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<br />e <br /> <br />. <br /> <br />~ <br /> <br />e <br /> <br />. <br /> <br />~. <br /><~ <br /> <br />e <br /> <br />For these inlets, it is necessary to compute the performance <br />of the face section (face control curve), the throat section (throat <br />control curve), and the barrel (outlet control curve), in order to <br />develop the culvert performance curve for a range of discharges. <br />The actual culvert performance curve, the hatched line of Figure 7, <br />represents the performance of the face, throat and barrel sections <br />in the ranges where their individual performance determines the <br />required headwater. In the lower discharge range, face control <br />governs; in the intermediate range, throat control governs; and <br />in the higher discharge range, outlet control governs. <br /> <br />Performance curves should always be developed for culverts <br />with side-tapered or slope-tapered inlets to insure that the designer <br />is aware of how the culvert will function over a range of discharges, <br />especially those exceeding the design discharge. It is important to <br />emphasize that outlet control may govern for the larger discharges, <br />and, as shown in Figure 7, the outlet control curve has a much <br />steeper sl' 'e - a more rapidly rising headwater requirement for <br />increasing discharges - than either the face or throat control <br />curve. It should be recognized that there are uncertainties in <br />the various methods of estimating flood peaks and that there is <br />a chance that the design frequency flood will be exceeded during <br />the life of the project. Culvert designs should be evaluated in <br />terms of the potential for damage to the highway and adjacent <br />property from floods greater than the design discharge. <br /> <br />As alternate culverts are possible using improved inlet design, <br />a performance curve should be plotted for each alternate considered. <br />The performance curve will provide a basis for selection of the <br />most appropriate design. <br /> <br />The advantages of various improved inlet designs are demonstrated <br />by the performance curves shown in Figure 8. These curves represent <br />the performance of a single 6 ft. by 6 ft. reinforced concrete box <br />culvert 200 ft. long, with a 4 ft. difference in elevation from the <br />inlet to the outlet. For a given headwater, the culvert can convey <br />a wide range of discharges, depending on the type of inlet used. <br /> <br />Curves 1 through 4 are inlet control curves for a 900 wingwall <br />with a square-edged inlet, a 1.5:1 bevel-edged inlet, a side-tapered <br />inlet, and a slope-tapered inlet with minimum FALL, respectively. <br />Curves 5 and 6 are outlet control curves. Curve 5 is for the square- <br />edged inlet and curve 6 is for the other three inlet types. As <br />previously discussed, curves 5 and 6 show that improved entrances can <br />increase the performance of a culvert operating in outlet control, but <br />the improvement is not as great as for culverts operating in inlet <br />control, as demonstrated by curves 1 through 4. <br /> <br />13-15 <br />