FLOOD09807

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: A Uniform Technique for Determining Flood Flow Frequencies
• Date: 12/1/1967
• Prepared By: Water Resources Council - Hydrology Committee
• Floodplain - Doc Type: Educational/Technical/Reference Information

El:):uation 7 gives the n value in terms of the boundary roughness,
no; the hydraulic radius, R; the effective drag coefficient, C*; and
the vegetation characteristics, LAi/AL. 'Ihe vegetation density, Vegd'
in the cross section is represented by the expression

LA.
1
Veg =-
d AL

(S)

The boundary roughness, no' can be determined from the following
equation.

no = nb + nl + n2 + n3 + n4'

(9)

Elquation 9 contains all of the roughness factors of equation 6, which
describes the boundary roughness. The definition of the roughness factors
nb and nl through n3 are the same as those in equation 6. The n4'
factor is for vegetation, such as brush and grass, on the surface of the
flood plain that could not be measured directly in the Vegd term. The
value for the above roughness factors can be determined using table 3.
The n4' factor would be defined in the small to medium range in table 3
because the tree canopy would prohibit a dense undergrowth in a densely
wooded area.

The hydraulic radius, R, is equal to the cross-sectional area of
flow divided by the wetted perimeter; therefore, in a wide flood plain
the hydraulic radius would be equal to the depth of flow. An effective-
drag coefficient for densely wooded flood plains can be selected from
figure 3, a plot of effective-drag coefficient versus hydraulic radius
for densely wooded flood plains.

Indirect Technique.--A vegetation resistivity value, VegR' can be
determined through indirect methods (petryk and Bosmajian, 1975). When
flood data that include a measured discharge and depth of flow are
available, hydraulic analysis can be made and the roughness coefficients
can be determined for a flood plain. By rearranging equation 7 and using
the hydraulic radius and n value computed from the discharge measurement
and an assumed no' the vegetation resistivity for the reported flood can
be determined from the following equation:

(n2_n 2)2g
o
=
(1.49) 2 R4/3

C*LA.
1
Veg =
R AL

(10)

The value of VegR determined at this known depth of flow can be
used to estimate VegR for other depths by estimating the change in the
density of growth. This can be done from pictorial or physical descrip-
tions of the vegetation. By evaluating the change in VegR' an evalua-
tion of the n value as a function of flow depth can be determined.

Direct Technique.--Tree trunks are major contributors to the rough-
ness coefficient in a densely wooded flood plain. Where trees are the
major factor, the vegetation density can be easily determined by measuring
the number of trees and trunk size in a representative-sample area. The
n value as a function of height can be computed using equation 7.

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