we'd love to hear from you.

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