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<br />unless the water surface elevation is above the top of one or both of the <br />levees, in which case flow area or areas outside the levee(s) will be in- <br />cluded. If this option is employed and the water slJrface elevation is <br />close to the top of a levee, it may not be possible to balance the assumed <br />and computed water surface elevations due to the changing assumptions of <br />flow area when just above and below the levee top. When this condition <br />occurs a note will be printed that states that the assumed and computed <br />water surface elevations for the cross section cannot be balanced. A water <br />surface elevation equal to the elevation which came closest to bal~ncing <br />will be adopted. It is then up to the program user to determine the ap- <br />propriateness of the assumed water surface elevation and start the computa+ <br />tion over again at that cross section if required. <br /> <br />It is important for the user to study carefully the flow pattern of <br />the river where levees exist. If, for example, a levee were open at both <br />ends and flow passed behind the levee without overtopping it, IEARA equals <br />zero or blank should be used. Also, assumptions regarding effective flow <br />areas may change with changes in flow magnitude. Where cross section eleva- <br />tions outside the levee are considerably lower than the channel bottom, it <br />may be necessary to set IEARA equal to ten to confine the flow to the chan- <br />nel. For further information on this option see Appendix IV, paragraph 8, <br />Effective Area Option. The effective flow capabilities of the bridge and <br />encroachment routines are described in the following paragraphs and in <br />Appendices II and IV, respectively. <br /> <br />5. BRIDGE LOSSES <br /> <br />Energy losses caused by structures such as bridges and culverts are <br />computed in two parts. First, the losses due to expansion and contraction <br />of the cross section on the upstream and downstream sides of the structure <br />are computed in the standard step calculations. Secondly, the loss through <br />the structure itself is computed by either the normal bridge or the special <br />bridge methods. <br /> <br />The, normal bridge method handles the cross section at the bridge just <br />as it would any river cross section with the exception that the area of the <br />bridge below the water surface is subtracted from the total area and the <br />wetted perimeter is increased where the water surface elevation exceeds the <br />low chord. The normal bridge method is particularly applicable for bridges <br />without piers, bridges under high submergence, and for low flow through <br />circular and arch culverts. Whenever flow crosses critical depth in a <br />structure, the special bridge method should be used. The normal bridge <br />method is automatically used by the computer, even though data was prepared <br />for the special bridge method, for bridges without piers and under low flow <br />control. <br /> <br />The special bridge method can be used for any bridge, but should be <br />used for bridges with piers where low flow controls, for pressure flow, and <br />whenever flow passes through critical depth when going through the structure. <br />The special bridge method computes losses through the structure for low flow, <br />weir flow and pressure flow or for any combination of these. Refer to Appendix <br />IV for a detailed explanation of HEC-2 bridge capabilities. <br /> <br />18 <br />