Choose channel shape, enter flow, slope, and roughness, then compute depth easily. Review hydraulic area, perimeter, velocity, and safety checks instantly for field decisions.
The calculator uses Manning’s equation for uniform, steady open-channel flow:
| Expression | Meaning |
|---|---|
| Q = (k/n) · A · R2/3 · S1/2 | Discharge from roughness, geometry, and slope. |
| R = A / P | Hydraulic radius equals area divided by wetted perimeter. |
| V = Q / A | Mean velocity in the section. |
| Fr = V / √(g · (A/T)) | Flow regime indicator using hydraulic depth A/T. |
Here, A is flow area, P is wetted perimeter, T is top width, S is energy slope approximated by bed slope, and n is roughness. The factor k equals 1.0 in metric units and 1.486 in US customary units.
For design, confirm assumptions, review lining limits, and apply project standards.
Sample values below illustrate typical drainage-channel inputs and resulting depths.
| Shape | Units | Q | S | n | Geometry | Output |
|---|---|---|---|---|---|---|
| Trapezoidal | Metric | 0.75 m³/s | 0.002 | 0.015 | b=1.2 m, z=1.5 | Normal depth ≈ 0.45 m |
| Rectangular | Metric | 0.50 m³/s | 0.0015 | 0.014 | b=1.0 m | Normal depth ≈ 0.52 m |
| Triangular | Metric | 0.20 m³/s | 0.003 | 0.020 | z=2.0 | Normal depth ≈ 0.34 m |
| Circular | US customary | 12.0 ft³/s | 0.002 | 0.013 | D=3.0 ft | Normal depth ≈ 1.30 ft |
Channel depth controls capacity, erosion risk, and freeboard. Undersized sections overtop, saturate subgrades, and damage formwork. Oversized sections add excavation, concrete, and lining cost. A quick depth check supports drainage planning, temporary works, and permanent conveyance.
The key inputs are discharge, slope, roughness, and geometry. Discharge comes from hydrology, dewatering pumps, or storm allowances. Slope is the channel bed grade. Roughness reflects surface texture, vegetation, or riprap. Geometry defines flow area and wetted perimeter. Include unit consistency and verify flow is not pressurized during design checks.
Rectangular channels suit lined drains and precast segments. Trapezoidal channels are common for earthworks because side slopes improve stability. Triangular channels fit shallow roadside ditches. Partially full circular sections represent culverts operating as open flow under gravity.
Manning’s equation estimates uniform, steady flow: Q depends on area A, hydraulic radius R, slope S, and roughness n. For a chosen shape, the calculator computes A and wetted perimeter P at a trial depth, then finds R=A/P and the resulting discharge.
When solving for depth, the calculator searches for the depth where computed discharge matches the target. A bracketing method narrows the interval until the difference is negligible. This approach is stable for typical open channels and avoids sensitivity to initial guesses.
Velocity helps assess lining need and scour potential. The tool also reports Froude number using hydraulic depth A/T, where T is top width. Values below one indicate subcritical flow, while values above one suggest rapid, supercritical conditions requiring energy control. Aim for velocities that match lining limits and sediment goals onsite.
Freeboard adds a safety margin above normal depth to account for debris, waves, construction tolerances, and uncertainty in discharge. Many projects specify a percentage or fixed allowance. The calculator outputs a suggested freeboard and a design depth for section sizing.
Use the calculator during planning meetings to compare alternatives quickly. Record assumptions for Q, n, and slope. Validate results with site constraints such as excavation limits, utilities, and lining specifications. Export CSV or PDF for submittals, inspections, and crew briefings. Recheck after grade changes, rain events, or material substitutions occur.
It estimates normal depth from a target discharge using Manning’s equation and the selected channel shape.
Use a value that matches the finished surface: smooth concrete is lower, rough concrete and riprap are higher. If unsure, run a sensitivity check with a reasonable range.
No. The circular option assumes gravity-driven, partially full flow. If the section runs full, use a closed-conduit method and confirm upstream and downstream control conditions.
Enter the average bed grade for the reach being designed. If grade changes along the alignment, evaluate each segment separately and confirm continuity at transitions.
Freeboard provides a safety margin for debris, waves, construction tolerance, and uncertainty in flow. The tool adds a percentage above computed depth to suggest a practical design depth.
It depends on lining and soil. Compare calculated velocity with project limits for erosion and lining durability, then add protection or energy dissipation if needed.
If the maximum search depth is too small, the target discharge may not be reachable. Increase the limit, review geometry and slope, and confirm that inputs are in consistent units.
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.