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<br />called reach lengths. Together they form a three-dimensional, digital <br />description of the flood plain and channel through which water must <br />flow - a digital model of the boundary geometry. <br />Usually a rigid boundary is assumed, and the geometric models <br />are invariably deterministic. Cross sections are often spaced as <br />close as 500 feet (150 meters) on major rivers such as the Missouri <br />and Arkansas Rivers in the United States which have top widths of 2 <br />to 6 times that distance. However, this is not a fixed rule as the <br />type of study to be performed must also be considered. For instance, <br />navigation-depth studies require closely spaced cross sections be- <br />cause local conditions are important: whereas the calculation of <br />water surface profiles for establishing top-of-levee elevations or <br />rating curves at various points along a stream can be made with sec- <br />tions spaced 1 or 2 miles (2 or 3 km) apart provided a proper formula- <br />tion of the friction loss equation is used. (Friction loss is discus- <br />sed in Chapter 5.) In sedimentation studies where future deposition <br />in reservoirs is being predicted. cross sections spaced 5 or 10 miles <br />(8 or 16 km) apart are often satisfactory. In any case, the final <br />spacing of cross sections for the calculations depends upon the com- <br />putation scheme, not the spacing selected for field measurement. It <br />is often desirable to conwine several measured sections into one <br /> <br />average section to better define average geometry over long distances. <br />Some computation schemes treat each cross section as represent- <br />ing a reach of the river and use only one section at the midpoint of <br />the reach to calculate losses through the entire reach. Other schemes <br />use cross sections to define break points in the geometry, and properties <br /> <br />4.02 <br />