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

procedures for estimating n values for vegetated
channels. In addition, many photographs are
presented for visual representation of the
substantial vegetation and channel changes that
may occur during flows in semiarid to arid
environments (See the section entitled "Basic
Data" at the end of the report). The photographs
also serve as a comparative aid for transferring the
results of this study to sites with similar channel
and vegetation characteristics.

This study was not an attempt to describe and
quantify all the complex hydrodynamic processes
that collectively affect vegetation and channel
conditions during flows. Data, coefficients, and the
semiempirical relation are presented in order to
address the potential for erroneous open-channel
conveyance calculations. Limerinos (1970) stated
that it is unlikely that the determination of n values
for natural channels will ever be an exact science,
and the determination of channel conveyance when
vegetation conditions are substantially altered by
flow will always be difficult. The primary focus of
this study was on vegetation growing in the main
channel of streams; however, the effect of flow on
overbank vegetation was evaluated at two of the
study sites. The effect of flow on streambank
vegetation was not assessed in this study.

Description of the Study Area

The basin and range topography of central
Arizona generally is characterized by steep
block-faulted mountains separated by gently
sloping valleys composed of material eroded from
the mountains. Elevation above sea level ranges
from about 5,000 ft in the mountains to about
1,000 ft at the valley floors. The composition and
stability of stream channels varies in the study
area. Natural channels range from unstable
predominantly sand channels to more stable
gravel-bed channels composed of cobble- to
boulder-sized bed material. Extremely stable
bedrock channels also are found in the study area,
but were not included in the investigation. Stream
channels in or near urban areas generally are
manmade and have soil cement, concrete, rip-rap,
grouted and wire-enclosed rock, grass, or a
combination of these materials (NBS Lowry

Engineers and Planners and McLaughlin Water
Engineers, 1992).
In the southern and western parts of central
Arizona, stream channels typically are composed
of sand-sized material, are ephemeral, and can
undergo substantial change in geometry during
flows. Vegetation often is found throughout the
main channels of these streams. The uncon-
solidated and highly erodible nature of sand-
dominated streams can result in root exposure and
vegetation removal during flows. For the sand
channels in this study, gradients range from 0.0036
to 0.0038 ftIft and median diameter of the bed
material ranges from about 0.6 to 1.0 mm.
Gravel-bed streams are predominant in the
northern and eastern parts of the study area and are
characterized by larger substrate material (median
diameter of bed material is greater than about
2 mm). Channel boundaries generally tend to be
stable during low to moderate flows in gravel-bed
channels; however, during large flows, large
amounts of bed material may be displaced. Several
of the gravel-bed channels used for this study are
in river reaches where flow is regulated by
reservoirs and diversions as well as in reaches that
have been channelized.
Mean annual precipitation in the study area
ranges from about 7 in. in the metropolitan area of
Phoenix to more than 25 in. in the nearby
mountains. Precipitation in central Arizona
generally occurs in summer (June through
October) and winter (December through March),
and rainfall for both seasons is about equal (Sabol
and others, 1990). In the summer, convective
thunderstorms produce precipitation that is intense
and short in duration and covers small areas.
Summer storms frequently produce flash floods
(Burkham, 1970). In the winter, regional-frontal
systems produce precipitation that is relatively low
in intensity, long in duration, and covers large
areas. These storms can result in substantial runoff
volumes and peak flows for large streams in the
study area. Dissipating tropical cyclones---a third
storm type-primarily occur in September and
October (Hirschboeck, 1985; Webb and
Betancourt, 1992). Although less frequent than the
other types of storms, dissipating tropical cyclones
have caused record floods that are regional in
extent (Aldridge and Eychaner, 1984; Roeske and
others, 1989).

4 _ID DII........ ~ of fIow.Induced VegelaIion Ch8nges on Channel Convet.- of Streams in CenIraI Arizona