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Area 8: Area 8 is the upper and eastern portion of the Lower Bench, located immediately west of the Upper Dump which is part of Area 7. Runoff from Area 8 will be collected in a middle Lower Bench channel, DS8, located at the lower boundary of Area 8, approximately in the middle of the Lower Bench. Water in this "middle channel" (DS8) ' will merge with the "east channel" (DS7) at discharge point DP4. Area 9: Area 9 consists of the lower and west portion of the Lower Bench and is defined by the bench area between channel DS8 and the west runoff channel (DS9), which will be constructed approximately 75 feet east of the brow of the Lower Bench. Runoff from ' Area 9 will be conveyed to the north by channel DS9 and discharged at a pour-off structure (DPS) located on the brow of the Lower Bench. Discharge from this point will also be received by Dry Gulch. Table 1.0 presents the characteristics of each subbasin including acreage and parameters required for solving the rational method equation. 2.2 Time of Concentration The rational method assumes that peak discharge occurs when all portions of a given watershed or subbasin are contributing flow to the inlet of the drainage structure. The time required for this to occur is referred to as the time of concentration, t~, or inlet time. The time of concentration is the measured time it takes for flow from the most remote point in the subbasin to the outlet discharge point including travel time in interbasin channels (channels within the subbasins), and interconnecting channels between subbasins. Where subbasins are connected by a common channel, the time of concentration is calculated by using the subbasin with the greatest overland flow travel time and adding the travel time required in the segment of the channel between the subbasin inlets. For more information on calculating tc, see Chow and others, (1988). For the rational method, it is assumed that the time of concentration is equivalent to the storm duration. Therefore, once time of concentration is calculated for a subbasin, a stone duration equivalent to the time of concentration is used to arrive at the associated storm intensity, either through the use of a specific equation developed for the region or by using intensity-duration-frequency (IDF) curves developed for the site of interest, as described in NOAA (1973) (See Section 2.4). The Federal Aviation Administration (FAA 1970) time of concentration formula was used to calculate the time of concentration for each subbasin (Chow and others, 1988). This formula is commonly used with the rational method. The FAA formula is as follows: t~ = 1.8(1.1-C)LO.SO~S,0.333 Western Water 8 Land Inc.