Stormwater Drainage

Flow Calculator

Manning's Equation Analysis

Calculate flow rates and hydraulic properties using Manning's equation for open channel flow in trench drains.

1
Channel Properties

2
Flow Rate Results

Cubic Feet/Second

0.53

CFS

Gallons/Minute

238.66

GPM

Liters/Second

15.06

L/s

Cubic Meters/Second

0.0151

m³/s

Flow Characteristics

Velocity

0.97

m/s

Froude Number

0.97

Fr

Flow Regime

critical

Fr = 1: Transition point between flow regimes.

Learn more about flow regimes →

Design interpretation

At the entered full-flow depth, this channel conveys approximately 0.53 cfs at 0.97 m/s. The result is near critical flow, where small geometry or depth changes can shift the hydraulic regime.

Manning capacity does not account for grate interception, inlet bypass, debris, freeboard, tailwater, or outlet restrictions.

About This Calculator

This calculator uses Manning's equation to determine flow rates and hydraulic properties for open channel flow. Results consider channel geometry, slope, and surface roughness.

Q = (1/n) × A × R^(2/3) × S^(1/2)

  • Q = Flow rate
  • n = Manning's roughness coefficient
  • A = Cross-sectional area
  • R = Hydraulic radius
  • S = Channel slope
Size a complete trench drainCalculate design runoff

Worked example

Six-inch HDPE channel

Enter a 6-inch channel width, 4-inch flow depth, 0.5% slope, and HDPE roughness (n = 0.009). The calculator converts the geometry to SI units before applying Manning's equation.

Capacity

0.53 cfs

238.66 gpm

Velocity

0.97 m/s

Froude number

0.97

Near critical

The capacity is the flow the open channel can convey at the stated depth, not the runoff it must receive. Compare it with a design flow from the runoff calculator or use the trench-drain sizing calculator to screen the channel and outlets together.