Open Channel Flow Calculator
Calculate Flow Rate, Velocity, and Froude Number for Rectangular, Trapezoidal, and Circular Channels
Free source-aware open channel screen for civil engineers, stormwater designers, and wastewater operators. Select rectangular, trapezoidal, or circular channel shape, enter dimensions, slope, and Manning's n to screen flow rate in CFS using Q = (1.486/n) x A x R_h^(2/3) x S^(1/2). Shows velocity, Froude number, flow-regime prompts, and source-boundary warnings.
The app applies a local steady uniform Manning equation fixture for prismatic channel or partially full pipe geometry. It does not replace surveyed geometry, design storm/runoff basis, HGL or backwater modeling, roughness selection, scour/erosion review, freeboard criteria, permits, AHJ requirements, or qualified hydraulic engineering review.
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Open Channel Flow Guide →How It Works
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Select Channel Shape
Choose rectangular (vertical walls), trapezoidal (sloped sides with specified z:1 ratio), or circular (partially full pipe). Each shape has different area, wetted perimeter, and hydraulic radius formulas.
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Enter Channel Dimensions
Input bottom width and water depth for rectangular/trapezoidal, or pipe diameter and flow depth for circular. For trapezoidal, enter side slope as horizontal to vertical (e.g., 2:1).
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Enter Slope and Roughness
Input channel slope in ft/ft and a Manning n value from a project source, field inspection, or local planning row. Built-in rows are not certified roughness data.
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Review Flow Results
See flow rate in CFS and GPM, average velocity, Froude number, and flow-regime prompts. Treat velocity and regime bands as review flags, not design approval.
Built For
- Civil engineers designing drainage ditches and channels for stormwater conveyance
- Stormwater designers checking capacity of existing channels under new development runoff
- Wastewater operators calculating flow in partially full gravity sewer pipes
- Irrigation engineers sizing canals and delivery channels for agricultural water supply
- Highway engineers checking culvert capacity for road crossing drainage
- Environmental engineers screening velocity before separate erosion, scour, and lining review
- Mine dewatering teams sizing drainage channels for surface runoff management
Features & Capabilities
Manning's Equation (US Customary)
Q = (1.486/n) x A x R_h^(2/3) x S^(1/2). Local steady uniform flow arithmetic with source warnings attached.
Three Channel Shapes
Rectangular, trapezoidal, and circular free-surface geometry. Reports area, wetted perimeter, hydraulic radius, and source-boundary caveats.
Froude Number and Flow Regime
Fr = V / sqrt(g x D_h). Flags local subcritical, critical, and supercritical review bands without approving channel stability.
Partially Full Pipe
Screens circular gravity-flow geometry and warns when entered depth reaches full-pipe or possible surcharge conditions.
Manning's n Reference
Built-in roughness rows are local placeholders. Verify current source tables, product data, and field condition before design use.
PDF Export
Export a source-aware screening report for review notes, not a sealed hydraulic design or permit submittal.
Assumptions
- Manning's equation: Q = (1.486/n) x A x R_h^(2/3) x S^(1/2) in US customary units, assuming uniform and steady flow
- Channel cross-section is prismatic (constant shape and slope) along the entire reach being analyzed
- Manning's n value is constant along the channel and across the cross-section (no composite roughness calculation)
- Turbulence, Reynolds effects, and transition behavior are not independently checked
- Circular pipe calculations assume gravity flow with a free water surface; surcharged or pressurized flow is outside the screen
- Entered slope is used as the uniform-flow energy slope proxy; real channels may require water-surface-profile analysis
Limitations
- Does not model gradually varied flow (backwater curves) or rapidly varied flow (hydraulic jumps, drops) - only uniform flow
- Composite channels (different roughness on bed vs. banks) require weighted n calculations not performed here
- Partially full pipe geometry uses analytical equations that lose accuracy at very shallow depths (d/D below 0.05)
- Does not account for sediment transport capacity, scour velocity limits, or permissible velocity for erodible channels
- Natural channels with irregular cross-sections, vegetation, and meandering cannot be accurately modeled with a single prismatic section
- Does not evaluate freeboard requirements, which depend on flow regime, channel lining, and local design standards
- Supercritical flow results should be used with caution - channel transitions and obstructions can cause hydraulic jumps with significant energy loss
References
- Chow, V.T. - Open-Channel Hydraulics, 1959 (Manning's n tables and uniform flow theory)
- FHWA HDS-4 - Introduction to Highway Hydraulics
- USGS WSP 2339 - Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains
- USGS OFR 88-707 - Basic Hydraulic Principles of Open-Channel Flow
- USACE HEC-RAS Hydraulic Reference Manual documentation
- NIST SP 811 Appendix B.8 - unit conversion context