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

indicate that the actual flow forces are more
directly controlled by the magnitude of hydraulic
conditions, such as channel slope and flow depth,
and probably not the absolute discharge.
The absolute magnitude of peak discharge is
not used directly in the derivation ofthe relation in
this report; however, discharge data are presented
for comparative purposes (table I). Discharge data
were' obtained by direct current-meter measure-
ment (Rantz and others, 1982), at a nearby USGS
streamflow-gaging station that had a well defined
stage-discharge relation, or by indirect peak-flow
measurement (Dalrymple and Benson, 1967).

Stream Power and Shear Stress

A fundamental assumption of this
investigation is that a critical stream power exists
for specific vegetation conditions, and that
vegetation will begin to bend when this critical
stream power value is exceeded. Stream power is a
measure of energy transfer in an open channel and,
according to Simons and Richardson (1966),
stream power is defined as:

SP = 62.4RS V,
w

where

SP = stream power, in foot-pounds per
second per foot squared,
62.4 = specific weight of water, in pounds per
cubic foot,
R = hydraulic radius, in feet,
Sw = slope of the water-surface profile, in
feet per foot, and
V = average velocity, in feet per second.

Boundary shear stress (shear stress) is the force
exerted on the bed by the moving water and is the
hydraulic component most related to substrate or
bedload movement (Vanoni, 1975; Carson and
Griffiths, 1985). Shear stress is used instead of
stream power to describe the potential for a flow to
degrade the channel substrate. Consequently, shear
stress is considered the predominant mechanism
that potentially can expose and weaken root
systems and remove vegetation. Shear stress, to' is

the force per unit area exerted on the bed by the
moving water and is defined (Davidian and Cahal,
1963) as:

to = 62.4RSo'

(2)

where

62.4 = specific weight of water, in pounds per
cubic foot;
R = hydraulic radius, in feet; and
So = bed slope, in feet per foot.

,

Vegetation Data

(I)

Adequately describing all the physical
components that collectively characterize
vegetation conditions in streams in Arizona is
complex and difficult. In the study by Phillips and
Hjalmarson (1994), the average height of
vegetation measured before flow was the only
component used to characterize vegetation
conditions. Their simplified approach required
various assumptions that ignored several important
factors such as the variable flexural strength of
different vegetation species.

In the course of this investigation, it was
determined that additional physical components
could more effectively model the effects of
flow-induced changes to vegetation. The physical
components include four vegetation attributes------{I)
the flexural strength of specific types and sizes of
vegetation, (2) the percent of flow blocked by
vegetation, (3) the distribution of the vegetation,
and (4) the depth of flow relative to the vegetation
height (table 2).

Vegetation-Susceptibility Index

The vegetation attributes that were determined
for each site (table 2) are incorporated into a single
parameter called the vegetation-susceptibility
index. The vegetation-susceptibility index is
defined by

Kv = Vj/exCblockingCdistCdepth' (3)

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