An open channel is any conduit that carries water (or another fluid) with a free surface exposed to the atmosphere. Rivers, irrigation ditches, storm gutters, and roadside culverts are all open channels. Unlike water flowing through a sealed pipe, water in an open channel moves primarily under the force of gravity rather than pressure from a pump, and the surface of the water is always at atmospheric pressure.
How Open Channels Differ From Pipes
The distinction comes down to what drives the water. In a closed pipe, flow happens because of a pressure difference between two points, typically created by a pump. The pipe is completely filled, and the water never contacts the atmosphere. In an open channel, the water surface is exposed to the air, and gravity pulls the water downhill along the channel’s slope. This means the shape of the water surface can change as conditions change, rising or falling depending on flow rate, obstructions, and channel geometry.
This difference matters in practice. Engineers designing a pipe system need to calculate pressure drops. Engineers designing an open channel need to account for how the water depth will vary along the channel’s length, which introduces a more complex set of variables.
Where Open Channels Are Used
Natural open channels include rivers, streams, and mountain torrents. Large rivers near their estuaries, like the Nile between Alexandria and Cairo, tend to flow slowly and calmly. Mountain rivers and rapids, like the cataracts of the Nile or the Zambesi rapids, carry fast, turbulent water. Both are open channel flow, just at different speeds.
Engineered open channels appear everywhere in civil infrastructure: irrigation canals distributing water to farmland, storm drains collecting rainwater from streets, sewers carrying wastewater to treatment plants, and culverts routing water beneath roads and railway lines. Water treatment plants also use open channel conveyors to move water between processing stages. Even decorative public fountains rely on open channel principles.
Channel Shapes and Efficiency
Open channels come in many cross-sectional shapes. The two most common in engineering are trapezoidal and circular. Trapezoidal channels are widely used for irrigation canals and drainage ditches because they’re relatively simple to construct in earth or concrete and they balance structural stability with hydraulic performance.
For any trapezoidal channel, there’s a mathematically optimal combination of bottom width and water depth that moves the most water for the least material. This is called the most efficient cross section. It minimizes the contact area between the water and the channel walls, which reduces friction. In practice, engineers calculate the optimal bottom width and depth based on the required flow area and the slope of the channel’s side walls.
What Controls Flow Speed
The most widely used formula for calculating flow in an open channel is Manning’s equation, introduced by Irish engineer Robert Manning in 1889. It relates flow rate to three main factors: the channel’s slope, its cross-sectional geometry, and a roughness coefficient that captures how much friction the channel surface exerts on the water.
The roughness coefficient (called Manning’s n) varies significantly by material. Smooth, trowel-finished concrete has a value around 0.013, meaning very little friction. An earthen channel that’s clean and recently dug comes in around 0.018. A gravel-bottomed channel is rougher at about 0.025, and corrugated metal storm drains range from 0.019 to 0.024 depending on the corrugation pattern and diameter. Higher roughness means more friction, which slows the water down for any given slope.
Under uniform flow conditions, where the water depth stays constant along the channel, the slope of the water surface matches the slope of the channel bottom. This is the simplest scenario to calculate and serves as the baseline for more complex situations.
Slow Flow vs. Fast Flow
Open channel flow falls into two major regimes based on speed relative to depth: subcritical and supercritical. The dividing line is defined by a ratio that compares the water’s velocity to the speed at which a small surface wave would travel in that depth of water.
When the ratio is below 1, the flow is subcritical. This is calm, relatively slow flow, sometimes called tranquil flow. You see it in large, flat rivers and gently sloped irrigation canals. Water in subcritical flow is deep relative to its speed, and disturbances on the surface can travel upstream. This means downstream conditions (like a dam or a narrowing) can influence the water level upstream.
When the ratio exceeds 1, the flow is supercritical. This is fast, shallow, rapid flow, typical of steep mountain streams and spillways. Surface waves can’t travel upstream against the current, so the flow is controlled entirely by what’s happening upstream. The water moves too quickly for downstream conditions to “communicate” back.
At a ratio of exactly 1, the flow is at the critical point, the boundary between these two behaviors. This critical condition corresponds to the minimum amount of energy needed to carry a given volume of water, which is an important design threshold.
Energy in Open Channel Flow
Engineers think about the energy in an open channel as a combination of two components: the depth of the water and the kinetic energy from its velocity. For any given flow rate, there are always two possible water depths that produce the same total energy: one shallow and fast (supercritical), one deep and slow (subcritical). The relationship between depth and energy forms a curve with a single minimum point, and that minimum corresponds to critical flow.
This matters for design because it tells engineers that pushing water through a channel at or near critical depth is inherently unstable. Small disturbances can cause the water to jump between the two possible depths, creating unpredictable surface conditions. Most engineered channels are designed to operate comfortably in either the subcritical or supercritical range, not at the boundary.
The Hydraulic Jump
One of the most dramatic phenomena in open channel flow is the hydraulic jump, a sudden, turbulent transition where fast, shallow (supercritical) flow abruptly becomes slow, deep (subcritical) flow. If you’ve ever seen water shooting out from under a sluice gate and then suddenly rising into a churning, foamy surface a short distance downstream, that’s a hydraulic jump.
A hydraulic jump always dissipates energy. The water downstream has less total energy than the water upstream, and the difference is lost as heat and turbulence. Engineers use this deliberately. At the base of dam spillways or downstream of gates, a hydraulic jump can be engineered to burn off excess energy that would otherwise erode the channel bed. The energy loss depends on the difference between the upstream and downstream water depths: the greater the difference, the more energy is dissipated.
Turbulence in Open Channels
Separately from the subcritical/supercritical distinction, open channel flow can be laminar (smooth, orderly) or turbulent (chaotic, mixed). Flow stays laminar when the Reynolds number, a ratio of the water’s momentum to friction forces, stays below about 500. Above roughly 1,000, the flow is fully turbulent. Between those values, bursts of turbulence appear intermittently, growing more frequent as the number climbs.
In practice, nearly all open channel flow you’ll encounter in the real world is turbulent. Laminar open channel flow exists only in very thin sheets of water moving very slowly, like a film of rainwater creeping down a gently sloped parking lot. Rivers, canals, gutters, and storm drains all operate well into the turbulent range, which is why Manning’s equation (an empirical formula developed from real-world observations of turbulent flow) works so well for everyday engineering calculations.