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<br />specified by the user. This is done by reading in effective-area eleva- <br />tions (ELLEA and ELREA) in fields 8 and 9 of the X3 card. If these el- <br />evations are not read in, elevations specified on GR cards for the left <br />and right bank stations will be used. <br />The effective area option applies to the left and right bank sta- <br />tions; therefore, those stations should coincide with the abutments of <br />the bridge. For cross sections 2 and 3, the left and right bank stations <br />should line up with the bridge abutments. An X3 card would be used with <br />these sections to call for the effective area option and to designate <br />effective-area elevations for the left and right bank stations. The <br />given elevations would correspond to an elevation where weir flow would <br />just start over the bridge. For the downstream section, the threshold <br />water surface elevation for weir flow is not usually known on the in- <br />itial run, so an estimate must be made. An elevation anywhere between <br />the low chord and top-of-road elevation could be used; so an average <br />of the two elevations might be a reasonable estimate. <br />Using the effective area option to define the effective flow area <br />allows the entire overbank to become effective as soon as the effective- <br />area elevations are exceeded. The assumption is that under weir flow <br />conditions, the water can generally flow across the whole bridge length <br />and the entire overbank in the vicinity of the bridge would be effective- <br />ly carrying flow up to and over the bridge. If it is more reasonable to <br />assume only part of the overbank is effective for carrying flow when the <br />bridge is under weir flow, then the cross section should be redefined <br /> <br />24 <br />