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<br />b. Subcritical or Supercritical Flow. The computation begins at a control <br />section (location of known water surface elevation) in the river channel and <br />proceeds upstream for subcritical flow or downstream for supercritical flow, <br />The direction of flow is specified by the user on card Jl (first job card) <br />by setting variable IDIR (direction) equal to 1 for super critical flow or <br />o (blank) for subcritical flow, In cases where flow passes from subcritical <br />to supercritical or vice versa, during computations, it is necessary to <br />compute the entire profile twice assuming alternately subcritical and super- <br />critical flow, From the above results the most likely water surface profile <br />csn be determined <br /> <br />c. Startin~ Elevation, The water surface elevation for the beginning <br />cross section may be specified in one of three.ways: (1) as critical depth, <br />(2) as a known elevation, (3) by the slope area method. By setting the <br />variable STRT on card Jl equal to -1, critical depth will be computed and <br />used as the starting water surface elevation, With variable STRT left blank <br />the starting water surface elevation is specified by variable WSEL on card Jl. <br />For beginning by the slope area method STRT is set equal to the estimated <br />slope of the energy grade line (must be a positive value) and WSEL is used <br />as the initial estimate of the water surface elevation, The flows computed <br />for the fixed slope and estimated depth are compared with the starting flow <br />and the initial depth is adjusted until the computed flow is within 1% of <br />the starting flow. The last assumption of initial water surface elevation <br />thus determined is then used as the starting water surface elevation for <br />water surface profile computations, <br /> <br />d. Flow, <br /> <br />(1) The river flow may be specified and altered in several ways, <br />The starting flow is normally specified as variable Q on card Jl when only <br />one flow is anticipated. If it is desired to use different flows for subse- <br />quent jobs using the same cross sections, variable INQ and card QT (discharge <br />table) may be used, The flows are input in fields 2 thru 10 on the first <br />QT card and 11 thru 20 on the second, Variable INQ for each job should equal <br />the field number of the flow on card QT to be used for that job, Use of <br />variable INQ and card QT overrides any flow specified for variable Q on <br />card Jl, However, variable Q on card JI will be used until a QT card is <br />encountered, <br /> <br />(2) Where it is desired to change the flow beginning at a certain <br />cross section such as a confluence with another river or stream, variable QNEW <br />on card X2 (second card describing specified cross section) may be used, QNEW <br />permanently changes the flow at any cross section for which this variable is <br />specified, <br /> <br />(3) Where it is necessary to increase or decrease flows specified on <br />cards QT and X2 by a factor, variable FQ on card JI is available, When a value <br /> <br />2 <br />