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j. River Cross Sections. (1) Cross sections are required at representative locations throughout the river reach. These are locations where changes occur in slope, cross sectional area, or channel roughness; locations where levees begin or end; and at bridges. In general, for rivers of flat slope and fairly uniform section (drop of three or four feet per mile) cross sections should be taken at least every mile. For steeper slopes and very irregular cross sections four or five cross sections per mile may be necessary. Where an abrupt change occurs in the cross section, several cross sections should be used to describe the change regardless of the distance. Every effort should be made to obtain cross sections that accurately represent the river geometry. (2) Each cross section in the reach is identified and described using cards Xl (first card for a cross section) and GR. Variable SECNO on card Xl is the cross section number which may correspond to stationing along the channel, mile points, or any fictitious numbering system, since it is only used to identify output and is not used in the computations. Each point in the cross section is given a station number corresponding to the horizontal distance from the first point on the left. The station number and corresponding elevation of each point are input as variables STA(I) and EL(I) on card GR. Up to 100 points may be used. Cross sections / may be oriented looking either upstream or downstream since the program considers the left side to be the lowest station number and the right side the highest. The left and right stations separating the channel from the overbank areas are specified as variables STCHL and STCHR on card Xl. End points of a cross section that are too low (below the computed water surface elevation) will automatically be extended vertically by the program and a message giving the vertical distance extended will be printed. (3) There are times when the user wishes to use the previous cross section as the current one (for uniform channels), with or without a modifi- cation, or to modify the current cross section (perhaps the surveyed cross section is moved upstream or downstream). To do this, variables NUMST, PXSECR and PXSECE on card Xl are available. A zero or blank for variable NUMST indicates that the previous cross section will be used for the current one, i.e., GR cards are omitted. When the GR cards are read in NOMST must equal the number of stations on the GR cards. When the horizontal dimen- sions of the previous (NUMST . 0) or current (NU}!ST = +) cross section are to be increased or decreased by a factor, the value of the factor is entered as variable PXSECR. All cross section stations except the first will then be multiplied by the factor. If the elevations of the previous or current cross sections are to be raised or lowered by a constant, the value is entered as variable PXSECE. During normal usage, when cross section data are read, NUt!ST will equal the number of stations on cards GR and PXSECR and PXSECE will be blank. 5