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<br />Photographs of the reaches were taken to reflect
<br />the major flow,retarding elements in each of the
<br />channels, For most n,verification measurements,
<br />photographs of the sites were taken dnring and after the
<br />flow.
<br />
<br />A particle,size distribution of the bed material
<br />was measured for most sites because energy losses can
<br />be influenced by the size of the bed material (Chow,
<br />1959). For alluvial channels, frequency distributions of
<br />bed-material size were determined by sieve analysis,
<br />For bed material too large to sieve, such as gravel,bed
<br />channels, frequency distributions were obtained by
<br />measuring the intermediate axis of particles selected at
<br />random from the study reach (Wolman, 1954; Benson
<br />and Dalrymple, 1967). These data were used to
<br />determine the median grain-size diameter (dSO) for
<br />most of the study sites.
<br />Suspended'sediment samples also were
<br />collected for many flows because large amounts of
<br />sediment can become entrained during flooding and
<br />may require substantial amounts of energy to transport
<br />the materiaL As indicated by several investigations
<br />(Chow, 1959; Costa, 1987; Jarrett, 1987; Glancy and
<br />Williams, 1994), suspended sediment can have
<br />discernible effects on the fluid characteristics by
<br />increasing its density and viscosity, which increase
<br />flow resistance and the verified n values. For most of
<br />the flows from which verification measurements were
<br />made, however, suspended sediment was considered
<br />wash load and probably had no effect on energy losses,
<br />
<br />COMPUTATION OF REACH PROPERTIES
<br />AND ROUGHNESS COEFFICIENTS
<br />
<br />The fundamental equations on which many
<br />open,channel hydraulic computations are based
<br />include the Manning's equation, the continuity
<br />equation, and the energy equation. The computer
<br />program NCALC, developed by Jarrett and Petsch
<br />(1985), is based primarily on these equations and was
<br />used to compute most of the values of Manning's n in
<br />this report. The equations are essentially identical for
<br />sites presented in this report for which verified n values
<br />were published previously, Manning's equation is
<br />defined as:
<br />
<br />v = L~86 R2/3 S}/2,
<br />
<br />(I)
<br />
<br />where
<br />
<br />v = mean velocity of flow, in feet per second;
<br />R hydraulic radius, in feet;
<br />Sf = energy gradient or friction slope, in feet
<br />per foot; and
<br />n = Manning's roughness coefficient
<br />
<br />The continuity equation is expressed as:
<br />
<br />Q
<br />
<br />AV,
<br />
<br />(2)
<br />
<br />where
<br />
<br />Q = discharge, in cubic feet per second;
<br />A = cross,sectional area of channel, in square
<br />feet; and
<br />V = mean velocity of flow, in feet per second,
<br />
<br />Substitution of equation 1 for V in equation 2 yields a
<br />variation of Manning's equation often used to compute
<br />discharge in open channels:
<br />
<br />Q = L~86 AR2I3( S/2).
<br />
<br />(3)
<br />
<br />The equation was developed for conditions of
<br />uniform flow in which the water-surface slope and
<br />energy gradient are parallel to the streambed, and the
<br />area, depth, and velocity are constant throughout the
<br />reach, Equation 3 is assumed for nonuniform reaches if
<br />the energy gradient is modified to reflect only the losses
<br />resulting from boundary friction (Barnes, 1967). The
<br />energy equation for a nonuniform stream channel reach
<br />between sections I and 2 (fig. 2) is:
<br />
<br />(h+h,Jj = (h+h,J2+ (hf)j_2+k(Ll.hv)1_2,(4)
<br />
<br />where
<br />
<br />h = elevation of the water surface at the
<br />respective section above a common
<br />datum, in feet;
<br />hv = velocity head at the respective section
<br />equals a V'/2g, in feet;
<br />hf = energy loss due to boundary friction in
<br />reach, in feet;
<br />
<br />Computation of Reach Properties and Roughness Coefficients 5
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