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<br />I <br />i <br />! <br />Ie <br />I <br />I <br />I <br />I <br />I <br />I ~ <br />I <br />I <br />i <br />I <br /> <br />i <br />I <br />Ie <br />I <br /> <br />. <br /> <br />. <br /> <br />'e <br /> <br />IV. FLOW TRANSITIONS <br /> <br />A flow transition, as discussed here, is a change of open <br />channel flow cross section designed to be accomplished in <br />a short distance with a minimum amount of flow disturbance. <br />The types of transitions are shown in figure IV-l. Of these <br />the abrupt (headwall) and the straight line (win~~all) are <br />the most corrunon. <br /> <br />Specially desi~ned open channel flow inlet transitions <br />(contractions) are normally not required for highway culverts. <br />The economical culvert is designed to operate with an upstream <br />headwater pool which dissipates the channel approach velocity <br />and, therefore, negates the need for an approach flow transi- <br />tion. The side and slope tapered inlets (see reference II-A-3), <br />are designed as submerged transitions and do not fall within <br />the intended limits of open channel transitions discussed <br />in this chapter. <br /> <br />Special inlet transitions are useful when the conservation <br />of flow energy is essential because of allowable headwater <br />considerations such as an irrigation structure in subcritical <br />flow (section IV-A-2), or where it is desirable to maintain <br />a small cross section with supercritical flow in a steep <br />channel (section IV-B-l). <br /> <br />Outlet transitions (expansions) must be considered in the <br />design of all culverts, channel protection, and energy <br />dissipators. Of interest to the highway engineer are the <br />standard wingwall-apron combinations which are abrupt expan- <br />sions and expansions upstream of dissipator basins (chapter <br />VII) . <br /> <br />Transition designs fall into two general categories: Those <br />applicable to culverts in outlet control (subcritical flow) <br />or those applicable to culverts in inlet control (supercritical). <br />For design, use section IV-A for culverts in outlet control <br />and section IV-B for culverts in inlet control. <br /> <br />IV-A. CULVERTS IN OUTLET CONTROL <br /> <br />Two types of design problems apply to culverts in outlet <br />control: abrupt expansions and gradial transitions. <br /> <br />ABRUPT EXPANSION <br /> <br />As a jet of water, which is not laterally constrained, <br />leaves a culvert flowing in outlet control, the water sur- <br />face plunges or drops very rapidly (see figure IV-A-l). As <br /> <br />IV-A-l <br />