Open channel flow is the movement of water (or any liquid) with a free surface exposed to the atmosphere. Unlike water flowing through a closed pipe under pressure, open channel flow is driven by gravity alone. Rivers, irrigation canals, stormwater ditches, and even the water running along a street gutter are all examples. The defining feature is that the water surface is open to the air, meaning atmospheric pressure acts on it rather than internal pressure forcing it along.
How Open Channel Flow Differs From Pipe Flow
In a closed pipe running full, water moves because of a pressure difference between two points. The pipe walls completely surround the flow, and the water has no contact with the atmosphere. Open channel flow works differently: the liquid has a free surface, and gravity pulling water downhill is the engine that keeps it moving. The slope of the channel bed and the roughness of the surface it flows over determine how fast the water travels.
A storm drain that’s only partially full, for instance, behaves as open channel flow even though it’s technically inside a pipe. What matters isn’t whether there’s a roof overhead but whether the water surface is free and unpressurized. This distinction is important in engineering because the equations and design methods for each type of flow are fundamentally different.
Laminar vs. Turbulent Flow
Water in an open channel can move in smooth, orderly layers (laminar flow) or in chaotic, swirling patterns (turbulent flow). The Reynolds number, a ratio comparing the water’s inertia to its viscosity, determines which state dominates. In open channels, the transition from laminar to turbulent happens at slightly higher Reynolds numbers than in closed pipes, with the exact threshold depending on the channel shape. In practice, nearly all real-world open channel flows are turbulent because the velocities and channel sizes involved are large enough to push past the laminar range.
Subcritical, Critical, and Supercritical Flow
Beyond turbulence, engineers classify open channel flow by a second number: the Froude number. This compares the flow velocity to the speed at which a small surface wave would travel in the channel. When the Froude number is below 1, the flow is subcritical, meaning it’s relatively deep and slow, and disturbances can travel upstream. When the Froude number equals exactly 1, the flow is critical. Above 1, the flow is supercritical: shallow, fast, and unable to send wave signals back upstream.
This classification has real consequences. A river flowing calmly through a valley is subcritical. Water shooting down a steep spillway is supercritical. When supercritical flow suddenly transitions to subcritical, the result is a hydraulic jump, a violent, turbulent surge of water that rises abruptly. You can see a small-scale version of this in your kitchen sink, where the thin, fast sheet of water hitting the basin suddenly jumps up into a thicker, slower ring.
Uniform vs. Varied Flow
When the water depth stays constant along the length of a channel, the flow is called uniform. This happens when gravity pulling the water downhill is perfectly balanced by friction from the channel bed and walls. In a long, straight canal with a consistent slope and cross-section, the flow eventually settles into this equilibrium depth, known as normal depth.
When the depth changes along the channel, the flow is non-uniform, or “varied.” Engineers split this into two categories:
- Gradually varied flow occurs when the water surface changes slowly over a long distance. A common example is the backwater effect upstream of a dam, where the water surface gradually rises as it approaches the obstruction. The drawdown near a sudden drop in a channel is another. Because the changes are gentle, the pressure at any point still behaves as though the water were standing still (hydrostatic), which simplifies the math considerably.
- Rapidly varied flow occurs when the depth changes sharply over a short distance. A hydraulic jump is the classic case. The water surface curves so steeply that the hydrostatic assumption breaks down, and the analysis requires different tools.
Manning’s Equation
The most widely used formula for calculating flow in open channels is Manning’s equation. It connects five variables: flow rate, channel cross-sectional area, how rough the channel surface is, the hydraulic radius, and the slope of the channel.
The hydraulic radius is a measure of how efficiently the channel shape moves water. It’s calculated by dividing the cross-sectional area of the flowing water by the “wetted perimeter,” which is the length of the channel surface actually touching the water. A wide, shallow channel has a large wetted perimeter relative to its area, so more of the water is in contact with the rough channel bed, and friction slows it down. A deep, narrow channel is more hydraulically efficient. In very wide rectangular channels, the hydraulic radius approaches the water depth itself.
The roughness coefficient (called Manning’s n) captures how much friction the channel surface creates. Smooth finished concrete has an n value around 0.010 to 0.013. Unfinished concrete formed against rough wood boards ranges from 0.013 to 0.020. Corrugated metal storm drains typically fall between 0.021 and 0.030. Natural earth channels with vegetation and irregular banks have even higher values. Choosing the right roughness coefficient is one of the most important, and most judgment-dependent, steps in any open channel design.
Specific Energy and Critical Depth
Specific energy is the total energy of the flow measured relative to the channel bottom rather than some external reference point. It combines the water depth with the kinetic energy of the moving water. For any given flow rate, there’s a minimum possible specific energy, and the depth at which that minimum occurs is the critical depth, the same depth where the Froude number equals 1.
This relationship produces a useful curve. At any energy level above the minimum, two possible depths exist: one shallow and fast (supercritical), one deep and slow (subcritical). Both carry the same flow rate at the same energy. Understanding this curve helps engineers predict what happens when a channel narrows, a bump rises from the bed, or water passes over a weir.
Measuring Flow in Open Channels
Two of the most common devices for measuring open channel flow are weirs and flumes.
A weir is essentially a barrier placed across the channel with a specially shaped opening, or notch, cut into it. Water backs up behind the barrier and then spills through the notch. By measuring the height of the water surface upstream, you can calculate the flow rate using established equations for that notch shape. Common notch shapes include V-notch (good for low flows), rectangular, and trapezoidal. Thin-plate weirs are among the simplest and least expensive flow measurement devices available.
A flume is a fixed structure built into the channel that forces the water through a narrowed section called the throat. It typically has three parts: a converging section that funnels the water in, the narrow throat where velocity increases, and a diverging section where the flow expands again. Parshall flumes, trapezoidal flumes, and cutthroat flumes are all variations of this design. Flumes are especially useful when debris in the water would clog a weir, or when you can’t afford the head loss that a weir creates.
Where Open Channel Flow Shows Up
Open channel flow principles govern the design of irrigation systems, urban stormwater networks, wastewater treatment plants, flood control channels, and hydropower spillways. Any time water flows with a free surface, whether in a concrete-lined canal or a rocky mountain stream, the same physics applies. Even partially full sewer pipes and culverts are analyzed as open channels rather than pressurized systems, since the water surface inside them is exposed to air.
The concepts scale from small roadside ditches a few inches deep to major rivers carrying thousands of cubic feet per second. The math stays the same; only the numbers change.