Estimate culvert discharge with depth, slope, and roughness inputs. Review capacity, velocity, and example values for better planning.
| Width (m) | Height (m) | Depth (m) | Slope | Manning n | Barrels | Estimated Flow (m³/s) |
|---|---|---|---|---|---|---|
| 2.5 | 2.0 | 1.4 | 0.004 | 0.015 | 1 | 6.40 |
| 3.0 | 2.5 | 1.8 | 0.003 | 0.016 | 2 | 15.08 |
| 2.0 | 1.8 | 1.2 | 0.005 | 0.014 | 1 | 4.91 |
This calculator uses the Manning open channel flow method for a rectangular box culvert under free surface flow conditions.
Area: A = b × y
Wetted Perimeter: P = b + 2y
Hydraulic Radius: R = A / P
Velocity: V = (1 / n) × R2/3 × S1/2
Discharge: Q = A × V
For multiple barrels, total discharge equals single barrel flow multiplied by the number of barrels.
This method is best for preliminary hydraulic checks. It does not model inlet control, outlet control, tailwater effects, or pressurized flow.
Box culverts carry runoff under roads, rail lines, and embankments. Designers need a quick way to estimate how much water can pass through a barrel. This calculator helps by using simple rectangular geometry and Manning flow theory.
Hydraulic flow estimates support safer drainage planning. An undersized culvert may back water up during a storm. An oversized culvert may increase construction cost without adding enough value. A clear estimate supports better decisions early in design.
The tool uses width, height, water depth, slope, roughness, and barrel count. It then calculates area, wetted perimeter, hydraulic radius, velocity, and discharge. It also shows full flow capacity for comparison. This makes the output useful for quick screening work.
Manning theory is common in open channel hydraulics. It links velocity to roughness, slope, and hydraulic radius. When the flow depth changes, the area and wetted perimeter also change. That changes the hydraulic radius and total discharge.
This calculator is useful for preliminary checks, concept layouts, and educational analysis. It works well when the culvert flows partly full and behaves like an open channel. It is also useful when comparing several culvert sizes before detailed modeling begins.
Real culvert performance can differ from a simple estimate. Inlet control, outlet control, debris, sediment, tailwater, and pressure flow can all affect capacity. Because of that, the result should be treated as a planning value, not a final hydraulic approval value.
Review total flow with velocity and percent full. High velocity may raise scour concerns. A low percent full may show spare capacity. A result close to full flow capacity may suggest the system needs deeper review under design storm conditions.
It estimates water discharge through a rectangular culvert using width, water depth, slope, roughness, and barrel count. It also reports velocity, hydraulic radius, and percent full.
It uses the Manning open channel equation. This method is widely used for preliminary hydraulic estimates where the culvert is flowing with a free water surface.
Yes. Enter the number of barrels. The calculator estimates single barrel flow first, then multiplies that value to give the total combined discharge.
Use slope as a decimal value. For example, 0.005 means a 0.5 percent slope. Do not enter slope as a percentage unless you convert it first.
Manning n is the roughness coefficient. It represents surface resistance to flow. Smoother culvert surfaces usually have lower values and carry water faster.
No. This version is for open channel style flow. It does not model pressurized conditions, submerged entrance effects, or detailed inlet and outlet control behavior.
Depth changes the flow area and wetted perimeter. That changes hydraulic radius and velocity. Even small depth changes can noticeably increase or reduce discharge.
Use it for screening, comparison, and learning. Final culvert design should also include storm hydrology, tailwater, headwater, site constraints, and local hydraulic 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.