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<br />bed streams in Canada (Bray, 1979), and 21 perennial
<br />channels in New York State (Coon, 1995), However,
<br />only a few n,verification measurements bave been
<br />obtained for dryland (Graf, 1988) stream channels in
<br />arid and semiarid regions of the southwestern United
<br />States (Aldridge and Garrett, ] 973). Roughness
<br />coefficients for a variety of channel conditions are
<br />needed to substantiate the validity of guidelines
<br />currently used by hydrologists and engineers to assess
<br />flow resistance for dryland channels (Aldridge and
<br />Garrett, 1973; Thomsen and Hjalmarson, 1991),
<br />
<br />Purpose and Scope
<br />
<br />The purpose of this report is to present verified
<br />Manning's roughness-coefficient values for 37
<br />discharge measurements at 14 selected stream sites in
<br />Arizona (fig. 1). The information includes geometry
<br />and roughness characteristics of the 14 sites. The
<br />selected sites represent a wide range of channel
<br />conditions that include unstable alluvial channels,
<br />high'gradient boulder,strewn channels, and manmade
<br />f1ood,control channels, Of the sites presented, five
<br />were published in previous n,value assessment reports
<br />and are included herein to increase the range of stream
<br />types and transferability of the information (Barnes,
<br />1967; Aldridge and Garrett, 1973),
<br />The verification-measurement data are used to
<br />develop empirical relations between channel and
<br />hydraulic components and Manning's n, The relations
<br />presented include an equation for gravel,bed streams
<br />that relates Manning's n to relative roughness and an
<br />equation to determine the effect of vegetation on total
<br />roughness, These relations can be used to transfer
<br />results to similar dryland stream channels in Arizona
<br />and the southwestern United States,
<br />This study is the second phase of a two-phase
<br />investigation to assess roughness coefficients for
<br />stream channels in Arizona. Thomsen and Hjalmarson
<br />(1991) concluded the first phase by establishing
<br />guidelines for determining roughness coefficients and
<br />presented estimated n values for 16 stream channels in
<br />central Arizona. Some of the 14 verification sites used
<br />in this report are on the same streams as those
<br />described by Thomsen and Hjalmarson (1991) but at
<br />different locations. This report is intended to be used in
<br />conjunction with Thomsen and Hjalmarson's report to
<br />aid hydrologists and engineers in assessing and
<br />estimating n values for channels in arid to semiarid
<br />enviromnents.
<br />
<br />Acknowledgments
<br />
<br />B.N, Aldridge, R.D, Jarrett, K.M. Nolan, U.S.
<br />Geological Survey; Joseph Tram and Ted Lehman,
<br />Flood Control District of Maricopa County; and H.W
<br />Hjalmarson and B.W. Thomsen contributed
<br />significantly to the technical substance of this report,
<br />Personnel in the USGS, Tempe Office assisted with
<br />field and office work during the study.
<br />
<br />DESCRIPTION OF STUDY AREA
<br />
<br />The basin and range topography typical in most
<br />parts of Arizona is characterized by steep block-faulted
<br />mountains separated by gently sloping valleys,
<br />Dryland streams in the study area cover a wide variety
<br />of conditions ranging from unstable alluvial channels,
<br />generally stable channels of cobble to boulder,sized
<br />bed material, and extremely stable bedrock channels.
<br />Sand-dominated streambeds commonly are
<br />characterized by unstable boundary conditions, high
<br />sediment loads, and long periods of low or no flow
<br />punctuated by brief floods that increase discharge
<br />several orders of magnitude within minutes (Parker,
<br />1995). Although generally more stable than sand
<br />channels, some gravel,dominated channels in Arizona
<br />also are ephemeral and subject to flooding for brief
<br />periods. Flash flooding and the general instability of
<br />channel beds of natural channels in Arizona can
<br />complicate the task of obtaining accurate f1ow,rate and
<br />channel,geometry measurements that represent
<br />conditions during peak discharge, Many stream
<br />channels in urban areas are relatively stable, manmade,
<br />and composed of either soil cement, concrete, riprap,
<br />grouted and wire enclosed rock, grass, or a
<br />combination of these materials (NBS Lowry Engineers
<br />and Planners and McLaughlin Water Engineers, LId"
<br />1992).
<br />The type, distribution, and density of riparian
<br />vegetation can vary in the study area, Vegetation types
<br />found in and along many streams in central Arizona
<br />include saltcedar, willow, cottonwood, mesquite, palo
<br />verde, and many brush and grass species, The spatial
<br />distribution and density of riparian vegetation mainly
<br />depend on water availability, characteristics of flow,
<br />and water quality. The few perennial stream channels
<br />in the study area have vegetation growing parallel to
<br />base,ftow channels; whereas vegetation can be found
<br />growing randomly throughout the main channel of
<br />ephemeral streams. In addition, many effluent,
<br />
<br />2 Verification of Roughness Coefficients for Selected Natural and Constructed Stream Channels in Arizona
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