Formula used
The equation estimates steady uniform gravity flow. Area and hydraulic radius change with water depth. Pressurised operation needs another hydraulic method.
| Symbol | Meaning | SI unit |
|---|---|---|
| Q | Discharge | m³/s |
| n | Manning roughness | dimensionless |
| A | Wetted area | m² |
| R | Hydraulic radius | m |
| S | Energy slope | m/m |
How to use the calculator
- Select the quantity that must be solved.
- Enter pipe dimensions, slope, and roughness.
- Choose units and optional design checks.
- Run the calculation and review all warnings.
- Export the result table when documentation is needed.
Example data
| Input | Example | Reason |
|---|---|---|
| Inside diameter | 600 mm | Common drainage pipe size |
| Flow depth | 360 mm | 60% depth ratio |
| Slope | 0.5% | Moderate gravity slope |
| Manning n | 0.013 | Typical concrete value |
Typical Manning coefficients
| Material | Typical n |
|---|---|
| PVC | 0.009 |
| HDPE | 0.010 |
| Concrete | 0.013 |
| Vitrified clay | 0.013 |
| Corrugated metal | 0.024 |
| Cast iron | 0.013 |
| Steel | 0.012 |
| Brick | 0.015 |
| Smooth laboratory pipe | 0.008 |
Depth and capacity reference
| Depth ratio | General behaviour |
|---|---|
| 25% | Small wetted area and modest hydraulic radius |
| 50% | Velocity equals full-pipe velocity under Manning flow |
| About 81% | Velocity reaches its maximum |
| About 94% | Discharge reaches its maximum |
| 100% | Open-channel assumption approaches its practical limit |
Frequently asked questions
What is part-full pipe flow?
It is gravity flow with an exposed water surface. The pipe behaves like an open channel. Pressure flow requires different equations.
Why can discharge exceed full-pipe discharge?
The hydraulic radius rises before the pipe becomes full. Manning capacity therefore peaks near ninety-four percent depth. The result assumes open-channel conditions remain valid.
Why does maximum velocity occur near eighty-one percent?
Velocity depends directly on hydraulic radius. That radius reaches its largest useful value earlier. The exact ratio follows circular geometry.
Which Manning coefficient should I use?
Choose a value matching material and condition. Age, joints, deposits, and damage increase roughness. Local standards should guide final selection.
Can this calculator handle pressurised pipes?
No, Manning mode targets free-surface gravity flow. A full pipe may become pressurised downstream. Use Darcy–Weisbach or network modelling then.
What is hydraulic radius?
Hydraulic radius equals wetted area divided by perimeter. It measures flow efficiency for the section. Larger values usually support greater capacity.
What does the Froude number show?
It compares flow inertia with gravity effects. Values below one indicate subcritical flow. Values above one indicate supercritical flow.
What does the Reynolds number show?
It compares inertial and viscous forces. Drainage pipes are usually turbulent during normal operation. Very shallow flow can behave differently.
How should results be used?
Use results for preliminary analysis and comparison. Confirm assumptions using project standards and survey data. Qualified review remains important for final design.