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<br />GENERAL MODELING GUIDELINES <br /> <br />Considerations in modeling the geometry of a reach of river in the <br />vicinity of a bridge are essentially the same for both the normal and <br />special bridge routine. Suggested techniques are presented in this <br />section and are applied in subsequent examples on bridge coding. <br /> <br />Cross Section Locations <br /> <br />Figure 6 shows in plan view the basic configuration of cross sections <br />for computing losses through bridges. For ease of discussion, assume a <br />subcritica1 profile starting downstream from the bridge. <br />Section 1 is sufficiently downstream from the bridge that flow is <br />not affected by the bridge. The flow has fully expanded, and the basic <br />input problem is to determine how far downstream from the bridge the <br />cross section should be located. A rule of thumb is to locate the <br />downstream section about four (4) times the average length of the side <br />constriction caused by the bridge abutments. Therefore, section 1 <br />would be located downstream from the bridge 4 times the distance AB or <br />CD shown in Fig. 6. Because the constriction of flow may vary with the <br />discharge, the downstream reach length should represent the average con- <br />dition if a range of discharges are used in the model. <br />Locating cross section 1 based on a 4:1 expansion of flow down- <br />stream from the bridge may provide a reach length to cross section 2 <br />that is too long for a reasonable estimate of friction loss. If in- <br /> <br />18 <br />