Flow Resistance Converter Nf Calculator
Free Flow resistance n↔f Calculator for hydrology & water resources. Enter variables to compute results with formulas and detailed steps.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
f = 8g x n^2 / R^(1/3)
The Darcy-Weisbach friction factor f relates to Manning n through: f = 8g x n^2 / R^(1/3), where g = 9.81 m/s2 and R is the hydraulic radius (m). The Chezy coefficient C = R^(1/6) / n = sqrt(8g/f). Manning velocity V = (1/n) x R^(2/3) x S^(1/2). Darcy-Weisbach velocity V = sqrt(8gRS/f).
Worked Examples
Example 1: Concrete-Lined Channel
Problem:Manning n = 0.013, hydraulic radius = 0.8 m, slope = 0.002.
Solution:Darcy-Weisbach f = 8 x 9.81 x 0.013^2 / 0.8^(1/3) = 0.01426\nChezy C = 0.8^(1/6) / 0.013 = 74.3\nVelocity = (1/0.013) x 0.8^(2/3) x 0.002^(1/2) = 2.96 m/s\nFroude = 2.96 / sqrt(9.81 x 0.8) = 1.06 (supercritical)
Result:f = 0.0143 | C = 74.3 | V = 2.96 m/s | Supercritical
Example 2: Natural Gravel Stream
Problem:Manning n = 0.035, hydraulic radius = 1.2 m, slope = 0.005.
Solution:Darcy-Weisbach f = 8 x 9.81 x 0.035^2 / 1.2^(1/3) = 0.0901\nChezy C = 1.2^(1/6) / 0.035 = 29.5\nVelocity = (1/0.035) x 1.2^(2/3) x 0.005^(1/2) = 1.83 m/s\nFroude = 1.83 / sqrt(9.81 x 1.2) = 0.53 (subcritical)
Result:f = 0.0901 | C = 29.5 | V = 1.83 m/s | Subcritical
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy