Inputs
Example Data Table
| Shape | Size | Depth | Slope S | n | Estimated Q (m³/s) |
|---|---|---|---|---|---|
| Circular | D = 1.20 m | y = 0.90 m | 0.005 | 0.013 | ≈ 1.05 |
| Circular | D = 0.90 m | Full | 0.004 | 0.015 | ≈ 0.65 |
| Box | b = 1.50 m, h = 1.20 m | y = 0.80 m | 0.006 | 0.014 | ≈ 1.15 |
These values are illustrative. Confirm assumptions for tailwater, inlet effects, and site constraints.
Formula Used
The calculator estimates capacity using Manning’s equation: Q = (1/n) · A · R2/3 · S1/2.
- Q = discharge (m³/s)
- A = wetted flow area (m²)
- R = hydraulic radius = A/P (m)
- P = wetted perimeter (m)
- S = slope (dimensionless)
- n = Manning roughness coefficient
For circular partial flow, the area and wetted perimeter are computed from the circular‑segment angle based on depth.
How to Use This Calculator
- Select your preferred units and the culvert shape.
- Choose partially full or full flow state.
- Enter section dimensions and the flow depth for partial flow.
- Provide the slope and roughness appropriate to the lining.
- Press Calculate to view capacity and velocity above the form.
- Use Download CSV or Download PDF to document results.
This tool estimates conveyance. For final design, review inlet control, tailwater, debris risk, and regulatory criteria.
Technical Article
1) What the calculator estimates
This tool estimates culvert conveyance by applying Manning’s equation to the wetted section. It returns discharge (m³/s and cfs) and average velocity (m/s and ft/s). The calculation assumes steady, uniform flow driven by slope, so it is best used for screening sizes and comparing alternatives. It does not add entrance, bend, or outlet loss coefficients.
2) Geometry and flow depth data
Capacity is sensitive to wetted area and perimeter. For circular culverts running partially full, the wetted area is a circular segment computed from the depth-to-diameter ratio. For box culverts, the wetted perimeter includes the bottom and both sidewalls. For full flow, the entire section is treated as wetted. Depth is capped at the section height to prevent unrealistic inputs.
3) Roughness and slope guidance
Manning roughness (n) represents lining resistance. Smooth pipes often fall near 0.011–0.015, while corrugated or debris-prone barrels can be higher. Slope is entered as a dimensionless value (rise/run), such as 0.005 for 0.5%. Small slope changes can create noticeable capacity shifts. When uncertain, test a low and high n to bracket outcomes.
4) Interpreting capacity and velocity
Discharge increases with area and hydraulic radius, but velocity helps identify operational risks. Higher velocity may indicate potential outlet scour and a need for energy dissipation. Low velocity can increase sedimentation risk. Use the reported area, perimeter, and hydraulic radius to cross-check reasonableness against field conditions.
5) Design checks beyond conveyance
Final culvert design typically requires inlet and outlet control evaluation, headwater limits, tailwater effects, and allowances for debris and sediment. Consider roadway overtopping criteria, fish passage requirements, and minimum cover. Use the download options to document assumptions, then validate with project standards and hydrology results. Apply a conservative margin and confirm with detailed hydraulics.
FAQs
1) Is this an inlet-control culvert design?
No. It estimates conveyance using Manning-based uniform flow. For design, you still need inlet and outlet control checks, headwater limits, and tailwater conditions.
2) What depth should I enter for partial flow?
Enter the expected normal depth in the barrel. If you only know peak stage, use a realistic barrel depth estimate and compare a few depths for sensitivity.
3) What roughness value should I use?
Choose n based on lining and condition. Smooth barrels are typically lower, while corrugated, aged, or sedimented culverts are higher. If unsure, run a conservative higher n.
4) Why does capacity change so much with small slope edits?
Manning discharge is proportional to the square root of slope. Small slope changes can materially affect capacity, especially for larger sections with significant hydraulic radius.
5) Can I use US customary dimensions?
Yes. Select US customary units and enter feet for dimensions. The calculator converts internally and reports both m³/s and cfs, plus velocities in m/s and ft/s.
6) Does the result include multiple barrels?
The computed discharge is for one culvert barrel. For multiple identical barrels in parallel, multiply the reported discharge by the number of barrels, then verify site hydraulics.
7) How should I document results for review?
Use the CSV for spreadsheets and the PDF for submittals. Record slope source, chosen roughness, and depth assumptions so reviewers can reproduce the calculation quickly.