FLOOD08739

• County: Statewide
• Community: Statewide
• Basin: Statewide
• Title: Determination of Roughness Coefficients for Streams in Colorado
• Date: 1/1/1985
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Educational/Technical/Reference Information

In
need to
reach.
area I
trees)

wide, flat sections of the overbanks, subdivisions based on roughness
be made where the roughness changes, if they occur throughout the
The left overbank shown in figure 4 needs to be subdivided into sub-
(pasture) having an n value of 0.05 and subarea 2 (dense growth of
having an n value of 0.12.

ROUGHNESS COEFFICIENTS

An understanding of the basic factors that affect roughness coefficients
in channels and overbanks are needed for the selection of Manning's n. In
general, the roughness coefficient is a measure of the effect that bed mate-
rial, channel geometry, vegetation type and density, and other factors have on
flow resistance. Consequently, any factors that increase flow resistance
increase the roughness coefficient, and factors that decrease flow resistance
decrease the roughness coefficient.

The roughness can vary greatly with changes in stage and discharge and
the seasonal changes in vegetation. The variation with stage and discharge is
particularly important in higher-gradient mountain streams. The general
approach to evaluate roughness outlined in this section consists of evaluating
the channel roughness (base n value), modifying the base value by adjustment
factors, and evaluating the roughness of the overbanks. The discussion is
further divided into natural, agricultural, and urban conditions.

General Approach

The general approach for estimating n values consists of the selection of
a base roughness value for a straight, uniform, smooth channel in the materi-
als involved, then, through a consideration of various factors, modifying
values are added to the base n value to obtain the n value for the channel
under consideration (Chow, 1959; Cowan, 1956).

The n value for a given depth of flow may be calculated by the general
equation

n = (nO+nt+n2+n3+n4) m

(4)

where
no is the base value for a straight uniform channel;
nt is the additive value due to the effect of cross-section irregularity;
nz is the additive value due to variations of the channel;
n3 is the additive value due to the relative effect of obstructions;
n4 is the additive value due to the type and density of vegetation; and
m represents a value for the degree of meandering used to multiply the
sum of the previous values.

Although the base and modifying values are interrelated to some extent,
it is important that each factor be examined and considered independently and
the effects not duplicated. Cowan (1956) indicates this method has not been
verified on channels where hydraulic radius exceeds 15 ft.

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