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where: C =rational method runoff coefficient <br />L =length of overland flow, ft <br />S =surface slope, <br />Table 1 shows the calculated t° for overland flow for each subbasin at the mine using the <br />FAA equation. <br />2.2.1 Total Flow Time <br />The previous section described the calculation of travel time for overland flow within a <br />subbasin. For subbasins with open channels, the total travel time also includes the <br />channel flow time. Channel flow time applies to channel or ditch flow in structures <br />constructed to collect runoff. This stormwater drainage plan calls for the construction of <br />a collection channel for every designated subbasin (see Figure 3.0 and Plates 1.0 and 2.0). <br />Therefore, channel flow within, or at the lower boundary of a subbasin was included in <br />the final calculation of tc. <br />Channel flow time was initially estimated by using the continuity equation to calculate a <br />dischazge (Q) associated with a10-year storm intensity for each subbasin (see Section <br />2.3) and a duration time equal to t° calculated with the FAA equation. Velocity was then <br />estimated using approximated channel designs and the Manning equation. With the <br />approximate length of each drainage structure (channel) known, the travel time was <br />' calculated for each channel. The channel time calculation results are shown in Table 2.0 <br />and Table 2.1. <br />In addition to channel flow time within a subbasin, channel or pipe flow time in <br />structures that connect upstream subbasins with downstream subbasins, or connect two <br />' adjoining subbasins, must also be taken into account. When using the rational method, in <br />cases where a mutual drainage structure conveys runoff water from an upstream subbasin <br />to a downstream subbasin, the time of concentration for the downstream subbasin is <br />chosen as the larger of: (1) the inlet time (including interbasin channels) of the upstream <br />subbasin(s), or (2) the inlet time (including interbasin channels) of the downstream <br />subbasin and pipe or channel travel time from the upstream subbasin. The lazger travel <br />time is then applied to the determination of a new storm intensity for discharge from the <br />downstream subbasin. The rainfall intensity applied to calculating discharge may <br />therefore change depending on the lazgest applicable travel time amongst the subbasins <br />being considered. <br />The drainage design for the Logan Wash Mine presents several situations where <br />adjoining subbasins share a common collection channel. The following paragraphs <br />describe the rationale used to estimate the total travel time for each subbasin and <br />associated drainage structure. Table 1.0 shows the corresponding storm intensity for each <br />subbasin t~ value as determined from Figure 4.0 (see Section 2.3.3). Tables 2.0 and 2.1 <br />summarize the identified discharge points, the associated runoff-contributing subbasins, <br />channel and total travel times, and final discharge calculation for each discharge system. <br />Western Water S Land Inc. <br />