FLOOD03686

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains
• Date: 4/1/1984
• Prepared By: USDOT - Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

INTRODOCTION

There has been increasing interest and activity in flood-plain
management, flood-insurance studies, and in the design of bridges and
highways across flood plains. Hydraulic computations of flow for such
studies involve roughness coefficients, which represent the resistance to
flood flows in channels and flood plains.

The Manning's formula, frequently used as a part of an indirect
conputation of streamflow, is

v = 1.486 R2/3S 1/2
n e

(1)

in which: V = mean velocity of flow, in feet per second;
R = hydraulic radius, in feet;
Se = slope of energy grade line;
and n = Manning's roughness coefficient.

When a large number of calculations are necessary in using Manning's
formula, it is sometimes convenient to use a conveyance term, where
conveyance is defined as

K = 1.4861'R2/3
n

in which: K = conveyance of the channel, in cubic
A = cross-sectional area of channel, in
R = hydraulic radius, in feet;
and n = Manning's roughness coefficient.

(2)

feet per second;
square feet;

The term K is known as the conveyance of the channel section and it is a
measure of the carrying capacity of the channel section.

Suggested values for Manning's n, tabulated according to factors
that affect roughness, are found in references such as Chow (1959),
Henderson (1966), and Streeter (1971). Roughness characteristics of
natural channels are given by Barnes (1967). Barnes presents pictorial
illustrations of typical rivers and creeks with their respective n values.

It would be impractical to record all that is known about the selec-
tion of the Manning I s roughness coeff icient in this guide, but many
textbooks and technique manuals contain discussions of the factors
involved in the selection. Three, which could be considered as supple-
ments to this guide, are Barnes (1967), Chow (1959), and Ree (1954).

Although much research has been done to determine roughness coeffi-
cients for open-channel flow (Carter and others, 1963), less has been
done for densely vegetated flood plains, coefficients that are typically
very different from those for channels.

The objective of this guide is to develop procedures to aid engineers
in the selection of roughness coefficients for channels and flood plains,

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