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<br />
<br />30
<br />
<br />TECHNIQUES OF WATER-RESOUIl{;ES INVESTIGATIONS
<br />
<br />10, Compute Q"/2g(h,-z)'b'G' and enter figure
<br />14. Obtain a value of ch/(hl- z) and com-
<br />pu te ch,
<br />11. Compute a,V,'/2g, h"_,, h,.., from latest
<br />values of Q, d, and do.
<br />12, Compute Q from equation 7. The computed
<br />, Q should closely check the assumed Q of
<br />step 9,
<br />13. If the discharge computed with equation 7
<br />is not equal to the discharge computed
<br />in step 9, the procedure given above must,
<br />be repeated until agreement within 1 per-
<br />cent is reached.
<br />14. The assumption of type 2 flow is checked
<br />as for circular sections.
<br />
<br />Irregular sections
<br />The same general rules apply to type 2 flow
<br />through irregular sections as apply to type 1
<br />flow, Special consideration should be given to
<br />roughness coefficients, slope of culvert, and
<br />change in shape of culvert cross section, How-
<br />ever, the general equations 1 and 2 for critical-
<br />depth flow may be used with equation 7 for
<br />computing discharge.
<br />Equation 13 should be used to determine
<br />depth of water ch at the inlet because many
<br />irregularly shaped culverts have rough barrels,
<br />In these computations, d, must be measured in
<br />a manner similar to the measurement of d"
<br />either as the depth to the average bottom or the
<br />depth to the lowest point in the section. The
<br />same criteria must be used in measuring hi
<br />and z,
<br />For certain conditions it may be necessary to
<br />use the routing method to compute discharge.
<br />
<br />Flow with backwater
<br />
<br />In flow with backwater, types 3 and 4, criti-
<br /><'Ill depth does not, occllr in the clllvert, and the
<br />lIp~t.rcam elevation of the water surfn,ce for n
<br />g-ivcn discharge i" a function of t,he S\lrrftec ele-
<br />vtlf.ion of the tailwo.ter.
<br />
<br />Type 3 flow
<br />
<br />Water-sllrface elevations h, and h, can nor-
<br />mally be established from highwater marks,
<br />and it is assnmed that h, equals h" The followc
<br />ing' procedure is recommended in computing
<br />discharge:
<br />
<br />1. Assume a discharge. A fair approximation is
<br />O.95A,v'2g(h,-h,),
<br />2, Determine the depth at the inlet ch by t,ria.!
<br />solution of equation 13 or directly from
<br />figures 12, 13, or 14 if the culvert has a
<br />circular, pipe-arch, or rectangular section.
<br />3. Compute the conveyance of the sections at
<br />the approach, the inlet, and the outlet.
<br />4. Compute the friction loss between the ap-
<br />proach and the inlet,
<br />
<br />h,,_,=L.(Q"/K,K,),
<br />
<br />and between the inlet and the outlet
<br />
<br />h,,_,=L(Q'/KaK,).
<br />
<br />5. Compute the approach velocity head,
<br />a,Vl'/2g.
<br />i 6. Compute the discharge with equation 8,
<br />7. If the discharge computed with equation 8
<br />is not equal to the assumed discharge, theu
<br />another discha.rge should be assumed and
<br />the procedure outlined above repeated,
<br />
<br />Type 4 lIow
<br />
<br />Generally for type 4 flow, ponded conditions
<br />exist, If water is not ponded, hI should be
<br />adjusted for velocity head in the approach sec-
<br />tion and friction loss between the approach
<br />section and the inlet,
<br />Discharge is computed directly from equation
<br />9, where Ao and Ro are the area and hydraulic
<br />radius, respectively, for a full culvert.
<br />A constant can be determined for any given
<br />culvert, so the discharge can be computed
<br />simply by multiplying the constant, which
<br />equals,
<br />
<br />~ 2g
<br />rAo 29C'n'L
<br />1+ Ro'"
<br />
<br />by t.he square root. of the difference between
<br />headwater and tailwater elevations, -,/h,-h"
<br />Note that. type 4 flow is independent of the
<br />culvert slope.
<br />
<br />: Flow under high head
<br />
<br />Type 5 or 6 flow will occur if the tailwat.er is
<br />below the crown at the outlet, and (h,- z) If) is
<br />
<br />-e
<br />
<br />-e
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<br />'>e
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