How this calculator works
For a full circular section, hydraulic radius is one quarter of internal diameter. Manning velocity uses that radius, energy slope and an empirical roughness coefficient. Drop means loss of energy head, which equals invert drop only under suitable uniform gravity-flow conditions. Material references are illustrative selections within EPA new-pipe ranges. This relation does not model a pump curve, transient pressurization or partially filled flow.
Formula
A = pi D^2/4; P = pi D; R = A/P = D/4. S = delta H/L. v = R^(2/3) sqrt(S)/n; Q = Av. In SI, Manning n has units s/m^(1/3).
Worked example
For D = 1 m, n = 0.01 and S = 0.01, v = 3.968502630 m/s and Q = 3.116854677 m3/s. Reducing slope to 0.0025 halves both values.
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Frequently asked questions
Is slope the geometric pipe slope?
It is the energy slope. Under uniform gravity flow it can equal the conduit slope; a pressurized system may differ.
Does the roughness inverse use the material preset?
It infers n from measured flow, diameter and slope. The material choice is ignored for that inverse mode.
Can the pipe run partly full?
This page uses a full circle. Use open-channel geometry and wetted perimeter for a partly full conduit.