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<br />MEASUREMENT OF PEAK DISCHARGE AT CULVERTS BY INDIRECT METHODS 29
<br />
<br />
<br />065[ I I I I I I
<br />0701-- . ~ L -J
<br />~
<br />0,75- i _'IP - ---9=
<br />0.80 -
<br />"
<br />-0"1 !.
<br />..r:; 0.85
<br />
<br />0.90 ---
<br />
<br />D
<br />....b-O<
<br />
<br />I
<br />-- -, - I-----'~-'---
<br />-- -f-- i--- -
<br />
<br />i
<br />o 95 ~,j----
<br />, i I
<br />1.000 0.02 0.04
<br />
<br />
<br />Figure 14.-Relalion between head and depth of woter at
<br />inlet with critical depth of outlet for culnrts of rectangular I
<br />sedion.
<br />
<br />used to compensate for the friction loss
<br />in the culvert.
<br />3. With this first approximation of do. use
<br />equation 4 to obtain a trial value of Q.
<br />4. Compute <l/2gC'(hl-z)(D') and enter fig-
<br />ure 12 with (h,-z)/D to obtain d,/D.
<br />Compute d,.
<br />5. Having trial values of Q, d" and d, (which
<br />is d,), compute
<br />
<br />(a) OI.,V,'/2g
<br />
<br />(b) h,,_,
<br />
<br />(c) h,...,.
<br />
<br />6. Compute H=h,+OI.,V,'/2g-h,,_,-h"_3'
<br />7. Use value from step 6 as numerator in
<br />ratio H/D, Use this ratio as ordinll.te in
<br />figure 10 to read d,/ D.
<br />8. From step 7, compute d,.
<br />9, Using value of d, from step 8 in equation 4,
<br />compute Q.
<br />10. Compute Q'/2gC'(hl-z)(D'), using Q from
<br />step 9.
<br />11. Use the value from step 10 in figure 12 to
<br />obtain d,/D and compute d,.
<br />12, Compute the velocity head and friction
<br />with latest values of Q, do. and d,. Also
<br />compute A, and K,.
<br />13. Compute Q from equation 7. The com-
<br />puted Q should closely check the as-
<br />sumed Q of step 9.
<br />
<br />14. If the diseharge computed with equation 7
<br />is not within 1 percent of the discharge
<br />computed in step 9, the assumed value
<br />of d, is incorrect. The correct nIue of d,
<br />must be determined by successin "p_
<br />proximation, repeating the procedure
<br />outlined above.
<br />15. After the discharge and the elevation of
<br />the water surface at critical depth are
<br />established, the llSSumption of type 2
<br />flow is checked by comparing the eleva-
<br />tion of critical depth with the tailwater
<br />elevation.
<br />If h,>h., type 2 flow occurred, If h,<h" type 3
<br />flow occurred, and the discharge may be com-
<br />puted as outlined in the next section,
<br />
<br />Pipe.arch sections
<br />Type 2 flow in a riveted pipe-arch is com-
<br />puted in exactly the same manner as for "
<br />circular section by using the pipe-arch data in
<br />figures 10 and 13. These curves will give ll.p-
<br />proximate values applicable to multiplll.te pipe-
<br />arch computations.
<br />
<br />Rectangular sections
<br />Computation steps:
<br />1. Compute C.
<br />2. Determine d, factor from the table on page
<br />
<br />.,-
<br />~D,
<br />
<br />3. Assume d, (which is d,) =d, factor times
<br />(h,-z). Note.-The d, factor from page
<br />25 may be reduced 0.03 to approximate
<br />the friction loss in the culvert.
<br />4, Compute Q from equation 3.
<br />5, Compute Q'/2g(h,-z)'b'C' and enter figure
<br />14, Obtain a value of d,/(h,-z) and
<br />com pu te d"
<br />6. Having trial values of Q, d,. and d, (which
<br />is d,) com pu te
<br />
<br />(a) OI.,V,'/2g
<br />(b) h,,_,
<br />(e) h"_,,
<br />
<br />7, Compute H=h,+o"V,'j2g-h"_,-h,...,,
<br />8. Assume d,=d, factor times value from
<br />step 7.
<br />9, Using value of d, from step 8 in equation 3,
<br />compute Q.
<br />
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