Open Channel Flow Calculator

How It Works

1. 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.

2. 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).

3. 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.

4. 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

Frequently Asked Questions