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location of these features, The total duration of the hypothetical storm is dirElctly related to the time of concentration of the watershed (the travel time from the upper portions of the watershed to the most downstream point of interest), For example, if the estimated travel time is 14 hours (determined from actual records or by computation) from the watershed boundary to the lower limits of the study area, the storm duration must be at least 14 hours and preferably more, For most applications, the duration would be set to an even day (24 hours), Since a storm duration of less than 14 hours would not allow all portions of the drainage basin to contribute direct runoff to the outlet simultaneously during the course of the storm, the peak discharge at the basin outlet would not be reflective of the rainfall event if the storm duration wore made less than 14 hours, Runoff from the lower portions of the basin from, say, a 6-hour event would have left the basin before the inflow from the upper portion reached thll outlet. Therefore, a minimum storm duration should be selected at least equal to, and preferably well in excess of, the estimated travel time (time of concentration) at the downstream-most point of interest. This selected duration should be increased considerably if total volume of runoff as well as peak discharge is of importance in the study, Drainage basins having an unusually large amount of flood plain storage (wide flood plains and/or large areas of swamps) may require a storm of longer-duration to capture the attenuation effect of these large natural storage areas, Reservoir studies require long-duration events for full assessment of the reservoir flood storage needed, Therefore, a maximum storm duration of 10 days may be used even if the travel time to the reservoir site is only 14 hours, Total storm duration is normally taken as some increment of a 24-hour day (3, 4, 6, or 12 hours), or a multiple of a day (1 to 10 days), 3.2. TIME INTERVAL Once the storm duration has been established, the time interval for subdivision of the total storm must be selected, The time interval must be small enough to accurately define the flood hydrograph (especially the peak); however, too small an interval will result in excess computations by the individual or the computer. The time interval will generally be established by the fastest peaking subarea of the overall basin model for which the peak discharge is required, I.e,. for later use in developing water surface profiles. to evaluate the effects of a flood control component, etc, The time interval must be small enough to define the rising limb and peak for the hydrograph for this subarea. It has been found from past experience that a time interval that gives at least 3 points on the rising limb of the hydrograph prior to the peak provides an estimation of the peak discharge that is accurate enough for most work, " Extending the previous example of the selection of a storm duration of 24 hours based on a travel time of 14 hours to the outlet, one can now select a time interval of 4 hours (14 hours divided by 3 points prior to peak = 4,67 hours, rounded down to 4 hours). This would be an appropriate time interval if one were interested only in the peak discharge at the outlet. However, if there is a subarea upstream for which a peak discharge estimate was also necessary, Ihlm the time interval fo'rthe entire drainage basin would be based on the requirements for this subarea, For example, if the time of concentration for this subarea is 70 minutes. the interval required would be 20 minutes (70 minutes/3 points prior to peak = 23,3 minutes, rounded down to 20 minutes), Note that the 24-hour storm would now be subdivided into 72 twenty-minute intervals to define the rainfall distribution for the basin, compared with the 6 four-hour intervals where only the Colorado Flood Hydrology Manual. DRAFT 7,3