Physics

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Open-Channel Flow Calculator.

Calculate uniform rectangular-channel flow with Manning's equation and explicit geometry, roughness, and slope.

On-device calculationNo signup

Set your values

Results update as you type.

02 / ANSWER

Channel width

Flow depth

Manning roughness n

Channel slope

Flow area

0.00000000m²

Wetted perimeter

0.00000000m

Hydraulic radius

0.00000000m

Mean velocity

0.00000000m/s

Flow rate

0.00000000m³/s

Results update automatically as you type.

Educational model with every parameter shown. No material, weather, equipment, or safety defaults are inferred.

Use Cases

Design of Irrigation Channels

Engineers can use this calculator to size rectangular irrigation channels by determining the flow capacity for given dimensions and slope, ensuring adequate water delivery.

Example: A channel 2 m wide, 1 m deep, n=0.015, slope=0.001 gives a flow rate of about 2.6 m³/s.

Stormwater Drainage Analysis

Urban planners can estimate the discharge capacity of rectangular stormwater drains to prevent flooding, using local slope and roughness values.

Example: Check if a 1.5 m wide drain with 0.8 m depth and slope 0.005 can handle a 10-year storm flow.

Frequently Asked Questions

What does the Open-Channel Flow Calculator do?: This calculator computes the flow rate and velocity for uniform flow in a rectangular open channel using Manning's equation. You input channel width, flow depth, Manning's roughness coefficient (n), and channel slope, and it calculates the discharge and average velocity based on the hydraulic radius and cross-sectional area.
What is Manning's roughness coefficient (n)?: Manning's n is a dimensionless coefficient that represents the resistance to flow caused by the channel surface. Typical values range from about 0.012 for smooth concrete to 0.035 for natural streams with vegetation. It is an empirical parameter used in Manning's equation to estimate flow velocity.
What units are used in this calculator?: The calculator uses metric units: channel width and flow depth are in meters (m), channel slope is in meters per meter (m/m) which is dimensionless, and Manning's n is unitless. The resulting flow rate is in cubic meters per second (m³/s) and velocity in meters per second (m/s).

Tips & Common Mistakes

Tips

Ensure all inputs are in consistent units (meters for dimensions, m/m for slope).
Select an appropriate Manning's n based on the channel material; consult standard tables for guidance.
For steep slopes, verify that the flow remains uniform and subcritical; otherwise, results may be approximate.
Use the calculated velocity to check for erosion potential; high velocities may require lining or energy dissipation.

Common Mistakes to Avoid

Using slope as a percentage instead of a dimensionless ratio (e.g., 1% should be entered as 0.01).
Confusing hydraulic radius with flow depth; for a wide rectangular channel, hydraulic radius is approximately equal to depth, but for narrow channels, it differs.
Ignoring that Manning's equation assumes uniform, steady flow; it may not be accurate for rapidly varied flow or non-uniform conditions.

Last updated: August 13, 2026

Use Cases

Design of Irrigation Channels

Engineers can use this calculator to size rectangular irrigation channels by determining the flow capacity for given dimensions and slope, ensuring adequate water delivery.

Example: A channel 2 m wide, 1 m deep, n=0.015, slope=0.001 gives a flow rate of about 2.6 m³/s.

Stormwater Drainage Analysis

Urban planners can estimate the discharge capacity of rectangular stormwater drains to prevent flooding, using local slope and roughness values.

Example: Check if a 1.5 m wide drain with 0.8 m depth and slope 0.005 can handle a 10-year storm flow.

Frequently Asked Questions

What does the Open-Channel Flow Calculator do?

This calculator computes the flow rate and velocity for uniform flow in a rectangular open channel using Manning's equation. You input channel width, flow depth, Manning's roughness coefficient (n), and channel slope, and it calculates the discharge and average velocity based on the hydraulic radius and cross-sectional area.

What is Manning's roughness coefficient (n)?

Manning's n is a dimensionless coefficient that represents the resistance to flow caused by the channel surface. Typical values range from about 0.012 for smooth concrete to 0.035 for natural streams with vegetation. It is an empirical parameter used in Manning's equation to estimate flow velocity.

What units are used in this calculator?

The calculator uses metric units: channel width and flow depth are in meters (m), channel slope is in meters per meter (m/m) which is dimensionless, and Manning's n is unitless. The resulting flow rate is in cubic meters per second (m³/s) and velocity in meters per second (m/s).

Tips & Common Mistakes

Tips

Ensure all inputs are in consistent units (meters for dimensions, m/m for slope).
Select an appropriate Manning's n based on the channel material; consult standard tables for guidance.
For steep slopes, verify that the flow remains uniform and subcritical; otherwise, results may be approximate.
Use the calculated velocity to check for erosion potential; high velocities may require lining or energy dissipation.

Common Mistakes to Avoid

Using slope as a percentage instead of a dimensionless ratio (e.g., 1% should be entered as 0.01).
Confusing hydraulic radius with flow depth; for a wide rectangular channel, hydraulic radius is approximately equal to depth, but for narrow channels, it differs.
Ignoring that Manning's equation assumes uniform, steady flow; it may not be accurate for rapidly varied flow or non-uniform conditions.

Use Cases

Design of Irrigation Channels

Engineers can use this calculator to size rectangular irrigation channels by determining the flow capacity for given dimensions and slope, ensuring adequate water delivery.

Example: A channel 2 m wide, 1 m deep, n=0.015, slope=0.001 gives a flow rate of about 2.6 m³/s.

Stormwater Drainage Analysis

Urban planners can estimate the discharge capacity of rectangular stormwater drains to prevent flooding, using local slope and roughness values.

Example: Check if a 1.5 m wide drain with 0.8 m depth and slope 0.005 can handle a 10-year storm flow.

Frequently Asked Questions

What does the Open-Channel Flow Calculator do?

What is Manning's roughness coefficient (n)?

What units are used in this calculator?

Tips & Common Mistakes

Tips

Ensure all inputs are in consistent units (meters for dimensions, m/m for slope).
Select an appropriate Manning's n based on the channel material; consult standard tables for guidance.
For steep slopes, verify that the flow remains uniform and subcritical; otherwise, results may be approximate.
Use the calculated velocity to check for erosion potential; high velocities may require lining or energy dissipation.

Common Mistakes to Avoid

Using slope as a percentage instead of a dimensionless ratio (e.g., 1% should be entered as 0.01).
Confusing hydraulic radius with flow depth; for a wide rectangular channel, hydraulic radius is approximately equal to depth, but for narrow channels, it differs.
Ignoring that Manning's equation assumes uniform, steady flow; it may not be accurate for rapidly varied flow or non-uniform conditions.