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<br />and the input requirements are presented in subsequent sections. <br />Section 3 represents the effective flow area just upstream from <br />the bridge. The reach lengths from section 2 to section 3 are generally <br />equal to the width of the bridge. The energy elevation computed by the <br />special bridge routine is applied to this section or, for the normal <br />bridge routine. a standard step solution from a section in the bridge <br />to this section provides the energy elevation. The energy loss computed <br />between sections 2 and 3 represents the loss through the bridge struc- <br />ture itself. <br />Section 4 is an upstream section where the flow lines are approxi- <br />mately parallel and the full cross section is effective. Because the <br />flow contraction can occur over a shorter distance than the flow expan- <br />sion. the reach length between sections 3 and 4 can be about one (1) <br />times the average bridge opening between the abutments (distance B-C in <br />Figure 6). However. this criterion for locating the upstream section <br />may result in too short a reach length for situations where the ratio <br />of the width of the bridge opening to the width of the flood plain is <br />small. An alternative criterion would be to locate the upstream cross <br />section a distance equal to the bridge contraction (distance AB or CD <br />in Figure 6). The program will compute the contraction portion of the <br />bridge loss over this reach length by the standard step calculations. <br /> <br />Effective Flow Area <br /> <br />A basic problem in setting up the bridge routines is the defini- <br /> <br />tion of effective flow area near the bridge structure. Referring to <br /> <br />21 <br />