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3. OFFSITE ANALYSIS AND MITIGATION <br /> Total Previous SFR: <br /> The original project proposed to construct approximately 6,235 sf of new/replaced <br /> impervious surfaces. The following table summarizes the PGIS and NPGIS proposed. <br /> Total (sf) <br /> Existing (TBR) 0 <br /> New + Replaced 6,235 <br /> Net New 6,235 <br /> This project will add the following: <br /> Total (sf) <br /> Existing (See Above) 6,235 <br /> New + Replaced 2,702 <br /> Site Total Imp. 8,937 <br /> The final impervious roof and driveway surfaces for this project will be mitigated using on- <br /> site stormwater management BMPs including an engineered conveyance system with a <br /> discharge at the base on the bluff adjacent to the BNSF railroad embankment. Soil <br /> management,will also be used to mitigate the portions of the site that have been graded, <br /> compacted or require soil amendments meeting BMP T5.13. <br /> A. UPSTREAM ANALYSIS <br /> During the site visit it was observed that due to the site grades and surrounding <br /> development/improvements that the primary area contributing runoff to the project site was <br /> limited to the roadway and parcel to the south. It was clear that all off-site flows will be <br /> allowed to pass through or around this site and will remain that way after construction. <br /> B. DOWNSTREAM ANALYSIS <br /> Runoff from the proposed disturbance areas (prior to original SFR construction) of the lots <br /> sheet flows north over flat landscaped slopes. The topography on the proposed lot is such <br /> that the runoff is directed primarily north toward to the steep slope. The runoff to the north <br /> continues north down the steep forested slopes. Near the toe of the slope is the BNSF <br /> R/W. Runoff to the west flows over vegetated steep slopes and is collected by a non-fish <br /> bearing stream is located approximately 100 feet to the west and it is believed that the <br /> runoff from both directions would enter this stream system. This stream flows north and <br /> passes under the BNSF R/R and then drains immediately into the Puget Sound. Based on <br /> the on/off site observations the Puget Sound would be considered the receiving water <br /> body for this project and the downstream analysis can be stopped at that point. <br /> Langus Garage 15-0403-L <br /> March,2022 Page 5 <br />