Worked example · Drain pipes
How much slope does a drain pipe need?
Change the grade and the pipe-roughness value for an example 4-inch pipe. The tool shows how far the pipe falls over its length and how the flow velocity changes.
A full-pipe worked example. It leaves out inlets, partial flow, blockages and the rest of a drainage system, so it isn't a capacity rating.
EXPLORE THE ROUTE
What a little slope changes
Flatten or tilt a pipe route. See the fall change, and compare idealized flow through different interior surfaces.
Water follows the route and grade you give it. Try a flatter slope, then change the interior surface.
CHANGE ONE THING
Small slope. Meaningful fall.
Watch the slope change. The vertical angle is exaggerated so a small real-world difference is visible.
Idealized full-pipe velocity: 3.15 ft/s. Actual flow depends on more than slope.
Behind the numbers
Fall in inches = length in feet × slope ÷ 100 × 12. The velocity illustration uses Manning’s equation, V = (1.486 / n) × R^(2/3) × √S, for a full, uniform 4-inch circular example, where R = diameter ÷ 4 in feet and S is the slope as a decimal.
Roughness values are example assumptions, not a statement about every product. Inlets, partial flow, tailwater, fittings, grade changes and blockage are not modeled. This tool does not choose a pipe, certify capacity or predict storm performance.
Method context: NDS exterior drainage guide. Roughness context: SDDOT culvert manual, Manning’s n table.