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<br />MEASUREMEl'."T OF PEAK DISCHARGE AT CULVERTS BY INDIRECT METHODS 21
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<br />AREA. IN SQUARE FEET
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<br />Figure 6.-Depth-orea curves for multipla'e: pipe-arc:he:s.
<br />
<br />In type 1 flow critical depth normally is
<br />assumed to occur at the inlet or upstream end
<br />of the culvert. However, a limited number of
<br />current-meter measurements have showh that,
<br />for mitered pipes on steep slopes, this assump-
<br />tion will show less water than is actually flowing
<br />through the culvert. For mitered pipes, assume
<br />that critical depth occurs at the point where
<br />the headwater elevation intersects tbe mitered
<br />entrance. For large culverts the difference in
<br />elevation between the inverts at each end of the
<br />miter may be several tenths of a foot. Computa-
<br />tions of type 1 flow for these two invert eleva-
<br />tions will show considerably different discharges.
<br />Normally, a critical depth is assumed which
<br />fixes the value of the remaining unknown terms.
<br />A good first approximation is d,=0.66(k,-z).
<br />Successive approximations of d, will quickly
<br />converge toward the solution.
<br />To check the assumption of type 1 flow, the
<br />critical slope for the culvert is computed as
<br />8,= (Q/K,)'. Here 8, is the critical slope and K,
<br />
<br />is the conveyance of the section of flow at
<br />critical depth at the inlet, If 8,<8, and k.<k"
<br />the assumption of type 1 flow has been proved
<br />and the correct discharge has been comput.cd.
<br />If k.>k" type 3 flow has been identified and the
<br />discharge may be computed as outlined on page
<br />30, If k.<k, and 8,>8" type 2 flow is
<br />assumed and the analysis is cont.inued as out-
<br />lined on pages 25-30,
<br />For circular, pipe-arch, and rectangular
<br />sections, t.he identification of flow type is
<br />simplified by figures 9, 10, and 11. The pro-
<br />cedure is outlined below,
<br />
<br />Circular seclions
<br />
<br />1. Compute (k,-z)jD and (8.D'I2)jn' and plot
<br />the point on figure 9. A point to the right
<br />of the curve indicates type 1 flow and "
<br />point to the left, type 2 flow,
<br />2, Determine d, from figure 10.
<br />3, Compare (d,+z) with k.,
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