Size channels faster with clear, field friendly inputs. Switch units and shapes for any project. See velocity and discharge instantly for safer builds operations.
| Section | Inputs | Slope S | n | Typical output |
|---|---|---|---|---|
| Trapezoidal | b=3.0, y=1.2, z=1.5 | 0.001 | 0.022 | V≈1.1 m/s, Q≈4.7 m³/s |
| Rectangular | b=2.5, y=1.0 | 0.0008 | 0.018 | V≈1.2 m/s, Q≈3.0 m³/s |
| Circular segment | D=1.2, y=0.6 | 0.0015 | 0.013 | V≈2.0 m/s, Q≈1.1 m³/s |
The calculator uses Manning’s equation to estimate average open-channel velocity:
V = (k / n) · R2/3 · S1/2
Discharge is computed as Q = V · A. Flow regime uses the Froude number Fr = V / √(g · Dh), where Dh = A / T and g = 9.80665.
Canal velocity is a core check for lined and unlined conveyance. Too much velocity can cause erosion, lining damage, joint opening, and unstable banks near structures. Too little velocity can encourage sediment deposition, vegetation growth, algae, and reduced hydraulic capacity. This calculator supports quick screening during layout, grading, and rehabilitation planning, before detailed hydraulic modeling is performed for preliminary sizing and cost estimates.
Manning roughness (n), slope (S), and hydraulic radius (R) dominate the outcome. Slope represents energy loss per unit length, and should reflect the design grade or water surface assumption. In flatter sites, small slope changes can significantly change velocity. R depends on both flow area and wetted perimeter, so geometry revisions can shift velocity even when depth stays constant. Include losses from bends and appurtenances later when moving to final design.
Select n based on the expected surface condition: finished concrete, troweled masonry, corrugated metal, compacted earth, or vegetated channels. Construction tolerances, surface wear, debris, and minor misalignment increase effective n. When uncertainty is high, evaluate a low and high n range to bracket velocity and discharge, then align the design with the conservative case.
For trapezoidal sections, side slopes affect wetted perimeter and top width, influencing hydraulic depth and Froude number. Rectangular and circular sections are sensitive to depth assumptions, especially near partial flow where R changes rapidly. Always confirm that calculated depth leaves adequate freeboard, allows maintenance access, and meets safety and overtopping criteria.
Use velocity (V) and discharge (Q) for capacity comparison, and review flow regime from the Froude number. Subcritical flow may backwater under downstream controls, while supercritical flow can create hydraulic jumps at transitions and outlets. Compare V to allowable limits for the lining or soil, verify scour protection at bends, and confirm that transitions, culverts, and check structures can pass the computed discharge safely.
It uses steady, uniform open-channel flow with Manning’s equation. It does not model backwater, transitions, bends, or unsteady effects. Use detailed modeling when controls, structures, or rapidly varied flow are important.
Use the option that matches the wetted cross-section at the design depth. For irregular sections, choose “Custom A & P” and enter measured flow area and wetted perimeter from your section drawing.
Start with the design channel invert slope. If water surface is controlled or the reach is short, ground grade may not represent energy slope. Confirm with hydraulic checks for controlled reaches and structures.
Side slopes change wetted perimeter and top width, which changes hydraulic radius and hydraulic depth. Those changes influence both Manning velocity and the Froude number, even when depth and bottom width stay the same.
It depends on soil type, vegetation, and erosion protection. Use project criteria or local standards for allowable velocity. When unsure, treat results as screening and add lining or protection where risk is high.
Yes, as an initial estimate of average velocity and discharge at a selected depth. For pump suction, intake, or diversion structures, also evaluate head losses, approach conditions, and debris control separately.
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.