Manning's n — Roughness Coefficient Reference

Quick reference for open-channel and partial-flow conduit calculations. All values from Chow (1959) Table 5-6 unless noted. Print and pin above your desk.

Lined / Constructed Channels & Pipes

Material	n	Range
Smooth concrete, trowel-finished	0.013	0.011–0.015
Concrete, float-finished	0.015	0.013–0.017
Concrete, unfinished (gunite)	0.019	0.016–0.023
PVC pipe, smooth	0.011	0.010–0.013
HDPE pipe, smooth	0.012	0.010–0.013
Cast iron, coated	0.013	0.011–0.014
Corrugated metal pipe (2⅔ × ½ in)	0.024	0.022–0.027
Corrugated metal pipe (3 × 1 in)	0.027	0.025–0.030
Asphalt, smooth	0.013	0.012–0.015
Asphalt, rough	0.016	0.015–0.020
Riveted steel	0.016	0.013–0.017
Vitrified clay (sewer)	0.013	0.011–0.017
Brick, glazed	0.013	0.011–0.015
Brick, in cement mortar	0.015	0.012–0.018

Earth & Excavated Channels

Description	n	Range
Earth, clean, recently completed, straight	0.018	0.016–0.020
Earth, clean, after weathering, straight	0.022	0.018–0.025
Earth, gravelly, with some weeds, straight	0.025	0.022–0.030
Earth, weedy with stones	0.035	0.025–0.040
Earth, dense weeds, deep flow	0.080	0.050–0.120
Earth, rock cuts, smooth, uniform	0.035	0.025–0.040
Earth, rock cuts, jagged, irregular	0.040	0.035–0.050
Riprap, well-graded	0.040	0.030–0.045

Natural Streams (Bank-full, Top of Bank)

Description	n	Range
Clean, straight, full stage, no rifts or pools	0.030	0.025–0.033
Same as above, but more stones & weeds	0.035	0.030–0.040
Clean, winding, some pools and shoals	0.040	0.033–0.045
Same as above, with weedy & stony banks	0.045	0.035–0.050
Sluggish reach, weedy, deep pools	0.070	0.050–0.080
Mountain streams, gravel/cobbles, few boulders	0.040	0.030–0.050
Mountain streams, cobbles & boulders	0.050	0.040–0.070

Floodplain (Overbank Flow)

Description	n	Range
Pasture, short grass	0.030	0.025–0.035
Pasture, high grass	0.035	0.030–0.050
Cultivated, no crop	0.030	0.020–0.040
Cultivated, mature row crops	0.035	0.025–0.045
Light brush and trees, winter	0.050	0.035–0.060
Light brush and trees, summer	0.060	0.040–0.080
Heavy brush, summer	0.100	0.070–0.160
Trees, dense willows, straight	0.150	0.110–0.200
Trees, heavy stand, flooded with branches submerged	0.120	0.080–0.200

Sources: Chow, V.T. (1959). Open-Channel Hydraulics, Table 5-6. USGS Water-Supply Paper 2339 (Arcement & Schneider, 1989). USACE EM 1110-2-1601 for floodplain values.