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

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 photographs and cross sections of
typical rivers and smaller streams with their respective n values.

For flood plains with relatively dense vegetation, Schneider and others
(1977) found that values of Manning's n ranging between 0.11 and 0.18 were
necessary to describe measured flood profiles using a step-backwater
procedure. Ree (1958) reported n values as high as 0.18 for flow through row-
planted vegetation, such as wheat and soybeans.

Ree and Crow (1977) conducted experiments over a 4-year period to
determine the roughness factors for earth channels having small slopes and
planted to wheat, cotton, sorghum, lespedeza, or grasses. The roughness-
factor data were intended for application to the design of diversion terraces.
The results of the experiments are presented according to the vegetation.
Photographs and brief descriptions of the vegetation and a tabulation of the
hydraulic elements are given. The reported n values can be applied directly
to a channel exactly like one of those tested, but this situation usually does
not happen. However, the n values reported can be used as a base to determine
the roughness values in flood plains with similar vegetation.

Several of the methods previously proposed for the determination of
roughness values in densely vegetated flood plains were examined. Robinson
and Albertson (1952), Sayre and Albertson (1961), Koloseus and Davidian
(1966), Herbich and Shulits (1964), Garton (1970), and Kowen and others (1969)
all made extensive experimental studies of the resistance of open-channel flow
over large, rigid roughness features. Unfortunately they were not able to
develop a general relationship that could be compared to an actual field
situation.

Other researchers, like Ramser (1929), Ree (1960), petryk and Bosmajian
(1975), Fenzel (1962), and Cowan (1956), have tried to develop methods of
determining roughness values in densely vegetated channels.

In this research study, four approaches to the evaluation of roughness
values were examined. They were a 'vegetation density" method developed by
petryk and Bosmajian (1975); a "roughness concentration" analysis reported by
Tseng and others (1974); a "regression analysis' developed by Garton (1970);
and an "estimating procedure" suggested by Cowan (1956). In addition to
presenting discussions of the above methods, this report also presents field
data related to roughness coefficients of wide, densely vegetated flood plains
used in the evaluation of the roughness-selection methods.

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