FLOOD06972

• County: Statewide
• Basin: Statewide
• Title: Method of Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances of Streams in Central Arizona
• Date: 1/1/1998
• Prepared For: Central Arizona
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

the end of the report). Because of logistical con-
straints, such as protruding branches, however, the
dynamometer was not always placed at this posi-
tion on the vegetation. The relation of bending
moment to height of vegetation was determined for
each vegetation type (fig. 3). Additionally, equa-
tions were developed by regression techniques of
the bending moment with height for each of the
four vegetation types (table 3).

Table 3. Regression equations relating bending
moment to vegetation height for saltcedar, willow,
mesquite, and palo verde

[8M. bending moment in foot pounds; H, height of vegetation in feet]

Vegetation type
Saltcedar ...............
Willow,.......,......."
Mesquite...............
Palo verde.............

Equation
BM = 100.102H + 0.880
BM = 100.122H + 0.581
BM = 100. 1 24H +0.935
BM= 100.17]H+0.848

Coefficient of
determination
(r')

0,87

,98

,88

.86

The bending moment (also referred to as
flexural strength or stiffness) of the vegetation at
varying heights can be estimated from the
equations in table 3, For example, a flexural

2,500
2,000

1,000
rn 700
0
z 500
:::J
0 400
a.
~ 300
0
0 200
LI.
;!;
,..:
z 100
w
::; 70
0
::; 50
(')
z 40
is 30
z
w
'" 20
10
7
6
2

strength of 63.2 fi-Ib is estimated for a ] O-foot-tall
willow; whereas, a flexural strength of 36] fi-Ib is
estimated for a 10-foot-tall palo verde. The
assumption, therefore, is that a lone palo verde in
midchannel, is thought to be substantially more
likely to resist bending when it is subjected to a
similar magnitude of stream power and similar
degree of submergence as a lone willow tree in
midchannel.

Although uncertainties are associated with this
simplified approach, data collected during this
investigation seem to support these assumptions,
For example, figure 4 shows a lone willow about
IS fi tall that was laid over during a flow of
6,590 fi3/S; whereas, figure 5 shows a lone
16-foot-tall palo verde that remained erect
throughout a flow of 9,760 fi3/S. Duration of flow
for the events was about equal. The magnitude of
stream power that affected the palo verde was
20.2 fi-Ib/s/ft'; whereas, the magnitude of stream
power to which the willow was subjected is equal
to 12.9 fi-lb/s/fi' (table I). These data indicate that
the large flexural strength of the palo verde enabled
it to resist the stream power that was substantially
larger than the computed stream power that altered
or laid over a willow with similar dimensions. The

"
,,"
"
J-/
,," .
"
"
."
" .
".
"
"
,," .
"
,,".

...

...

.

.

.

... PALO VERDE

. MESQUITE
. SAL TCEDAR
... WILLOW

4

8

20

10

12

6

16

14

18

VEGETATION HEIGHT, IN FEET

Figure 3. Bending-moment values for four types of vegetation in central Arizona.

10 Method to Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances of Streams in Central Arlzone