Table of Contents
What Is Open Channel Flow?
Open channel flow is the flow of liquid with a free surface exposed to atmospheric pressure. Rivers, canals, irrigation ditches, and storm drains are all examples. Unlike pipe flow (pressurized), open channel flow is driven by gravity and the channel slope. The flow behavior depends on channel geometry, roughness, slope, and depth.
Manning's equation is the most widely used formula for calculating open channel flow velocity and discharge. Developed by Robert Manning in 1889, it relates flow velocity to channel properties through an empirical roughness coefficient (Manning's n). The equation works well for uniform, steady flow conditions in natural and artificial channels.
Manning's Equation
Where V is velocity (m/s), n is Manning's roughness coefficient, R is hydraulic radius (A/P), S is channel slope, Q is flow rate (m³/s), A is cross-sectional area, and P is wetted perimeter.
Manning's n Values
| Channel Type | Manning's n |
|---|---|
| Smooth concrete | 0.012 |
| Unfinished concrete | 0.014 |
| Earth, straight | 0.020-0.025 |
| Natural stream, clean | 0.030-0.040 |
| Floodplain with trees | 0.10-0.15 |
Frequently Asked Questions
What is the Froude number?
The Froude number (Fr = V/√(gy)) classifies flow regime: Fr < 1 is subcritical (tranquil, deep), Fr = 1 is critical, and Fr > 1 is supercritical (rapid, shallow). Hydraulic jumps occur when flow transitions from supercritical to subcritical.
What is hydraulic radius?
Hydraulic radius (R = A/P) is the ratio of cross-sectional flow area to wetted perimeter. It characterizes how efficiently a channel section conveys flow. A wider, shallower channel has a smaller hydraulic radius than a narrower, deeper one with the same area.