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Manning’s n-values

Below is a table of n-values for a variety of open channel situations. Note that single values are offered for entire overbanks and channels with unique bottom and sides. Hopefully this will alleviate a need for using a multitude of n-values across a section.

MaterialManning's n
CULVERTS
Reinforced Concrete0.012
HDPE pipe0.012
Corrugated aluminum pipe
0.031
Corrugated steel pipe0.024
Polyvinyl chloride (PVC).011
Corrugated Poly0.024
MAIN CHANNELS
Gravel beds, Straight0.025
Gravel beds, large boulders0.040
Earth, straight, some grass0.026
Earth, winding, no vegetation0.030
Earth, winding, stones and weeds0.050
Sluggish reaches, weedy, deep pools0.070
Weedy reaches, heavy underbrush0.100
MOUNTAIN STREAMS
Steep banks, no vegetation, cobbles0.040
Steep banks, no vegetation, boulders0.050
OVERBANKS, FLOODPLAINS
Pasture, no brush, short grass0.030
Pasture, no brush, tall grass0.035
Cultivated, no crop0.030
Cultivated, mature row crop0.040
Heavy stand of timber, little undergrowth0.100
Light brush0.050
Dense brush0.100
EXCAVATED CHANNELS
Clean, recently completed 0.018
With short grass and weeds0.027
Earth bottom, rubble banks0.030
Stony bottom, weedy banks 0.035
Not maintained, weeds high as flow depth0.080
LINED or CONSTRUCTED CHANNELS
Concrete, trowel finish0.013
Concrete, unfinished0.017
Gravel bottom, riprap banks0.033
Source: Ven Te Chow (1959)