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Manning's n of vegetated surfaces ^{[1]}
n value range

Good condition turf grass

Other

0.002  0.010

Where the average depth of flow is at ≥ 2 times the height of the vegetation

Supple tree seedlings such as willow or cottonwood growing where the average depth of flow is at ≥ 3 times the height of the vegetation

0.010  0.025

Where the average depth of flow is between 1  2 times the height of the vegetation

Moderately dense weeds, or tree seedlings growing where the average depth of flow is from 2 3 times the height of the vegetation

0.025  0.050

Where the average depth of flow is about equal to the height of the vegetation

8 to 10yearold willow or cottonwood trees intergrown with some weeds and brush (none of the vegetation in foliage)

0.050  0.100

Where the average depth of flow is < 0.5 times height of the vegetation

bushy willow trees about 1 year old intergrown with weeds along side slopes (all vegetation in full foliage)

References
 ↑ Arcement, G.J., Schneider, R. Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains U.S.G.S. WATERSUPPLY PAPER 2339, 1989 http://eh.sdsu.edu/usgs_report_2339.pdf Accessed Dec 19 2017