Watercourse Capacity Calculator

Input Options

Choose a cross-section and enter channel properties.

Unit system

Use consistent units across inputs.

Cross-section shape

Geometry controls area and wetted perimeter.

Roughness (Manning n)

Typical range: 0.010–0.070.

Slope input mode

Example: 0.002 equals 0.2%.

Slope (S)

Unitless value, such as 0.0015.

Slope (%)

Percent grade, such as 0.2.

Flow depth (y) m

Normal depth used for capacity estimate.

Bottom width (b) m

Required for rectangular and trapezoidal shapes.

Side slope (z = H:V)

Example: 1.5 means 1.5H to 1V.

Diameter (D) m

Used for circular conduits.

Reset

Tip: If you need a higher capacity, adjust depth, width, slope, or lining.

Example Data Table

Sample scenarios show how changes affect discharge.

Scenario	Shape	n	S	Key dimensions	Depth	Expected notes
Roadside drain	Trapezoidal	0.030	0.002	b=1.2, z=1.5	0.75	General earth lining conditions.
Lined canal	Rectangular	0.015	0.001	b=2.0	1.00	Smoother lining increases capacity.
V-ditch	Triangular	0.035	0.004	z=2.0	0.50	Steeper slope raises velocity.
Storm conduit	Circular (partial)	0.013	0.003	D=1.0	0.60	Partial depth uses circular geometry.

Units match your selection when you calculate.

Formula Used

This calculator uses the Manning equation for steady, uniform open-channel flow:

Q = (1 / n) × A × R^2/3 × S^1/2

Q = discharge (flow rate)
n = Manning roughness coefficient
A = cross-sectional flow area
R = hydraulic radius = A / P
P = wetted perimeter
S = energy slope (approximated by bed slope)

Use consistent units. The equation works in both metric and imperial inputs.

How to Use This Calculator

Select the unit system that matches your project drawings.
Choose the cross-section shape for the watercourse or conduit.
Enter Manning n, slope, and the expected normal flow depth.
Provide the geometry fields that apply to your chosen shape.
Press Calculate Capacity to view results above the form.

Use the download buttons to store results as CSV or PDF.
Review notes when velocities seem unusually high.
Consider freeboard, sediment, and maintenance in final design.

Design intent and scope

This calculator estimates steady, uniform watercourse capacity for construction planning, drainage checks, and temporary works. It supports metric (m, m³/s) and imperial (ft, cfs) inputs to align with drawings and site measurements. Use it to compare cross-sections before detailed hydrologic routing.

Inputs that drive capacity

Capacity rises with flow area and hydraulic radius, and with steeper slope, while roughness reduces discharge. Because Q scales with S^0.5, doubling slope increases Q by about 41%. Likewise, lowering n from 0.035 to 0.030 increases Q by about 17%. Typical Manning n ranges include 0.012–0.015 for finished concrete, 0.020–0.025 for corrugated metal, 0.030–0.035 for clean earth, and 0.040–0.050 for riprap.

Geometry and lining benchmarks

Rectangular and trapezoidal channels are common for diversions, canals, and cut-off ditches. A trapezoid with z = 1.5 (1.5H:1V) often balances stability and excavation volume; z = 2.0 can improve maintainability and reduce sidewall sloughing. Circular conduits suit road crossings and culverts; partial-flow calculations represent realistic operating depths below the crown.

Interpreting velocity and regime

Velocity helps flag erosion risk and lining needs. Many earthen channels aim for roughly 0.6–1.8 m/s (2–6 ft/s) depending on soil, vegetation, and maintenance; lined sections typically tolerate higher velocities. The Froude number uses hydraulic depth; Fr < 1 indicates subcritical flow, Fr > 1 indicates supercritical flow, and near‑critical values deserve attention at transitions, bends, and outfalls.

Documenting results for submittals

Use CSV and PDF exports to attach assumptions to RFIs, method statements, and temporary drainage packages. Record unit system, n, slope input method, normal depth, and geometry, then compare alternatives using the history table. For final design, apply freeboard, debris allowance, entrance and junction losses, and local criteria, and verify slopes and dimensions with survey or as‑built checks. For QA, rerun cases with conservative n and reduced depth to test resilience.

FAQs

1) What does this calculator estimate?

It estimates steady, uniform open-channel discharge using Manning’s equation for a selected cross-section, slope, roughness, and normal depth. It is intended for screening and planning, not replacing final hydraulic design.

2) Which Manning n value should I choose?

Use a value consistent with the expected lining and maintenance condition. Concrete is typically lower, earth and riprap are higher. When uncertain, run a conservative (higher) n and document the assumption in your export.

3) Can I enter slope as a percent?

Yes. Select percent slope and enter the grade (for example, 0.2%). The calculator converts it to a decimal slope internally. Ensure the slope represents the energy slope you want to approximate.

4) Is the circular option valid for pressurized flow?

No. The circular options apply Manning-based open-channel assumptions (including a full conduit as uniform flow). For pressurized pipes, use appropriate pressure-flow methods such as Darcy–Weisbach or Hazen–Williams.

5) What does the Froude number tell me?

It indicates flow regime: subcritical (Fr < 1) tends to be calmer and depth-controlled downstream, while supercritical (Fr > 1) can be rapid and requires care at transitions, drops, and outlets.

6) Why does my capacity look unusually high or low?

Check units, slope magnitude, and roughness first. Confirm the selected shape matches your geometry and that depth is realistic. Small changes in n and slope can materially change Q, so test sensitivity with the history feature.