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<br />bridge below the water surface is subtracted from the total area, and <br />the wetted perimeter is increased where the water is in contact with <br />the bridge structure. The bridge deck is described either by entering <br />the constant elevations of the top of roadway and low chord as varia- <br />bles ELTRD and ELLC respectively on card X2, Dr by specifying a table <br />of roadway stations and elevations, and corresponding low chord eleva- <br />tions, on BT cards. When only ELLC and ELTRD are used, these elevations <br />are extended horizontally until they intersect the ground line defined <br />on GR cards. Pier losses are accounted for by the loss of area and the <br />increased wetted perimeter of the piers as described in terms of cross <br />section coordinates, usually on the GR card. <br /> <br />Special Bridge Routine <br /> <br />The special bridge routine computes losses through the structure <br />for either low flow, pressure flow, weir flow, or for a combination of <br />these. The profile through the bridge is calculated using hydraulic <br />formulas to determine the change in energy and water surface elevation <br />through the bridge. <br />Low Flow. The procedure used for low flow calculations in the spe- <br />cial bridge routine depends on whether the bridge has piers. Without <br />piers, the low flow solution is accomplished by standard step calcula- <br />tions in the normal bridge routine. The transfer to the normal bridge <br />routine is necessary because the equations used in the special bridge <br />routine for low flow are based on the obstruction width due to the piers. <br /> <br />3 <br />