The following Manning’s n Roughness Coefficient table is from the United States Department of Transportation – Federal Highway Administration: Hydraulic Engineering website.


Table B.2. Manning's Roughness Coefficients for Various Boundaries.

Rigid Boundary Channels

Manning's n

Very smooth concrete and planed timber

0.011

Smooth concrete

0.012

Ordinary concrete lining

0.013

Wood

0.014

Vitrified clay

0.015

Shot concrete, untroweled, and earth channels in best condition

0.017

Straight unlined earth canals in good condition

0.020

Mountain streams with rocky beds

0.040 -0.050

MINOR STREAMS (top width at flood stage < 30 m)

Streams on Plain

1. Clean, straight, full stage, no rifts or deep pools

0.025–0.033

2. Same as above, but more stones and weeds

0.030–0.040

3. Clean, winding, some pools and shoals

0.033–0.045

4. Same as above, but some weeds and stones

0.035–0.050

5. Same as above, lower stages, more ineffective slopes and sections

0.040–0.055

6. Same as 4, but more stones

0.045–0.060

7. Sluggish reaches, weedy, deep pools

0.050–0.080

8. Very weedy reaches, deep pools, or floodways with heavy stand of timber and underbrush

0.075–0.150

Mountain Streams, no Vegetation in Channel, Banks Usually Steep, Trees and Brush Along Banks Submerged at High Stages

1. Bottom: gavels, cobbles and few boulders

0.030–0.050

2. Bottom: cobbles with large boulders

0.040–0.070

Floodplains

Pasture, No Brush

1. Short Grass

0.025–0.035

2. High Grass

0.030–0.050

Cultivated Areas

1. No Crop

0.020–0.040

2. Mature Row Crops

0.025–0.045

3. Mature Field Crops

0.030–0.050

Brush

1. Scattered brush, heavy weeds

0.035–0.070

2. Light brush and trees in winter

0.035–0.060

3. Light brush and trees in summer

0.040–0.080

4. Medium to dense brush in winter

0.045–0.110

5. Medium to dense brush in summer

0.070–0.160

Trees

1. Dense willows, summer, straight

0.110–0.200

2. Cleared land with tree stumps, no sprouts

0.030–0.050

3. Same as above, but with heavy growth of sprouts

0.050–0.080

4. Heavy stand of timber, a few down trees, little undergrowth, flood stage below branches

0.080–0.120

5. Same as above, but with flood stage reaching branches

0.100–0.160

MAJOR STREAMS (Topwidth at flood stage > 30 m)

The n value is less than that for minor streams of similar description, because banks offer less effective resistance.

Regular section with no boulders or brush

0.025–0.060

Irregular and rough section

0.035–0.100

Alluvial Sand-bed Channels (no vegetation)

Tranquil flow, Fr < 1

Plane bed

0.014–0.020

Ripples

0.018–0.030

Dunes

0.020–0.040

Washed out dunes or transition

0.014–0.025

Plane bed

0.010–0.013

Rapid Flow, Fr > 1

Standing waves

0.010–0.015

Antidunes

0.012–0.020

Overland Flow and Sheet Flow

Smooth asphalt

0.011

Smooth concrete

0.012

Cement rubble surface

0.024

Natural range

0.13

Dense grass

0.24

Bermuda grass

0.41

Light underbrush

0.40

Heavy underbrush

0.80