Solve box culvert discharge using dimensions, slope, and roughness. View radius, velocity, and flow quickly. Download tables and validate assumptions using clear example data.
| Scenario | Width (m) | Height (m) | Depth (m) | Slope (%) | n | Barrels | Total Flow (m³/s) |
|---|---|---|---|---|---|---|---|
| Urban drain | 2.40 | 1.80 | 1.10 | 0.40 | 0.013 | 1 | 8.8700 |
| Highway crossing | 3.00 | 2.00 | 1.40 | 0.55 | 0.014 | 2 | 35.8823 |
| Estate outfall | 2.00 | 1.50 | 0.95 | 0.70 | 0.015 | 2 | 13.1230 |
| Service road line | 1.80 | 1.50 | 0.80 | 0.60 | 0.013 | 1 | 4.8389 |
| Flood bypass | 4.00 | 2.50 | 1.80 | 0.35 | 0.014 | 3 | 88.0452 |
This calculator uses Manning's open channel flow relationship for a rectangular concrete box culvert under gravity flow conditions.
Flow area: A = b × y
Wetted perimeter: P = b + 2y
Hydraulic radius: R = A / P
Velocity: V = (1 / n) × R2/3 × S1/2
Discharge per barrel: Q = A × V
Total discharge: Qtotal = Q × number of barrels
Froude number: Fr = V / √(g × Dh)
Here, b is culvert width, y is flow depth, n is Manning roughness, S is slope in decimal form, and Dh is hydraulic depth.
A concrete box culvert carries stormwater under roads, tracks, and embankments. Its hydraulic behavior changes with depth, slope, roughness, and barrel count. This calculator gives a quick way to estimate discharge, average velocity, and flow regime for a rectangular culvert section. It is practical for screening work, checking concept layouts, and reviewing alternative drainage options.
The tool uses box width, box height, flow depth, longitudinal slope, and Manning roughness. It then calculates area, wetted perimeter, hydraulic radius, hydraulic depth, velocity, discharge per barrel, and total discharge. It also reports the Froude number. That value helps you judge whether the flow is subcritical, near critical, or supercritical.
Hydraulic radius links the shape of the water section to friction losses. A larger radius usually improves conveyance because less perimeter is resisting the flow for each unit of area. In a box culvert, changing the water depth changes both the area and the wetted perimeter. That changes velocity and discharge at the same time.
The target discharge field adds a design check. When you enter a target flow, the calculator estimates the depth needed to pass that discharge with the given geometry and roughness. If the target is higher than the full-depth capacity, the tool flags that condition. This is helpful during sizing studies and early drainage reviews.
The Plotly graph shows how total discharge rises as depth increases. This makes part-full behavior easier to understand. The CSV and PDF exports let you save the result summary and example table for reports, markups, or internal review. Always confirm final culvert design with full hydraulic criteria, inlet control checks, outlet control checks, tailwater conditions, and local code requirements.
It represents gravity driven flow in a rectangular concrete box culvert using Manning based open channel assumptions. It is best for preliminary analysis and screening.
Yes. Enter the number of barrels working in parallel. The calculator multiplies the discharge of one barrel by the active barrel count.
Roughness directly affects resistance. A higher Manning n lowers velocity and reduces discharge for the same geometry, slope, and flow depth.
The Froude number compares flow inertia with gravity effects. Values below one are usually subcritical, near one is transitional, and above one indicates supercritical flow.
No. Inlet control needs additional entrance, headwater, and geometry checks. This page focuses on section conveyance from Manning based flow relationships.
Enter dimensions in meters, slope in percent, and discharge in cubic meters per second. Keep every input in consistent SI units.
It helps you see whether the chosen culvert size can carry a design discharge before the water reaches the crown. That is useful for early sizing studies.
No. Final design should also review inlet control, outlet control, tailwater, debris, sediment, scour, structural limits, and local drainage standards.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.