Model streams, canals, and pipes with confidence now. Choose geometry, slope, and roughness inputs precisely. Export results to share, verify, and document fast anywhere.
Use consistent units. Choose a geometry, enter roughness and slope, then calculate discharge and velocity.
These examples illustrate typical inputs and computed discharge. Values are approximate because roughness and slope vary in real channels.
| Case | Geometry | n | S | Key dimensions | Q (approx) |
|---|---|---|---|---|---|
| 1 | Rectangular | 0.013 | 0.0010 | b = 2 m, y = 1 m | 3.90 m³/s |
| 2 | Trapezoidal | 0.025 | 0.0008 | b = 3 m, z = 1, y = 1.2 m | 4.46 m³/s |
| 3 | Circular full | 0.012 | 0.0020 | D = 1.0 m | 2.59 m³/s |
Manning’s equation estimates steady, uniform open-channel flow based on channel geometry and roughness:
Here, A is flow area, P is wetted perimeter, S is energy slope, and n is Manning roughness. In many US references, C = 1.486; otherwise C = 1.0.
Manning’s equation is a standard screening tool for steady, uniform open-channel flow. It links channel roughness, slope, and cross-section to discharge. Engineers use it for preliminary sizing of drains, canals, and culverts, then confirm with site data and hydraulic checks.
The roughness coefficient n represents boundary friction and irregularity. Small changes can shift discharge noticeably. Smooth finished concrete often falls near 0.012–0.015, while natural earth channels may range roughly 0.020–0.035 depending on vegetation, stones, and bends.
Slope S is dimensionless and commonly mirrors bed slope under uniform flow. Typical design slopes can span from 0.0001 (very mild) to 0.01 (steep) depending on terrain. Because Q ∝ √S, quadrupling slope roughly doubles discharge, all else equal.
The calculator derives area A and wetted perimeter P for common shapes, then computes hydraulic radius R = A/P. Larger R means more area per boundary contact, reducing friction losses per unit flow. This is why wide channels or full pipes often carry more flow at the same slope.
Mean velocity V = Q/A helps compare against practical limits. Very low velocity can promote sedimentation, while high velocity may cause erosion of unlined channels. Many projects also compare velocity with allowable shear stress or lining guidance, especially in earth or riprap channels.
Conveyance K = (C/n)·A·R^(2/3) captures geometry and roughness in one value, so discharge becomes Q = K·√S. This is helpful for quick sensitivity studies: if only slope changes, you can scale Q without recomputing section properties.
SI calculations use C = 1.0 with meters and cubic meters per second. In many US references, a constant C = 1.486 is applied with feet and cubic feet per second. This tool lets you toggle that constant so your results match the convention you use.
Start by selecting a realistic section and lining, then choose n from a trusted reference and verify slope from drawings or survey. Review V for erosion or deposition risk, and document inputs and results using the CSV/PDF exports for reports and peer review.
It is primarily for open-channel, gravity-driven flow. A pipe flowing full can be approximated in special cases, but pressurized systems are usually analyzed with Darcy–Weisbach or Hazen–Williams methods.
S is the energy grade slope under uniform flow. In many practical designs, S is taken as the channel bed slope, assuming steady conditions and gradually varied effects are small.
Select n from published tables for the lining and condition, then adjust for vegetation, joints, stones, or irregularity. If possible, calibrate using measured flow or a known rating curve.
Hydraulic radius R = A/P measures how efficiently a section carries flow relative to its wetted boundary. Larger R typically reduces friction losses, increasing Q for the same n and slope.
Use the Custom option. Enter A and P directly to compute R and Q. This is useful for surveyed sections, irregular channels, or cases where geometry is defined from CAD or field measurements.
Many US customary formulations multiply by 1.486 to align units when using feet and seconds. Some references omit it by defining n differently. Toggle it to match your handbook or project standard.
Compare V with allowable ranges for the lining and soil. Low V can indicate sediment deposition risk, while high V can suggest erosion or lining damage. Use local guidance, shear checks, and safety factors.
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.