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<br />MEASUREMENT OF PEAK DISCHARGE AT CULVERTS BY INDIRECT METHODS 37
<br />
<br />Add the conveyances of the flow sections of
<br />all culverts to determine the total conveyance
<br />at section 2. Then use this with the total con-
<br />veyance at section 1 to compute the approach
<br />friction loss. Subtract this friction loss from the
<br />energy head at section 1 to obtain the energy
<br />head at section 2. This energy head is applicable
<br />to each of the culverts.
<br />The percent of channel contraction is an-
<br />other factor in which the entire approach area
<br />and the combined total of culvert flow areas are
<br />used together. Because the total area of flow at
<br />the terminal sections of multiple culverts is
<br />used, it is possible that one or more of the
<br />areas used are located at section 2, and the
<br />others are at section 3.
<br />
<br />Coefficients of Discharge
<br />
<br />Coefficients of discharge, 0, for flow types
<br />1-6 were defined by laboratory study and are
<br />applicable to both the standard formula and
<br />routing methods of computation of discharge.
<br />The coefficients vary from 0.39 to 0.98, and
<br />they have been found to be a function of the
<br />degree of channel contraction and the geometry
<br />of the cuI vert en trance.
<br />For certain entrance geometries the, discharge
<br />coefficient is obtained by multiplying a base
<br />coefficient by an adjustment factor such as k,
<br />or k.. If this procedure results in a discharge
<br />coefficient greater than 0,98, a coefficient of
<br />0.98 should be used as a limiting value in com-
<br />puting the discharge through the culvert,
<br />The coefficients are applicable to both single-
<br />barrel and multibarrel culvert installations, If
<br />the width of the web between barrels in a multi-
<br />barrel installation is less than 0.1 of the width
<br />of a single barrel, the web should be disregarded,
<br />in determining the effect of the entrance geom-
<br />etry. Bevels are considered as such only within
<br />a range of 0.1 of the diameter, depth, or width
<br />of a culvert barrel. Larger sizes are not consid-
<br />ered as bevels but as wingwalls.
<br />Laboratory tests also indicate that the dis-
<br />charge coefficient does not vary with the
<br />proximity of the culvert floor to the ground
<br />level at the entrance. Thus in types 1, 2, and 3
<br />flow, the geometry of the sides determines the
<br />value of 0; similarly, in types 4, 5, and 6 flow
<br />
<br />the value of 0 varies with the geometry of the
<br />top and sides. If the degree of rounding or
<br />beveling is not the same on both sides, or on the
<br />sides and the top, the effect of r or w mnst be
<br />obtained by averaging the coefficients deter-
<br />mined for the sides, or for the sides and top.
<br />according to the type of flow. One exception is
<br />noted: if the vertical sides of the culvert are
<br />rounded or beveled and the top entrance is
<br />square, multiply the average coefficient (deter-
<br />mined by the procedure just described) by
<br />0.90 for type 5 flow and by 0.95 for types 4 and
<br />6 flow, using the coefficient for the square
<br />entrance as the lower limiting value.
<br />The discharge coefficient does not vary with
<br />culvert skew,
<br />The radius of rounding or degree of bevel of
<br />corrugated pipes should be measured in the
<br />field. These are critical dimensions that shonld
<br />not be chosen from a handbook and accepted
<br />blindly.
<br />The ratio of channel contraction, m, is asso-
<br />ciated with horizontal contraction typical of
<br />flow types 1, 2, and 3. The effect of side .(.1\t,rac-
<br />tion becomes negligible for flow t~'pes 4, 5, and 6
<br />in which vertical contraction is more important.
<br />Therefore, no adjustment for contraction ratios
<br />I less than 0,80 is warranted for flow types 4, 5,
<br />or 6,
<br />In listing the discharge coefficients, it is con-
<br />venient to divide the six flow types into three
<br />groups, each group having a discharge equation
<br />of the same general form. Thus, flow types 1, 2,
<br />and 3 form one group; types 4 and 6 another;
<br />and type 5 a third, The coefficient C is descrip-
<br />tive of the live-stream contraction at the inlet
<br />and its subsequent expansion in the barrel of
<br />the culvert. Hence, base coefficients for types I,
<br />2, and 3 flow should be identical for identical
<br />geometries, as should coefficients for types 4
<br />and 6,
<br />In a systematic presentation of the coeffi-
<br />cients, the entrance geometries have been
<br />classified in four general categories: (1) flush
<br />setting in vertical headwall, (2) wingwall
<br />entrance, (3) projecting entrance, and (4)
<br />mitered pipe set flush with sloping embankment,
<br />The four classes have been subdivided as
<br />necessary, but they all are common to the three
<br />flow-type groups.
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