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<br />The type, distribution, and density of
<br />vegetation in streams in the study area are highly
<br />variable. Vegetation types found in and along many
<br />streams in central Arizona include saltcedar,
<br />willow, cottonwood, mesquite, palo verde, and a
<br />variety of brush and grass species. The spatial
<br />distribution and density of vegetation near streams
<br />may depend on the availability, quality, and flow
<br />characteristics of the water. For example, in the
<br />few perennial streams, vegetation grows parallel to
<br />base-flow channels; whereas, in ephemeral
<br />channels, vegetation often grows randomly
<br />throughout the main channel.
<br />
<br />Previous Investigations
<br />
<br />Previous investigations have attempted to
<br />quantify the retarding effects of certain types of
<br />vegetation on flow. Most of these studies, however;
<br />were limited primarily to unchanging
<br />main-channel vegetation (Chow, 1959; Aldridge
<br />and Garrett, 1973; Thomsen and Hjalmarson,
<br />1991), unchanging flood-plain vegetation (Petryk
<br />and Bosmajian, 1975; Arcement and Schneider,
<br />1989; Fathi-Maghadam and Kouwen, 1997), and
<br />unchanging streambank vegetation (Jarrett, 1985;
<br />Masterman and Thome, 1992; Coon, 1995). Some
<br />investigators have attempted to assess the effect of
<br />flow on grass growing in manmade channels and
<br />developed relations useful for estimation of the
<br />corresponding change in flow resistance associated
<br />with laid-over grasses (Ree and Palmer, 1949;
<br />Phelps, 1970; and Kouwen and Unny, 1973). Apart
<br />from a preliminary relation between flow and the
<br />flow-induced changes in vegetation conditions
<br />developed by Phillips and Hjalmarson (1994), and
<br />several other investigations that only marginally
<br />address this problem (Li and Shen, 1973;
<br />Burkham, 1976; and Ohlemutz, 1992), no
<br />comprehensive data set or methodologies exist for
<br />estimating the changes in channel conveyance that
<br />result from flow-induced changes in the condition
<br />of brush and small trees that commonly grow
<br />throughout the main channel of natural and man-
<br />made streams in semiarid to arid environments.
<br />Phillips and Hjalmarson (1994) developed a
<br />simple empirical relation that describes the effects
<br />of flow on vegetation and is based on vegetation
<br />and flow data collected before, during, and after 13
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<br />flows at II sites in central Arizona. Average
<br />vegetation height was determined for preflow
<br />conditions, and the flow was described in terms of
<br />stream power. Stream power, which is a measure of
<br />energy transfer, was computed at cross sections on
<br />the basis of postflow-channel conditions. Phillips'
<br />and Hjalmarson (1994, fig. I) indicate that the
<br />effect of the flow (no effect, little effect, laid-over,
<br />or removed) on vegetation is related to stream
<br />power and vegetation height. Although the relation
<br />between stream power, vegetation height, and the
<br />effect of flow on the vegetation correlated well,
<br />there was a clear need for more detailed research
<br />and study of the physical characteristics of
<br />vegetation and the variable effects of flow on these
<br />conditions. The information and relation presented
<br />in this report are an extension of the work by
<br />Phillips and Hjalmarson (1994).
<br />
<br />DATA COLLECTION AND ANALYSIS
<br />
<br />Hydraulic data, vegetation data, and substrate
<br />data were collected for this investigation.
<br />Hydraulic data included measured or estimated
<br />discharge, water-surface profiles, and channel
<br />cross-section characteristics. Vegetation condi-
<br />tions, such as average height and density, were
<br />measured or estimated, described, and photo-
<br />graphed before and after peak flows; thus, flow-
<br />induced changes to vegetation were documented
<br />shortly after flow subsided. Substrate data, which
<br />consist of the median diameter of bed material,
<br />were obtained either by visual examination or by
<br />measuring the intermediate axis of a representative
<br />population of particles.
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<br />Hydraulic Data
<br />
<br />Although the determination of discharge is
<br />essential for many hydraulic studies, the actual
<br />power and force of the flow are better indicators of
<br />flow-induced changes in vegetation and substrate
<br />conditions. For example, Costa (1987) suggests
<br />that some floods in small basins with low unit
<br />discharges can produce values of shear stress and
<br />stream power per unit area of channel bed that are
<br />substantially greater than those for floods in large
<br />rivers. Additionally, Costa's (1987) findings
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<br />Data Collection and Analysis 5
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