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<br />e <br /> <br />e <br /> <br />e <br /> <br />MEASUREMENT OF PEAK DISCHARGE AT CULVERTS BY INDIRECT METHODS 41 <br /> <br />Bo. culverts <br />The discharge coefficient for box culverts set <br />flush in a vertical headwall is a function of the <br />J<'roude number. The Froude number for flow <br />types 1 and 2 is always 1.0, and the correspond- <br />ing discharge coefficient is 0.95. Determine the <br />discharge for type 3 flow from figure 23 after <br />computing the Froude number, VI"fgd, at th,~ <br />downstream end of the culvert. If necessary, <br />figure 23 may be extrapolated with reasonable <br />safety to Froude numbers of 0.1 to 0.2. <br />If the entrance to the box is rounded or <br />beveled, compute the discharge coefficient by <br />multiplying the 'coefficient for the square-ended <br />box by an adjustment factor, k, or kw. Deter- <br />mine these adjustment factors, applicable to <br />flow types 1, 2, and 3, from figure 21 or 22, <br />respectively, <br /> <br />Wingwall entrance <br /> <br />Pipe culverts set flush with vertical headwall <br />The addition of wingwalls to the entrance of <br />pipes set flush in a vertical headwall does not <br />affect the discharge coefficient, which can be <br />determined as shown previously under "Flush <br />Setting in Vertical Headwall," on page 38. <br /> <br />Box culverts <br />Compute the discharge coefficient for box <br />culverts with a wingwall entrance by first <br />selecting a coefficien t from figure 23 and then <br /> <br />1.00 <br /> <br />'-' <br /> <br />UJ <br />'" <br />a: <br />~ 0.90 <br />() <br />(Jl <br />o <br />"- <br />o <br />~ 0.80 <br />UJ <br />U <br />;;: <br />"- <br />UJ <br />o <br />(,) 0.70 <br />0,3 <br /> <br />multiplying this coefficient by an adjustment <br />factor k" which can be determined from figure <br />24 on the basis of an angle 9 of the wingwall. If <br />the angle of the wingwall is not the same on <br />each side, determine the value of C for each <br />side independently and average the results, <br />Where the web between culvert barrels is wide <br />enough (0.1 b or greater) to affect the entrance <br />geometry, treat it as a wingwall. Consider a <br />web corner of less than a right angle as a square <br />en trance. <br /> <br />/' <br />/ <br />~ <br /> <br />Projecting entrance <br />Corrugated~metal pipes and pipe.arches <br />Determine the discharge coefficient for pipes <br />and pipe-arches that extend beyond a headwall <br />or embankment by first computing a coefficient <br />as outlined for pipes set flush in a vertical head~ <br />wall and then multiplying the coefficient by an <br />adjustment factor, k.. The adjustment factor is <br />a function of L.ID where L. is the length by <br />which the culvert projects beyond the headwall <br />or embankment. The adjusted C to which kL is <br />applied must not be greater than 0.98, as this <br />is the limiting value of C, <br />An acceptable method for determining kL is <br />to measure L. at various points around the pipe <br />entrance, between the invert and headwater <br />elevation, then weight L. for each side of the <br />pipe on the basis of vertical distance and obtain <br />the average Lp before computing kL. The <br /> <br />Types 1 and 2 <br />, <br />F = 1.0. C = 0.95 <br /> <br /> <br />Box culvert having square entrance <br /> <br />1.0 <br /> <br />0,5 0,6 0.7 0,8 0.9 <br />FROUOE NUMBER ( F = vlVsdl AT SECTION 3 <br /> <br />0,4 <br /> <br />Figure 23.-Base coefficient of discharge for types 1 , 2, and 3 How in box culverts with square entrance mounted <br />flush in vertical headwall. <br />