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<br />
<br />36
<br />
<br />TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS
<br />
<br />Calculations showing the standard and rout-
<br />ing methods for various types of flow conditions
<br />are given under "Examples."
<br />
<br />Unusual conditions
<br />
<br />There will he, of course, occasional culvert-
<br />flow problems of unusual complexity, examples
<br />of which have not been covered in detail in this
<br />chapter. In most of the problems, results may
<br />be computed by reverting to the basic funda-
<br />mentals of the subject or to literature covering
<br />the speeial conditions. Undoubtedly there will
<br />be occasional problems where no reliable solu-
<br />tion is possible.
<br />One exception to the general flow classifica-
<br />tion is that type 1 flow can occur with h. slightly
<br />greater than h" or with h, greater than D. In
<br />this case, type 1 flow is proved by trial compu-
<br />tation of a backwater curve that extends from
<br />the known ta.ilwater surface to the upstream
<br />end of the culvert. If the computed water-
<br />surface elevation is found to be below d" the
<br />type 1 flow existed.
<br />Another rare type of part-full flow may
<br />sometimes occur where the headwater is below
<br />1.5/J at the entrance, but the tailwater is above
<br />the crown at the outlet.
<br />Type 3 flow can occur with any bottom slope.
<br />In a culvert with a steep slope it might occur
<br />with h.ID> 1.0.
<br />The fact that both (h,-z)ID and h,/D are
<br />greater than 1.0 does not positively indicate
<br />type 4 flow. (h,-z) mustexceedDby an amount
<br />equal to Vo'/2gC', where Vo is full culvert
<br />velocity, or the pipe will not be full at, the
<br />upstream end.
<br />A culvert has to be short (L= aboll t 10 diam-
<br />eters) to allow type 5 flow on a mild slope,
<br />In order for type 6 flow to occur on a steep
<br />slope, the culvert must have a large rID ratio,
<br />perhaps as great as 0,06.
<br />Examples of unusual conditions that might
<br />he experienced are (1) culverts of nonuniform
<br />barrel geometry, (2) submerged culverts with
<br />flared outlets, (3) culverts with drop inlets.
<br />(4) cases where flow is rl1pid in the approach
<br />,ection, and (5) culverts flowing full part way.
<br />EXl1ll1ples 2-4 cl1nnot be sl1tisfactorily com-
<br />puted,
<br />
<br />Other conditions which are not so \lIlu",al,
<br />but which must be given special consideration,
<br />are (1) partly buried culverts, (2) culverts in I1n
<br />aggraded section of a strel1m, (3) culverts pl1rtly
<br />plugged with debris, and (4) culverts partly
<br />plugged with ice.
<br />For a culvert flowing full part way, the
<br />length of part--full flow at the downstream end
<br />may be determined by writing two equations,
<br />each of which will determine the point at which
<br />full flow ceases, and solving them simultl1ne-
<br />ously, Elevation of the culvert crown at the
<br />outlet plus the rise in the culvert crown (usually
<br />the same as the bottom) in the unknown length
<br />must equal the water-surface elevation at the
<br />outlet pIllS the friction 105' in the unknown
<br />length plus the change in velocity head in the
<br />unknown length. The equation is
<br />
<br />D+Sox~=d3+Q!xIK"K3+ V,'/2g- Vo'/2g, (16)
<br />
<br />in which x is the length of part-full flow,
<br />
<br />Multiple culverts
<br />
<br />A multiple-culvert installation is one in
<br />which the culvert barrels are separated by
<br />more than 0.1 the width or diameter of either
<br />barrel. This should not be confused with a
<br />multibarrel culvert which generally consists of
<br />t,wo or more barrels, separated by thin webs, in
<br />a single structure.
<br />Two or more culverts may be used as the
<br />drainage structure. In many places the culverts
<br />will be (1) made of different materials" (2) laid
<br />at different slopes, I1nd (3) installed with dif-
<br />ferent in,,'ert elevations. A common occurrence
<br />is for different flow t.ypes to occur. For example,
<br />a small culvert ml1Y be flowing under high head
<br />while a larger one is flowing I1S type 3,
<br />At multiple culverts special consideration
<br />must be given to the computation of approach
<br />friction loss I1nd velocity head, Assume the
<br />total discharge and estimate the total culvert,
<br />area occupied by flow in order to compute
<br />these factors, If the assumed values I1re greatly
<br />different from the final result, recompnte.
<br />Compute the veloeity head for the entire
<br />npproach section I1nd I1dd to the water-surface
<br />elevation to determine the energy head Ilt sec-
<br />tion 1 applicable to each culvert.
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