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<br />between the cross sections to give the energy loss caused by the transition. <br />Where the change in river cross section is small, coefficients CEHV and CCHV <br />are on the order of 0.3 and 0.1, respectively. When the change in cross <br />sections .is abrupt such as at bridges, CEHV and CCHV may be as high as 1.0 <br />and 0.6. These values may be changed at any cross section by inserting a <br />new NC card, however, these new values will be used until changed again by <br />another NC card. <br /> <br />p. Bridge Losses. <br /> <br />(1) Energy losses caused by structures s~ch.-,,_s,.~ridges and culverts <br />a);:e . comp.u,~~<i .in_two...llill:~.s. .F.ir'!.h. the lQ.!l,ses_ dU~L t(L~..P.'!lls!gn .and contraction <br />of the cross section on the upstream and downstream sides of the structure <br />are computed (see exhibits 3 and 4 for required cross sections). Variables <br />CEHV and CCHV discussed in the previous section are used to specify the <br />expansion and contraction coefficients. Secondlv. the 10"" thrnngh.the <br />struct\!r\!.itselL.i,,--~QmPutedJ).Y~.!theL.t\1."'.llgrmal bridge .I.Qudne or the <br />S1l.~cialJH"j_Qg~ routine. <br /> <br />(2) The normal routine handles the cross section at the bridge <br />just as it would any river cross section with the exception that the area <br />of the bridge below the water surface is subtracted from the total area and <br />the wetted perimeter is increased where the water surface elevation exceeds <br />the low chord. Th\! bridge deck is described by entering the elevation of <br />the toP. "f.. roadway .and 10w:.ch()rd.<i.!l_v.a...riaHes _J>LTRD, all<L!lLLC. respectively <br />on card X2 or by specifying a table of roadway elevation and station and <br />co!:respondingl()w.,c,horde.1eyatiJIDa (HT ~"rd,,). \fuen only ELLC and ELTRD are <br />used, these elevations are extended horizontally until they ,intersect the <br />g:round line., Pier losses areaCCo1J!l.t~.<i..f"or.J>.ythe increased wetted perimeter <br />of the piers as described on card GR. The .!!Q!Jl!f!l,rQ\ltIn.e is particularly <br />applicable for !>ri<iM!l,.w:!.th()ytl'ie!.s, bridges under high submergence, and for <br />low flow through circular and arch culverts. '-Whenever flol. cro.ss'es-critical <br />depth in a structure, the 'special bridge-routine should be used. The normal <br />bridge is automatically used by the computer, even though data was prepared <br />for the special bridge routine, for bridges without piers and under low flow <br />control. <br /> <br />(3) The special bridge routine computes losses through the structure <br />for low flow, weir flow and pressure flow or for any combination of these. <br />The type of flow is determined by a series of comparisons as shown on exhibit 1 <br />and as described below. First, the energy grade line elevations are computed <br />assuming alternately low flow and pressure flow control. The higher energy <br />grade line elevation determines the appropriate type of flow. If pressure <br />flow appears to control and the energy grade line is above the minimum top <br />of roadway elevation, then a combination of pressure flow and weir flow exists. <br />If the energy gradient is below the minimum top of roadway then pressure flow <br />alone controls. If low flow appears to control, and the corresponding energy <br /> <br />8 <br />