FLOOD07520

• County: Statewide
• Basin: Statewide
• Title: Roughness Coefficients for Densely Vegetated Flood Plains
• Date: 1/1/1987
• Prepared By: USGS
• Floodplain - Doc Type: Floodplain Report/Masterplan

while a - NWey/B6X is a proper expression characterizing the roughness
concentration of the channel, its determination requires the prior knowledge
of depth. In some cases depth is not known and is a dependent variable that
must be determined. Without the knowledge of water depth, however, the
roughness field can be physically represented by some type of roughness
density, A, where:

and a

I..

NWe
B....X

(22)

2:l.
we

(23)

Roughness density is a parameter used for measuring the number of
roughness elements of a typical size per unit area of channel bottom.

F.~tirnatinq Pro~p-dure

Cowan (1956) developed an estimating procedure for the determination of
Manning's n for natural channels. This procedure was developed assuming that
realistic estimates of n could be made through the recognition of five primary
factors. These basic factors are: irregularity of the surface of the channel
sides and bottom; variations in size and shape of cross section; obstructions;
vegetation; and meandering of channel. In this procedure, the value of n may
be computed by the equation.

where:

n

(nb + nl + n2 + n3 + n4)m

(24)

nb

base value of n for a straight uniform,
smooth channel in natural materials;
value added for the effect of surface irregularities;
value added for variation in shape and size of the
channel cross section;
value added for obstructions;
value added for vegetation; and
correction factor for meandering of the channel.

nl
n2

n3
n4
m

The base n value will vary only with the materials forming the sides and
bottom of the channel. Cowan gives suggestions for the selection of base n
values for channels of different materials.

The selection of modifying values of n due to surface irregularity (nll is
based on the degree of roughness or irregularity of the channel sides and
bottom. Actual surface irregularity comparable to the best surface to be
expected of the natural materials involved would call for a modifying value of
zero. Higher degrees of irregularity would cause turbulence and would call
for increased modifying values. Cowan describes four degrees of irregularity.
In considering changes in size of cross sections for the selection of a
modifying n value (n2), greater turbulence is associated with alternating
large and small sections where changes are abrupt. variations of cross
sections should be compared to an average section. Cowan lists three
different degrees of change in size and shape of cross sections.

The selection of a modifying value for obstructions (n3) is based on the
presence and characteristics of obstructions such as debris deposits, stumps,
exposed roots, boulders, and fallen logs. In judging the relative effect of
obstructions, consider (a) the degree to which the obstructions occupy or
reduce the average cross-sectional area, (b) the character of obstructions

8