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<br />2-2 <br /> <br />The runoff volume (Q) in inches is computed using the SCS runoff <br />equation outlined in Chapter 10, NEH-4. If the user requests, the CN <br />will be adjusted for the antecedent moisture condition limits (AMC) I <br />or III according to Table 10.1 NEH-4. If the user needs to adjust the <br />runoff volume within the AMC limits, either the CN's or rainfall will <br />need to be adjusted. <br /> <br />~,! <br /> <br />Hydrograph Development <br />An incremental unit hydrograph is developed for each subwatershed as <br />shown in Chapter 16, NEH-4. The unit hydrograph time increment (80) is <br />calculated as a function of the time of concentration (T). Coordinates <br />c <br />of the incremental unit hydrograph are determined as shown in Example <br />1, Chapter 16, NEH-4 using 80, T and the drainage area. The incremental <br />c <br />runoff volume (L\Q) is determi'ned for each 80. The composite flood <br />hydrograph is computed by summing the incremental.hy?r~grap~ ordinates <br /> <br />. .'....t.." (:'OJ ..1..0, l <br />as shown in thf7same Example. The time increment' usedl'is the main <br />time increment\,'as specified by the user on the INCREM statement. <br /> <br />A maximum of 300 ordinates (discharge values) can be 'stored for any <br />composite flood hydrograph. The peak flow value of the composite flood <br />hydrograph is computed by a separate routine that utilizes the <br />Gregory-Newton forward difference formula for fitting a 2nd degree <br />polynomial thru the three largest consecutive hydrograph values saved <br />at the main time,increment. (Wylie, p. 94). In multiple peaked <br />hydrographs up to ten peaks may be computed. <br /> <br />.ce <br /> <br />The primary concern when selecting the main time increment is to <br />provide an adequate definition of the hydrograph. Composite flood <br />hydrographs from subwatersheds are developed using 80 as a time interval, <br />but are converted to the main time incremenf'specified by the user <br />before being stored for subsequent use. The main time increment <br />should be about 0.1 to 0.2'T of the smallest subwatershed. Hydrograph <br />c <br />definition decreases when the time increment is larger than this range <br />of values. Generally, the main time increment need not be smaller <br />than 0.1 hour. <br /> <br />'- <br /> <br />Several factors must be considered by the user when selecting the main <br />time increment. ~he user must consider the longest hydrograph, with <br /> <br />~. <br />