2019-040 - Mainplace Mall Transformation

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Kimley>»Horn Discharge C 58 CFS 59 CFS +1.7% Discharge D 29 CFS 29 CFS 0% Discharge E 21 CFS 21 CFS 0% Total 233 CFS 234 CS 0.4% Discharge areas A and B drain toward the Santa Ana River, which is not a facility that is not susceptible to hydromodification. The combined flows from A and B are the same as the existing condition. Areas C, D, and E, which drain to the hydromodification- susceptible Santiago Creek collectively have a collective 0.9% increase (less than the allowable increase of 5%), which meets the County hydromodification criteria. In addition, the pipes will be sized according to the table below based on the pipe hydraulic calculations. The flowrate tributary to a pipe size should not exceed the tabulated values listed below without more detailed study: Table 2: Pipe Hydraulics Table Pipe Size Material Capacity at 0.3% Friction Slope 12" HDPE (n=0.011) 2.3 CFS 18" HDPE (n=0.011) 6.8 CFS 24" HDPE (n=0.011) 14.6 CFS 30" HDPE (n=0.011) 26.6 CFS 36" HDPE (n=0.011) 43.2 CFS 42" HDPE (n=0.011) 65.1 CFS 48" HDPE (n=0.011) 93.0 CFS At this time, since there is not precise site plan, the pipe locations have not been determined. The table above provides guidance for future project. More detailed hydraulics analyses will be required once precise site plans are determined. 6. CEQA Threshold Analysis The following standard CEQA thresholds are applicable to hydrology and drainage: Threshold (c) Would the project substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river, in a manner which would result in substantial erosion or siltation on- or off -site? (CEQA Environmental Checklist: Hydrology and Water Quality): No streams or channels are affected by the Project. The proposed drainage system and pattern largely matches the existing system and pattern. Sediment Page 9 of 14