Solve open channel depth using Manning based methods. Test rectangular, trapezoidal, and triangular sections easily. See results, tables, graphs, exports, formulas, and guidance instantly.
Engineering tool for steady uniform open channel flow analysis.
The graph shows how discharge changes with depth for the entered geometry and Manning inputs.
| Section | Q (m³/s) | n | S | b (m) | z (H:V) | yₙ (m) |
|---|---|---|---|---|---|---|
| Trapezoidal | 8.50 | 0.028 | 0.0012 | 3.00 | 1.50 | 1.69 |
| Rectangular | 6.20 | 0.016 | 0.0009 | 2.40 | 0.00 | 1.59 |
| Triangular | 2.80 | 0.030 | 0.0015 | 0.00 | 1.00 | 1.29 |
| Trapezoidal | 15.00 | 0.025 | 0.0022 | 4.50 | 2.00 | 1.62 |
Normal depth is the depth of steady, uniform open channel flow. This calculator solves Manning’s equation by matching the target discharge with the discharge produced by a trial depth.
Manning discharge equation:
Q = (1 / n) × A × R^(2/3) × S^(1/2)
Where:
Geometry used for a trapezoidal section:
A = y × (b + z × y)P = b + 2y√(1 + z²)T = b + 2zyRectangular channels use z = 0. Triangular channels use b = 0. The page solves for depth using a stable bisection routine.
Question: Calculate the normal depth, to the nearest 0.01 m, for a triangular earthen channel. Use n = 0.03.
Answer: A unique numerical depth cannot be found from n = 0.03 alone. You also need discharge, channel slope, and side slope. For a triangular channel, set b = 0, enter the missing values, solve for yₙ, then round the result to the nearest 0.01 m.
Normal depth is the flow depth that occurs during steady, uniform flow in an open channel. At that depth, gravity driving force and boundary resistance are in balance for the given geometry, slope, roughness, and discharge.
This page handles rectangular, trapezoidal, and triangular open channels. A trapezoidal form is the general case. Rectangular flow uses zero side slope, while triangular flow uses zero bottom width.
Manning n represents channel roughness. Higher roughness reduces carrying capacity, so a deeper flow is usually needed to pass the same discharge. Earthen channels commonly use larger n values than finished concrete channels.
Roughness alone is not enough. A single normal depth also depends on discharge, channel slope, and side slope. Without those inputs, many different triangular channels could satisfy the same roughness value.
Enter the side slope as horizontal to vertical, written as H:V. For example, 1.5 means 1.5 horizontal to 1 vertical on each side. Use project drawings or design standards for the correct value.
The Froude number indicates whether flow is subcritical, critical, or supercritical. A value below 1 suggests subcritical flow, near 1 indicates critical flow, and above 1 suggests supercritical flow.
The graph helps you see sensitivity. Small depth changes can produce large discharge changes, especially in broad or steep channels. It is useful for checking whether your solution lies in a stable and realistic range.
Yes, it works well for preliminary checks and educational use. For final design, also review freeboard, erosion resistance, sediment effects, local standards, and whether uniform flow is a valid assumption.
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.