Enter Culvert Details
Formula Used
Manning discharge equation: Q = (1 / n) × A × R2/3 × S1/2
Where: Q is discharge, n is Manning roughness, A is flow area, R is hydraulic radius, and S is slope as a decimal.
Hydraulic radius: R = A / P, where P is the wetted perimeter.
Velocity: V = Q / A
Circular partial flow: area and wetted perimeter are determined from circular segment geometry using the entered diameter and depth.
Box culvert flow: area = width × water depth, and wetted perimeter = width + 2 × water depth for partial flow.
Adjusted discharge: Total discharge = Manning discharge × barrels × (1 − entrance loss factor).
How to Use This Calculator
- Select the culvert shape and unit system.
- Enter diameter for a circular culvert, or width and height for a box culvert.
- Provide the actual water depth inside the culvert.
- Enter culvert slope as a percent and add Manning roughness.
- Set the number of barrels and any entrance loss reduction.
- Press Calculate Flow Rate to display results above the form.
- Review discharge, velocity, hydraulic radius, and section fullness.
- Use the CSV or PDF buttons to export the result summary.
Example Data Table
| Shape | Size | Depth | Slope | n | Barrels | Estimated Flow |
|---|---|---|---|---|---|---|
| Circular | 1.20 m diameter | 0.90 m | 1.20% | 0.013 | 1 | 1.624 m³/s |
| Box | 1.80 m × 1.20 m | 0.85 m | 0.80% | 0.015 | 2 | 4.514 m³/s |
| Circular | 4.00 ft diameter | 2.80 ft | 1.50% | 0.014 | 3 | 51.873 ft³/s |
FAQs
1. What does this calculator estimate?
It estimates culvert discharge capacity using geometry, water depth, slope, and Manning roughness. It also reports velocity, hydraulic radius, and cross-section fullness.
2. Does it support partial flow conditions?
Yes. Circular culverts use circular segment geometry for partial depth. Box culverts use the entered flow depth up to the culvert height.
3. Which equation is used for flow rate?
The calculator uses Manning’s equation for open-channel flow. This is appropriate when the culvert is not pressurized and the flow behaves as gravity-driven channel flow.
4. Why is Manning roughness important?
Roughness represents internal resistance from material texture. Higher roughness reduces velocity and discharge, while smoother culverts generally carry more flow under the same slope.
5. Can I use this for multiple barrels?
Yes. Enter the number of parallel barrels. The calculator multiplies the adjusted single-barrel discharge to estimate total system capacity.
6. What does entrance loss reduction mean?
It is a simple allowance that reduces theoretical Manning discharge to reflect additional losses near the entrance. It helps produce a more conservative planning estimate.
7. Is this suitable for pressurized culvert design?
No. Full pressure flow, inlet control, outlet control, and backwater conditions need more detailed hydraulic analysis beyond this simplified drainage calculator.
8. What inputs most affect the result?
Water depth, culvert size, slope, and roughness have the greatest influence. Increasing area or slope usually raises discharge, while higher roughness lowers it.