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<br />Reach Length <br /> <br />Although the cross section represents the hydraulic geometry at a spe- <br />cific location, the section needs to represent the typical conditions in a <br />reach of the channel and its adjacent overbanks. In this context, a reach is <br />a length of a channel and its adjacent overbanks that are reasonably uniform <br />with respect to discharge, depth, slope, channel and flood-plain geometry, <br />roughness characteristics, and cultural features. In figure 2, a hypothetical <br />stream has been subdivided into three study reaches in which the cross sec- <br />tions represent typical conditions within that reach. The reach that includes <br />anyone section is considered to extend halfway to the next section, shown in <br />figure 2. If channel flow is fairly steady and uniform, an upper limit of <br />cross-section spacing needs to be about 75 to 100 times the mean depth for the <br />largest discharge to be considered. Ideally, the fall between cross sections <br />generally needs to be equal to or greater than the larger of 0.50 ft or the <br />velocity head. <br /> <br />If water-surface profiles for several discharges are to be computed, the <br />lengths between any two cross sections may have to be computed differently for <br />different discharges. Small discharges would stay entirely within banks and <br />follow the meanders of the main channel. The length for the subreach would be <br />a maximum. Large discharges may have flood-plain flows, and their effective <br />flow distances would be shorter. <br /> <br />For overbank flows, a weighted or effective subreach length needs to be <br />used. The center of flow in the subsections of each cross section are <br />determined and connected through the subreach by curvilinear or straight <br />lines. One line will follow the main channel, and the others will be along <br />the flood plains. The lengths of the main channel and overbank subareas are <br />measured separately. Profile computations for a range of discharges may <br />require ooe set of subreach lengths for all discharges within banks and <br />another set of subreach lengths for discharges with overbank flow. <br /> <br />Subdivision <br /> <br />Cross-section subdivision usually is needed to satisfy the criteria for <br />uniform flow. Under normal conditions, roughness can vary significantly <br />between the main channel and the overbanks. Subdivision needs to be made <br />primarily at major changes in cross-sectional geometry so that the velocity in <br />each subarea is basically uniform in each stream reach. Typically, a cross- <br />section subdivision is made where overbank flow first occurs, resulting in a <br />channel and left and right overbank subareas as shown in cross-section 1 of <br />figure 3. The cross section needs to be divided on the basis of geometry and <br />separate n values assigned, even though roughness may be the same in the <br />channel and overbanks. <br /> <br />7 <br />