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<br />6. CROSS SECTION GEOMETRY <br /> <br />Boundary geometry for tbe analysis of flow in natural streams is <br />specified in terms of ground surface profiles (cross sections) and the <br />measured distances between them (reach lengths). Cross sections are <br />located at intervals along a stream to characterize the flow carrying <br />capability of the stream and its adjacent flood plains. They should <br />extend across the entire flood plain and should be perpendicular to <br />the anticipated flow lines (approximately perpendicular to contour <br />lines). Occasionally it is necessary to layout cross sections in a <br />curved or dog-leg alignment to meet this requirement. Every effort <br />should be made to obtain cross sections that accurately represent <br />the stream and flood plain geometry. However, ineffective flow areas <br />of the flood plain such as stream inlets, small ponds or indents in <br />the valley floor should generally not be included in the cross section <br />geometry. <br /> <br />Cross sections are required at representative locations throughout <br />a stream reach and at locations where changes occur in discharge, slope, <br />shape, or roughness; at locations where levees begin or end and at bridges <br />or control structures such as weirs. Where abrupt changes occur, several <br />cross sections should be used to describe the change regardless of the <br />distance. Cross section spacing is also a function of stream size, slope, <br />and the uniformity of cross section shape. In general, large uniform <br />rivers of flat slope normally require the fewest number of cross sections <br />per mile. The purpose of the study also affects spacing of cross sections. <br />For instance, navigation studies on large relatively flat streams may <br />require closley spaced (e.g., five hundred feet) cross sections to <br />analyze the effect of local conditions on low flow depths, whereas cross <br />sections for sedimentation studies to determine deposition in reservoirs <br />may be spaced at intervals of up to five or ten miles. <br /> <br />The choice of friction loss equation may also influence the spacing <br />of cross sections. For instance, cross section spacing may be maximized <br />when calculating an Ml profile with the average friction slope equation <br />or when the harmonic mean friction slope equation is used to compute <br />M2 profil es. <br /> <br />Each cross section in an HEC-2 data set is identified and described <br />by Xl and GR cards. Variable SECNO on the Xl card is the cross section <br />identification number which may correspond to stationing along the channel, <br />mile points, or any frictious numbering system, since it is only used <br />to identify output and is not used in the computations. Each data point <br />in the cross section is given a station number corresponding to the <br />horizontal distance from a zero point on the left. The elevation and a <br />corresponding station number of each data point are input as variables <br />EL(I) and STA(I) on GR cards, Up to one hundred data points may be used <br />to described cross section geometry for most program applications. When <br />the encroachment options are utilized, no more than nintey-five data pOints <br />should be used, since they generate additional data points automatically <br />to define the encroachment limits. The channel improvement option also <br />should be used with fewer than one hundred data points since it will <br />generate data points (four or more depending on the geometry. <br /> <br />14 <br />