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<br />3-13 <br /> <br />~ <br /> <br />there is a need for the hydrographs for the two smaller subwatersheds <br />(e.g., a potential structure site), only one hydrograph should be <br />developed for the total drainage area of 17.4 square miles. <br /> <br />Another consideration in subdividing watersheds is the reach length <br />used in routing. A cross section is generally used to represent the <br />stream or valley reach through which hydro graphs will be flood routed <br />using the Modified Att-Kin routing procedure. The reaches should be <br />as long as practical when considering the detail for which the watershed <br />hydrology is analyzed. In general, each reach will extend from one <br />major tributary to another. See Appendix H for further explanation. <br /> <br />3.5 SCHEMATIC DRAWING <br /> <br />~ <br /> <br />A schematic drawing is a flow diagram that shows subwatershed delineation <br />and the relationships of subwatersheds, structures, and reaches. The <br />schematic drawing for the watershed is an important aid in compiling <br />input data and is especially important to those checking and handling <br />the data through processing and documentation. A sample schematic <br />drawing including a legend showing suggested symbols is shown in <br />Appendix C, page C-2. A schematic drawing should be completed before <br />preparing the input data. <br /> <br />The location of all potential structures to be considered should be <br />shown together with all cross sections that represent reaches and <br />junctions. The structures and cross sections should be numbered in <br />the sequence in which they will be routed. This sequence facilitates <br />setting up the job, checking and verifying. The drainage area above <br />each structure and the area of local drainage to each reach are inserted <br />as shown on page C-2. The reach length, time of concentration (T ), <br />c <br />drainage area and the runoff curve number should also be included for <br />ready reference. <br /> <br />. <br /> <br />A cross section representing a rating section for a reach should be <br />shown at the downstream end of the reach, even though the surveyed <br />section may be at another location. The rating section gives the <br />average cross-sectional flow and end area of a routing reach. If more <br />than one rating is available for a reach, the ratings may be averaged <br /> <br />