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<br />the need for channel protection. A change in size of culvert does not <br />change outlet velocities appreciably in most cases. <br /> <br />e <br /> <br />Outlet velocities for culverts flowing wi th inlet control may be <br />approximated by computing the mean velocity for the culvert cross sec- <br />tion using Manning's equation <br /> <br />v = 1.49 R2/3 <br />n <br /> <br />S 1/2 <br />o <br /> <br />Since the depth of flow is not kno~~ the use of tables or charts <br /> <br />is recommended in solving this eqUatioJt. The outlet velocity as <br />computed by this method will usually be high because the normal depth, <br />assumed in using Manning's eqpation, is seldom reached in the rela- <br />ti vely short length of the average culvert. Also, the shape of the <br />outlet channel, including aprons and wingwalls, have much to do with <br />changing the velocity occurring at the end of the culvert barrel. <br />Tailwater is not considered effective in reducing outlet velocities <br />for most inlet control conditions. <br /> <br />In outlet control, the average outlet velocity will be the dis- <br />charge divided by the cross-sectional area of flow at the outlet. <br />This flow area can be either that corresponding to critical depth, <br />tailwater depth (if below the crown of the culvert) or the full cross <br />section of the culvert barrel. <br /> <br />Performance Curves <br /> <br />e <br /> <br />Although the procedure given in this circular is primarly for <br />use in selecting a size of culvert to pass a given discharge at a <br />given headwater, a better understanding of culvert operation can be <br />gained by plotting performance curves through some range of discharges <br />and barrel slopes. Such curves can also be used to compare the per- <br />formance of different sizes and types of culverts. The construction <br />of such curves is described in Appendix A, page 5-45. <br /> <br />Inlets and Culvert Capacity <br /> <br />Inlet shape, edge geometry and skew of the entrance affects cul- <br />vert capacity. . Both the shape and edge geometry have been investiga- <br />ted by recent research but the effect of skew for various flow condi- . <br />tions has not been examined. Results show that the inlet edge geometry <br />is particularly important to culvert performance in inlet-control flow. <br />A comparison of several types of commonly used inlets can be made by <br />referring to charts 2 and 5. The type of inlet has some effect on <br />capacity in outlet control but generally the edge geometry is less <br />important than in inlet control. (See reference 6.) <br /> <br />. <br /> <br />g; See references page 5-14. <br /> <br />5-12 <br /> <br />e <br />