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<br />termediate cross sections are required, the 4:1 expansion rate could <br /> <br /> <br />be used to locate the lateral extent of intermediate sections. The <br /> <br />user should carefully review the program output to determine if an <br /> <br /> <br />adequate number of cross sections are used. A change in energy slope <br /> <br /> <br />by a factor of two between the two sections and a relatively long <br /> <br /> <br />reach would indicate a need for intermediate sections. <br /> <br /> <br />Section 2 is a river cross section immediately (i.e., within a <br /> <br /> <br />foot or two) downstream from the bridge. The section should repre~ <br /> <br /> <br />sent the effective* flow area just outside the bridge and its loca- <br /> <br /> <br />tion could be considered as the downstream face of the bridge. It is <br /> <br /> <br />important to work with effective flow area because it is assumed in <br /> <br /> <br />the application of the energy equation that the mean downstream velocity <br /> <br /> <br />for each subsection can be determined from Manning's equation. The <br /> <br /> <br />method used to define the effective flow area at this cross section is <br /> <br /> <br />discussed under Effective Flow Area. The standard step solution at sec- <br /> <br /> <br />tion 2 would include determination of the expansion loss from section 2 <br /> <br />to section 1. <br /> <br />The bridge loss occuring from section 2 to section 3 is determined <br />by either the special bridge routine with the S8 card or by standard <br />step calculations through one or two sections that define the bridge <br />opening (normal bridge routine). The selection of the bridge routine <br /> <br />*Effective flow is that portion of flow where the main velocity is nor- <br />mal to the cross section and in the downstream direction. <br /> <br />20 <br />