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<br /> <br />7 <br /> <br />Figures 23, 24, and 25 show the improvement in capacity, <br />reduction in required headwater, and possible barrel size reduction, <br />respectively, which may be obtained through the use of improved <br />culvert inlets for specific site conditions. The example shown in <br />Figure 25 is solved in detail in HEC No. 13. <br /> <br />Multiple Culvert Barrels <br /> <br />The design procedures of BEC No. 13 are limited to two barrels <br />for the side- and slope-tapered designs. <br /> <br />Performance Curves <br /> <br /> , <br /> ~ <br />I <br />I <br />I <br />~ <br /> e <br /> I <br /> 1 <br /> I <br /> I <br /> I <br /> ~ <br /> I <br /> , <br /> ; <br /> <br />Performance curves are plots of required headwater depth versus <br />culvert discharge rate. If such curves are plotted for a given <br />improved inlet design configuration, a series of performance curves <br />will result, one for each control section (face, throat, outlet). <br />It will be found that for one range of discharges, the face will <br />govern, for another, the throat, and so on. The combination of the <br />governing sections of the individual section curves results in the <br />culvert performance curve. <br /> <br />The culvert performance curve is an excellent method of under- <br />standing how a culvert will function over a range of discharges. For <br />instance, it is clear that as outlet control is reached at higher <br />discharges, the required headwater rises rapidly, and may quickly <br />become critical if the design discharge is exceeded. An example <br />of a series of culvert performance curves for various inlets on a <br />5' x 5' RCB is shown in Figure 26. <br /> <br />~ <br />