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<br />dominated streams in the study area (the lower
<br />Hassayampa River, for example) contain elevated
<br />nutrient levels resulting in increased vegetation
<br />growth. Vegetation conditions in dryland streams in
<br />Arizona can be the primary factor in determining total
<br />resistance to flow.
<br />Mean annual precipitation in the study area
<br />ranges from about 7 in, near Phoenix to more than 30
<br />in, in the adjacent mountain regions, Precipitation in
<br />Arizona occurs mainly during two seasons, summer
<br />(June through October) and winter (December through
<br />March), and rainfall is about equal in each period
<br />(Sabol and others, 1990), Summer precipitation
<br />normally is produced by convective thunderstorms.
<br />These storms are characterized by rainfall of high
<br />intensity and short duration, which usually cover small
<br />areas and may result in flash floods (Burkham, 1970),
<br />Winter precipitation normally is produced by regional
<br />frontal systems that are characterized by low-intensity
<br />rainfall of long duration that covers a large areal extent
<br />These storms often result in substantial runoff volumes
<br />and create the potential for major floods. Dissipating
<br />tropical cyclones, a third storm type in Arizona, occur
<br />primarily in September and October (Hirschboeck,
<br />1985; Webb and Betancourt, 1992), These storms can
<br />cause record floods of regional extent (Aldridge and
<br />Eychaner, 1984; Roeske and others, 1989), Dissipating
<br />tropical cyclones and winter,frontal storms can
<br />generate runoff volumes large enough to require
<br />discharge from reservoirs in Arizona, Twelve of the 37
<br />verification measurements presented in this report were
<br />made during regulated release of waters from upstream
<br />reservoirs.
<br />
<br />FIELD MEASUREMENTS FOR VERIFIED
<br />ROUGHNESS COEFFICIENTS
<br />
<br />Site Selection
<br />
<br />The verification-site locations were selected to
<br />meet, as closely as possible, criteria presented by
<br />Dalrymple and Benson (1967) for indirect
<br />measurement of discharge by the slope,area method,
<br />and guidelines presented by Jarrett and Petsch (1985)
<br />and Coon (1995) for required hydraulic conditions and
<br />reach characteristics for accurate n-verification
<br />measurements, The site conditions that relate to the
<br />specified criteria are summarized as follows,
<br />
<br />I, Discharge mainly stayed within the channel banks,
<br />and extensive flow in flood plains or overbank
<br />areas did not exist Also, the cross-sectional area
<br />of the channels was fully effective and carrying
<br />water in accordance with the computed convey-
<br />ance.
<br />2. For verification measurements in which a well-
<br />defined stage,discharge rating was used to obtain
<br />discharge, good water,surface elevation indica,
<br />tors or high, water marks were available to define
<br />the water,surface profile,
<br />3. Channels generally were straight and uniform for
<br />some distance upstream and downstream from the
<br />reach, Severe channel bends and channel expan'
<br />sions were avoided.
<br />4. Reaches were long enough to develop adequate fall
<br />according to at least one of the following criteria.
<br />a, Length of the reach was equal to or greater than
<br />75 times the mean depth.
<br />b. Fall in the reach was equal to or greater than
<br />the velocity head.
<br />c, Fall in the reach was equal to or greater than
<br />0.50 fL
<br />
<br />Data Collection
<br />
<br />Discharge used for each verification
<br />measurement was obtained by the current,meter
<br />method or determined from a well,defined stage,
<br />discharge relation (Rantz and others, 1982), For
<br />verification measurements in which discharge was
<br />measured with a current meter, the water surface was
<br />marked with flagging at the time the discharge
<br />measurement was taken, Adjustments were made to the
<br />elevation of the markers if the stage or water surface
<br />was rising or falling during the current,meter
<br />measurement (Coon, 1995). Crest'stage gages were
<br />installed at several sites to aid in obtaining accurate
<br />peak water-surface elevations at cross sections,
<br />A transit, stadia survey was conducted for each
<br />reach either at the time of the current,meter
<br />measurement or soon after flow subsided to obtain
<br />accurate water' surface elevations and channel,
<br />geometry data, Standard surveying techniques were
<br />employed throughout the study and are described in
<br />detail by Benson and Dalrymple (1967). The
<br />information obtained from the surveys was used to plot
<br />the channel-geometry data and to determine the
<br />required channel'geometry components for
<br />computation of Manning's roughness coefficient.
<br />
<br />4 Verification of Roughness Coefficients for Selected Natural and Constructed Stream Channels in Arizona
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