9427 19TH AVE SE 2018-01-02 MF Import

Step 2. Time of Ccncentration The time of concentration, or Tc, is the time required for runoff to travel from the most distant point, eased on flow time of the tributary area to reach the outlet. Figure 1 lists the approximate velocity with which runoff can be expected to travel across various surfaces. This velocity is then divided into the distance of travel, multiplied by 60 to obtain minutes and finally 10 minutes are added to allow for the lag between the time rainfall starts to the time runoff begins. Step 3. Rainfall Intensity After Tc is determined, the rainfall intensity, "i", can be found by consulting figure 2, the intensity - duration - frequency curve. Enter the horizontal axis at the previously determined Tc, draw a vertical line until it intersects the applicable frequency curve, then draw a horizontal line from this intersection to the vertical axis. The point at which the line intersects the vertical axis is the design intensity. In most cases, a 10-year frequency will be used, however, if the site is within a critical drainage area, a frequency other than 10-year may be required. Step 4. Tributary Area. Pi most cases, the tributary area, A, is the area of the site. However, if the detention facility is planned to be in a natural drainage channel, (which is rarely allowed) the tributary area includes the entire area the channel drains upstream of the site; or if there are off -site areas from which surface runoff flows onto the site, they must be included in the tributary area.