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I <br /> Table 3BI <br /> Proposed Conditions—Powdermill Gulch Creek <br /> Area(AC) Description Grade Runoff Coef I <br /> 0.41 Rooftops <br /> 0.13 Parking Lot Flat (0%-5%) 0.90 <br /> 0.64 Parking Lot Moderate (5%-15%) 0.90 I <br /> 0.19 Landscaping Flat (0%-5%) 0.25 <br /> 1.37 Total Basin Area 0.81 <br /> 0.092 Southern Parking Lot Flat 0.90 I <br /> All runoff draining to the Powdermill Gulch Creek basin that cannot be feasibly collected onsite or <br /> does not required runoff treatment is described in Table 3C below: Il <br /> Table 3C <br /> Proposed Conditions—Powdermill Gulch Creek <br /> Area(AC) Description Grade Runoff Coef <br /> 0.15 Rooftops <br /> 0.29 Landscaping Flat (0%-5%) 0.25 <br /> 0.44 Total Basin Area 0.81 ' <br /> The above areas (1.37-acres + 0.44-acres = 1.81-acres) are equal to the full onsite Powdermill <br /> Gulch Creek basin plus the offsite tributary area (1.65-acres + 0.16-acres = 1.81-acres) I <br /> Hydrologic Modeling <br /> As discussed in the Minimum Requirements section of this report, the Narbeck Creek basin will be I <br /> modeled using a separate methodology than the Powdermill Gulch Creek basin since the total <br /> developed basin area is less than one acre. I <br /> Narbeck Creek: <br /> A hydrologic analysis for the Narbeck Creek basin was performed using the computer-modeling , <br /> program "StormShed 2G". The program effectively models pre-developed and developed runoff <br /> conditions using hydrographs. <br /> Powdermill Gulch Creek: I <br /> The hydrologic analysis for the Powdermill Gulch Creek basin was performed using the computer- <br /> modeling program, Western Washington Hydrology Model (WWMH3), based on matching flow <br /> durations. The program effectively models predeveloped and post-developed runoff conditions I <br /> using flow duration curves for a basin and a given area. <br /> I <br /> I <br /> 6/16/2011 Stormwater Site Plan Page 24 I <br />