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Table 2-2 shows the values of C that may be used in formula 2-1 _ for certain composite areas in lieu of computing a composite value by the above method. 2 .2 .2 Tributary Area,_A The tributary area for each point of computation shall be based on actual field reconnaissance or use of appropriately scaled maps that clearly depict the drainage boundaries . All of the area that will contribute runoff to the drainage system shall be considered, regardless of the limits - of the particular development under consideration. 2 .2 .3 Critical Flow Time The critical flow time is that time which results in the maximum flow rate fora iv point in drainage sys ximum low occurs when the product of the intensity and the contributing area corresponding to the flow time is a maximum. The time required for water to flow from the most hydraulicly remote point in the watershed to the point in question is called the time of concentration. Using the intensity corresponding to this time and the entire drainage area in the Rational Formula usually results in the maximum flow rate for the point . The nomograph shown in Figure 2-2 shall be used to determine the time of concentration to the first inlet of the drainage system. If this time is less than the minimum inlet time shown in Table 2-2 , the values shown in the table shall be used. ` When any part of the storm waters are conveyed to the point in question by pipes or open channels , the flow time in these conveyances shall be added to the inlet time to compute the total 2-4