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<br />Cross section data is traditionally oriented looking downstream <br />since the program considers the left side of the stream to have the <br />lowest station numbers and the right side to have the highest. The <br />left and right stations separating the channel from the overbank areas <br />are specified as variables STCHL and STCHR on the Xl card. End points <br />of a cross section that are too low (below the computed water surface <br />elevation) will automatically be extended vertically and a note indicat- <br />ing the extension amount will be printed. <br /> <br />Numerous program options are available to allow the user to easily <br />add or modify cross section data. For example, when the user wishes <br />to repeat a surveyed cross section. a single Xl card may be input to <br />identify the cross section and to provide reach length information. <br />Xl card variables, PXSECR and PXSECE. allow the user to modify the <br />horizontal and vertical dimensions of the repeated cross section data. <br />Other program options to modify cross section data to model improved <br />channel sections, encroachments and ineffective flow areas are described <br />in detail in the following chapter. <br /> <br />7. REACH LENGTHS <br /> <br />The measured distances between cross sections are referred to as <br />reach lengths. The reach lengths for the left overbank. right overbank <br />and channel used in computations are specified on the Xl card by variables <br />XLOBL. XLBOR, and XLCH. respectively. Channel reach lengths are typically <br />measured along the thalweg. Overbank reach lengths should be measured <br />along the anticipated path of the center of mass of the overbank flow. <br />Often these three values will be equal. There are however, conditions where <br />they will differ. such as at river bends. or where the channel meanders <br />considerably and the overbanks are straight. Where the distance between <br />cross sections for channel and overbanks are different. a discharge-weights <br />reach length is determined based on the discharges in the main channel <br />and left and right overbank segments of the reach (see Equation 3, page <br />4). <br /> <br />15 <br />