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<br />to the peak provides an estimation of the peak discharge that is accurate enough for <br />most work. <br /> <br />Extending the previous example of the selection of a storm duration of <br />24 hours base don a travel time of 14 hours to the outlet, one can now select a time <br />interval of 4 hours (14 hours divided by 3 points prior to peak = 4.67 hours, rounded <br />down to 4 hours). This would be an appropriate time interval if one were interested <br />only in the peak discharge at the outlet. However, if there is a subarea upstream for <br />which a peak discharge estimate was also necessary, then the time interval for the <br />entire drainage basin would be based on the requirements for this subarea. For <br />example, if the time of concentration for this subarea is 70 minutes, the interval <br />required would be 20 minutes (70 minutes/3 points prior to peak = 23.3 minutes, <br />rounded down to 20 minutes). Note that the 24-hour storm would now be subdivided <br />into 72 twenty-minute intervals to define the rainfall distribution for the basin, <br />compared with the 6 four-hour intervals where only the discharge hydrograph at the <br />outlet was needed. Only one time interval can be specified for a given HEC-1 <br />simulation. This means that the smaller intervals must be carried throughout the <br />hydrograph construction, routing, and combining operations within the HEC-1 <br />simulation. This means that the smaller intervals must be carried throughout the <br />hydrograph construction, routing, and combing operations within the HEC-1 model, <br />and this greater number of intervals will require more effort and computer time. <br />Extreme cases occasionally occur, such as a large drainage area with a few of its <br />subareas very small in size. Although the storm duration may be 2 days or more to <br />reflect travel time to the outlet, a time interval of 5 minutes may be required to <br />accurately capture the peak discharge from the smallest subareas. It may be more <br />economical to construct a separate HEC-1 model (most likely with a short-duration <br />storm) for each small subarea requiring a 5-minute subdivision, thereby allowing the <br />model of the large area to use multi-hour time increments for the longer storm <br />duration. <br /> <br />3.3. DATA EXTRACTION FROM NWS PUBLICATIONS. The <br />methodology for hypothetical storm development for the Western states will be <br />described separately from that for the remainder of the United States. Procedures for <br />developing hypothetical storms in Hawaii and Alaska are similar to procedures for the <br />other states. <br /> <br />3.3.1. EASTERN AND CENTRAL UNITED STATES. Once the storm <br />duration and computation time interval have been established, the rainfall depths for <br />key durations and each desired return period are taken from the appropriate NWS <br />publications. For a 6-hour storm duration and 15-minute increment, for example, the <br />2-year hypothetical storm data for the area of interest would be obtained from TP-40 <br />and HYDRO-35. TP-40 gives isopluvial maps of the 2-, 3- and 6-hour duration 2-year- <br />return-period total rainfall. The 30-minute and 1-hour maps in TP-40 have been <br />superseded by the procedures given in HYDRO-35. By determining the location of the <br /> <br />7-14 <br />