Channel Flow Calculator & Canal Discharge (Manning's)

Manning's equation estimates the velocity of water in an open channel: V = (1/n) × R^(2/3) × S^(1/2), where n is the roughness coefficient, R the hydraulic radius (area ÷ wetted perimeter) and S the bed slope. Discharge Q = V × A. It's the standard formula for designing canals, drains and channels, and this tool applies it.

Find the flow area and wetted perimeter from the channel's shape and water depth, compute the hydraulic radius, then apply Manning's equation for velocity and multiply by area for discharge. The tool does it for rectangular and trapezoidal channels and reports Q in L/s, m³/s and m³/h.

It's the flow cross-sectional area divided by the wetted perimeter (the length of channel boundary in contact with water). A larger hydraulic radius means the water 'feels' less friction relative to its volume, so it flows faster. It's central to Manning's equation.

The roughness coefficient — higher for rougher boundaries. Typical values: concrete-lined ≈ 0.013, brick/masonry ≈ 0.015, clean earth ≈ 0.022, gravel ≈ 0.025, weedy earth ≈ 0.030, natural stream ≈ 0.035. Smoother linings (lower n) carry more water at the same slope and size. Pick your lining in the tool.

Bed slope (S) is the fall of the channel bed per unit length, e.g. 0.001 means 1 m drop over 1,000 m. Measure the level difference over a known length and divide. Irrigation channels are usually gently sloped (0.0005–0.002) to move water without scouring.

Aim for a non-scouring, non-silting velocity — roughly 0.3–1.5 m/s for earthen canals. Below ~0.3 m/s sediment settles and weeds grow; above ~1.5–2 m/s earth canals erode (lined channels tolerate more). The tool flags silting or scouring risk for your design.

Trapezoidal channels (sloped sides) are the norm for earthen canals because vertical earth banks collapse; the side slope (z, horizontal per vertical) keeps them stable. Rectangular sections suit lined or masonry channels. The tool handles both — choose the shape and, for trapezoids, the side slope.

Make it bigger (deeper/wider), steepen the bed slope, or smooth the lining (lower n). Lining an earthen canal with concrete cuts roughness and seepage, raising capacity and letting you use a higher velocity. Test each change in the tool to see the discharge it delivers.

Overflow means the flow exceeds the channel's capacity — enlarge it or reduce inflow. Silting means the velocity is too low — steepen the slope, narrow the section, or clean and reshape it. The velocity and discharge here tell you which problem you have and how to fix it.

Yes — Manning's equation applies to any open channel. Use the appropriate n (higher for natural, weedy or stony beds) and approximate the section as rectangular or trapezoidal. For very irregular natural channels the estimate is rougher, but still a useful first approximation.