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<br />MEASURY,MU,'T 0>' PEAK DISCHARGE AT CULVERTS BY INDIRECT METHODS 25
<br />
<br />)6 n_--T--' 1"
<br />I
<br />1.4 I
<br />
<br />1.2 I
<br />I
<br />1.0' .,., i I
<br />JaOB~f, '1
<br />~I
<br />0,6 '.r l
<br />
<br />0.4 ~--- +
<br />
<br />02
<br />
<br />
<br />I
<br />j
<br />. ,~
<br />
<br />I
<br />r-
<br />Pipe)
<br />
<br />o
<br />
<br />
<br />:: .=l-~~~ =~r.~~! ,. .
<br />1,0 .J .j .+-
<br />~Ia:: .~~ .. -1 . [U .
<br />
<br />-T
<br />I
<br />I
<br />I
<br />
<br />0,4
<br />
<br />0,2
<br />
<br />Figure 10.-Relatlon befween head Clnd critical depth in I
<br />pipe and pipe..atch culnrts.
<br />
<br />6. Compute Q from equation 5, Generally this
<br />computed Q will closely check the as-
<br />sumed Q from step 4, If it does not, repeat
<br />steps 2-6, using
<br />
<br />[hl+a,VI'/2g-h'I.,J for [hI! in step 3,
<br />
<br />Computation of type 1 flow with ponded
<br />headwater for rectangular hox culverts set
<br />flush in a vertical headwall is simplified by the
<br />fact that 0 is limited to values from 0.95 to
<br />0.98, The factor 0.66 in the formula d,=O,66
<br />(h,- z) can be refined t.o give a final result
<br />with one computation, The following table
<br />gives factors for various values of 0:
<br />
<br />c
<br />O,98__n '._"'''.''un.hh.'...._
<br />.97__.._......._....._.........._.
<br />.96_....__....._...____.........__
<br />.95,_....__.._..__........__..___.
<br />
<br />dc/aetor
<br />0.658
<br />.653
<br />.648
<br />.643
<br />
<br />Irregular sections
<br />Arches and all other culverts t.hat have ir.
<br />regularly shaped bottoms or tops (including
<br />rectangular shapes with fillets but excluding
<br />pipe.arches) are considered in this category, The
<br />same general procedure is used in compH ting
<br />discharge for irregular shapes, except that
<br />equation 1 or 2 must be used with equation 5
<br />to obtain the unique solution, For rectangular
<br />culverts with fIllets, a variation of equation 3,
<br />in the form of Q=5.67Tdm"', may be used,
<br />If n number of discharge computations will
<br />be made for n given irregularly shaped culvert,
<br />prepare graphs of area, wetted perimeter, and
<br />conveyance.
<br />
<br />09
<br />
<br />Type 2 flow
<br />
<br />If type 2 flow L, correctly assumed, the
<br />critical depth should occur at the outlet. The
<br />flow equations used for the computation of
<br />type I flow are also applicable here, with the
<br />further provision that the barrel friction loss
<br />must be aceounted for in the energy equation
<br />since the control section has shifted to the
<br />outlet,
<br />The discbarge and the critical depth must,
<br />be computed by solution of equation 7 and the
<br />llPplicable critical.depth equlltion I, 2, 3, or 4,
<br />The solution is tedious, because to compute
<br />the barrel friction loss, h"., , the height of thc
<br />water surface at the inlet must be estllblished,
<br />The complete equation for determining the
<br />depth d, at the inlet is
<br />
<br />Vl (I ) Vi O!IVI~
<br />h,-z=d,+ 2[j + 0,-1 2g +h".,- 2(/ ,
<br />(13)
<br />
<br />Even though entrance losses actually are a fun('.
<br />tion of the terminal velocity rather than o(
<br />the entrance velocity, the above equation may
<br />be simplified for most computat.ions by assum.
<br />ing V3= V2. Under average conditions this is It
<br />fair approximation. Also, IlS a general rule,
<br />where ponded conditions exist above the
<br />eulvert, or the approach velocity head and the
<br />approach friction loss are compensating, these
<br />factors may be neglected, Thus, the equation
<br />is simplified to
<br />V'
<br />h,-z=d,+2[jCi (14)
<br />
<br />,/,
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