FLOOD01296

• County: Statewide
• Title: Method to Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances of Streams in Central Arizona
• Date: 9/16/1998
• Prepared For: Central Arizona
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

quite substantial and can even exceed the estimated
value of nb' Main-channel vegetation for this site,
however, can be removed by flow magnitudes
slightly larger than the 2-year flow (Garrett and
Gellenbeck, 1991, p. 578, and table I), which
makes the selection of n4 for this site difficult. Use
of the relation presented in this report, however,
can dramatically decrease the uncertainty
associated with the selection of n4 for sites where
vegetation conditions may be altered by flow.

Table 9. Estimated preflow and postflow roughness
coefficients and computed changes in water-surface
elevations

Estimated
preflow
Site roughness
number coefficient

10

Ib
Ie
20

2b

3

4

5a

5b
6
7
8
9a
9b
ge
to
II

t2
t3
t4

15
16
17
18
190
19b

0.030
.035
.045
.048
.038
.030
.00t
.041
.038
.034
.040
.050
.040
.050
.035
.075
.034
.080
.040
.200
.040
.050
.046
.045
.035
.048
.035
.045

Estimated
poslflow
roughness
coefficient

'0.030
.032
.037
1048
1035
'.026
'.041
.026
M~-
.029
.033
.035
.025
.025
.032
.060
.029
.080
.035
.200
.030
.050
.041
.040
.033
.043
.025
.025

IVerified n values (Phillips and Ingersoll, 1998).

Change in
water-surface
elevation,
In feel

0.00
.34
2.17

o

.t2
.55

o
I.7t
.20
1.32
.45
1.00
2.5t
2.60
.22
1.73
1.22
o
.43
o
2.63
o
.36
.43
.04
1.23
1.13
1.18

Example case.--Consider a rectangular
channel where computation of the flood elevation
of the 100-year flow is needed. The channel reach
is uniform in shape and bed-material composition.
The bed material is dominated by cobbles with a
d50 of 0.30 ft. Brush and willow are present
throughout the study reach and distributed
randomly in the channel. The average height of the
brush is 3.0 ft, and the average height of the willow
is lOft. The percentage of cross-section area of
flow that is blocked by each vegetation type is
about 30 percent (table 10). These component
values are in the range of data used to develop the
relation in this report.

Table 10. Channel and vegetation components for the
example case

Vege-
tation
type

Brush
Willow

Median
diam-
eter
dso,
in feet

Aver-
age
vege-
tation
height,
in feet

Vege-
tation-
flexi-
bility
factor,
In toot
pounds

8.85
63.2

Amount
otllow
blocked
by vege-
tation,
in percent

Vege-
tation
distri-
bution

3
to

30
30

Random
Random

0.3
.3

Use of the relation (fig. 9) presented in this
report to estimate the effect of flow on vegetation
conditions requires quantification of the stream
power as well as the vegetation-susceptibility
index. Information obtained from a detailed
description of the channel and vegetation
conditions in the reach as well as a survey of the
study reach can be used to compute the
vegetation-susceptibility index. For the hydraulic
computations, the standard-step method
(Shearman, 1990) is used. This method requires
solution of the energy equation, continuity
equation, and Manning's equation. Components
required to compute stream power are obtained
from the standard-step computational results.

A stepwise procedure is suggested for using
the information presented in this report. For
the purposes of this example case, discharges that
correspond to the 2-, 10-, 50-, and 100-year
flows will be used. The magnitude of these
discharges were determined arbitrarily (table II).

,

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22 Method to Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances ot Streams in Central Arizona

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