FLOOD04629

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Managing Floodplain Development in Approximate Zone A Areas
• Date: 4/1/1995
• Prepared By: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

Appendix 5

Manning' s 'In" Values

The value of "n" may be computed by

n = (110 + 0, + 02 + 0) + n4)mj

where: no = basic Un" value for a straight, uniform, smooth channel
n, = value added to correct for the effect of surface
irregularities
n, = value added for variation in the shape and size of the
channel cross section
n, = value added for obstructions
n. = value added for vegetation and flow conditions
m, = correction factor for meandering of the channel

Proper values of 110 to n. and m, may be selected from the following table
according to the given conditions:

Channel conditions Values
Earlh 0.020
Material Rock cut 0.025
involved n.
Fine gravel 0.024
Coarse gravel 0.028
Smooth 0.000
Degree of MinOT 0.005
irregularity "
Moderate 0.010
Severe 0.020
Gradual 0.000
Variations of
channel cross Alternating occasiona.lly n, 0.005
section
Alternating frequently 0.OHHl.015
Negligible 0.000
Relative Minor 0.010--0.015
effect of .,
obstructions Appreciable 0.020-0.030
Severe 0.040-0.060
Low 0.005-0.010
Medium 0.010-0.025
Vegetation n,
High 0.025-0.0.50
Very high 0.050--(). 100
Minor 1.000
Degree of Appreeiable m, 1.150
meanderic.g
Severe 1.300

1.

Chow, Yen Te, Ph.D.:
New York, 1959, pp.

REFERENCE
"Open-Channel Hydraulics, II McGraw-Hill Book Company,
106-114.

The computed "n" values should be compared with the typical Un" values from the
following pages, or with those in the U.s. Geological Survey Report (Reference
2) or the Federal Highway Administration Report (Reference 3).

A5-1