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 | 0.013–0.016 |
| Concrete, unfinished (against forms) | 0.017 | 0.014–0.020 |
| Gunite (shotcrete), good section | 0.019 | 0.016–0.023 |
| Concrete pipe (RCP), smooth wall | 0.012 | 0.011–0.013 |
| PVC pipe, smooth | 0.011 | 0.010–0.013 |
| HDPE, dual-wall (smooth interior) | 0.012 | 0.010–0.013 |
| HDPE, single-wall (corrugated interior) | 0.024 | 0.020–0.027 |
| 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.
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More cheat sheets & worked examples
Doing pressurized pipe? Hazen-Williams C reference covers PVC, ductile iron, steel and aged values. For drainage, Rational Method C, the Tc methods comparison, and weir discharge coefficients are now live too. See it all together in this end-to-end driveway culvert worked example. For a full channel-and-network model with a sealed report, see Hydraulic Notebook 003 — why your Manning's n is probably wrong.