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<br />MEASUREMENT OF PEAK DISCHARGE AT CULVERTS BY INDIRECT ~ETHODS 31
<br />
<br />equal to or greater than \.5. Approach velocity
<br />head and friction loss fire included in the com-
<br />putations when appropriate,
<br />The type of flow is dependent largely on the
<br />amount of beveling or rounding of the culvert
<br />entrance. For this and other reasons previously
<br />mentioned, the criteria for identifying type 5
<br />or type 6 flow must be considered approximate,
<br />Generally there is a transition from low-head to
<br />high-head flow that must be considered, This
<br />item is discussed under "Ratings with Transi-
<br />tion between Flow Types," on pa!l;e 47.
<br />
<br />Concrete culverts
<br />
<br />Figure 15 may be used to classify type 5 or 6
<br />flow in concrete culvert barrels by the procedure
<br />outlined below.
<br />\. Compute the ratios LID, riD, or wiD, and So'
<br />2, Select the curve of figure 15 corresponding
<br />to riD or wiD for the culvert. Sketch in an
<br />interpolated curve for the given riD or
<br />wiD, if necessary.
<br />3. Plot the point defined by So and LID for the
<br />cui vert.
<br />4. If the point lies to the right of the curve
<br />selected in step 2, the flow was type 6;
<br />if the point lies to the left, the flow was
<br />type 5.
<br />The use of the figure 15 is restricted to square,
<br />rounded, or beveled entrances, either with or
<br />without wingwalls. Wingwalls do not affect the
<br />flow classification, as the rounding effect they
<br />provide is offset by a tendency to produce
<br />vortexes that supply air to the culvert entrance,
<br />For culverts with wingwalls, use the geometry
<br />of only the top side of the entrance in computing
<br />the effective radius of rounding, r, or the effec-
<br />tive bevel, w, in using figure 15.
<br />
<br />Corrugated-pipe culverts
<br />
<br />Figure 16 may be used to classify type 5 or 6
<br />flow in rough pipes, both circular and pipe-arch
<br />sections, mounted flush with a vertical headwall,
<br />either with or without wingwalls, as outlined
<br />below. Figure 16A should be used in classifying
<br />the flow if the pipe projects from a headwall or
<br />emhankment.
<br />\. Determine the ratio riD for the pipe.
<br />2. From figure 16, select the graph correspond-
<br />ing to the value of riD for the culvert.
<br />
<br />3, Compute the ratio 29n'(h,-z)IR0413 I\nd
<br />select the corresponding curve on I h~
<br />graph selected in step 2, Sketch in 1\11
<br />interpolated curve for the computed rutin,
<br />if necessary.
<br />4. Plot the point dcfined by So I\nd LID for the
<br />culvert.
<br />5, If the point plots to the right of the cnrve
<br />selected in step 3, the flow was type 6;
<br />if the point plots to the left of the curve,
<br />the flow was type 5,
<br />
<br />Type 5 flow
<br />
<br />In type 5 flow the culvert entrance is sub-
<br />merged, and the tail water is below the crown
<br />at the outlet. The flow is rapid near the entranre
<br />to the culvert. The discharge may be computed
<br />directly from equation 10,
<br />
<br />Type 6 flow
<br />
<br />In type 6 flow the water surface is assumed to
<br />bc at the top of the culvert at the outlet, but
<br />the culvert is not submerged and free outfall
<br />prevails. The following procedure may be used
<br />to compute discharge:
<br />1. Compute the ratio h,fD, Select the discharge
<br />coefficient, G, applicable to the cui vert
<br />geometry,
<br />2, From figure 17, determine the value of QIAo{n
<br />corresponding to 29n'LIRo"3= \.
<br />3. Compute the ratio 29n'LIRo'13 for the culvert
<br />under study.
<br />4. From figure 17, using the computed ratio
<br />29n'LJRo4!f and the coefficient G, find the
<br />correction factor, k,.
<br />5, Multiply the value of QIA.{ij from step 2 by
<br />the value of kJ from step 4, thus deter-
<br />mining an adjusted ratio QIA.,,'D,
<br />6. Determine the value of Q from the adjusted
<br />ratio.
<br />
<br />Routing method
<br />
<br />The previously described computation pro-
<br />cedures cover the standard conditions found in
<br />a great majority of culvert installations, The
<br />methods for computation of types 1-4 flow are
<br />inappropriate for some nonstandard conditions,
<br />Some examples of such conditions are:
<br />\. Approach velocity head or friction loss of
<br />appreciable amoun t,
<br />
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