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� � <br /> ' Step 2. Time of Ca-:entration <br /> The time of concentration, or Tc, is the time required for runotf ta travel <br /> from the most distant point, based on flow time of the tributary area <br /> to reach the outlet. Figure 1 lists the approximate velor_ity with which <br /> runoff can be ex,�ectc�d to travel across various surfaces. This velocity <br /> is then divided into the distance of travel, multiplied by 60 to obtain minutes <br /> and finally !0 minutes are added to allow for the .':�.g between the time <br /> rainfall starts to the time runoff b�gins. <br /> Step 3. Rainfall Intensity <br /> After Tc is determined, the rainfal! intensity, "i", can be found by consulting <br /> iigure 2, tfie intensity- duration •- frequency curve. Enter the horiwntal <br /> axi: at the previously determined Tc, draw a vertical line i�ntil it intersects <br /> the applicable frequency curve, then,draw a horizontal line irom Uvs <br /> intersection to the vertir.al axis. 1'he point at which the line intersects <br /> the vertical. axis is the design intensity. <br /> In most cases, a !0-year frequency will be used, however, if the site is <br /> within a critical drainage area, a frequency other than !0-year may t+e <br /> rcyuired. <br /> Siep 4. Tributary Area. <br /> � In rnost cases, the tributary area, A, is the area of the site. liowever, <br /> if the detention facilit� is planned to be in a natural drainage channel, <br /> (which is rarely allowed) the tributary area includes the e��tire area 4he <br /> channel drains upstream of the site; or if tt ere are off-site areas from <br /> which surface rur.off flows on:o the site, they must be included in the <br /> tribatary area. . <br /> � <br />