4110 CLIFF DR 2022-01-26 - Laserfiche WebLink

This project is not able to protect 65% of the site area as forest or native conditions and still have a viable project. Therefore, BMP T5.30 — Full Dispersion is Not Feasible. Permeable Pavement Rain Gardens or Bioretention - Not Feasible This project is located within the City's "Infiltration in -feasible" red zone so all infiltration type BMPs are therefore deemed Not Feasible. Sheet Flow Dispersion - Not Feasible Per BMPs T5.12 feasibility criteria require use of this BMP for projects dispersing flows over slopes flatter than 20%. This project has slopes on -site in the area of the driveway steeper than 20% which makes this BMP Not Feasible. Based on this information provided above, the drainage mitigation was laid out using the Selected/Feasible BMPs. The soils for this project were found to be Alderwood gravelly sandy loam. These soils have characteristics that do not allow infiltration. Due to the steepness of the on -site slopes being greater than 15 percent, dispersion of the runoff is not feasible. Therefore, since the project area is less than 22,000 sf, the use of a perforated stub -out connection is allowed. The downspouts of the proposed SFR will be tightlined together to a perforated section of pipe. The pipe will discharge into the existing, vegetated drainage ditch on the northwestern side of Olympic Blvd., onto a rock pad. During the site visit, it appeared the downspouts on the existing SFR were tightlined and discharged at some unknown location. This discharge point is to verified by the project contractor. It if proposed that the pipe outfall from Lot 2 of the existing SFR be extended along the northern side of Lot 1 and connect to the proposed storm -pipe system to discharge at the same location within the right-of-way of Olympic Blvd., onto a rock pad. A total of approximately 880 sf of PGIS is proposed for the short plat and an additional 600 sf for the frontage improvements. The runoff generated by the proposed impervious driveway surfaces and additional pavement within the RNV shall be allowed to flow west over the driveway and then drain into the roadside swale at the low point in the driveway approach as shown on the plans. Finally, any area -of the site not proposed to be covered with impervious surfaces but graded or compacted will be treated to meet the criteria for DOE BMP T.5.13. Please refer to the engineering plans for the locations of the soil amendments and additional details. Refer to Appendix A for Soil amendment quantity calculation. Reid'SP 15-0714 July 2017 Page 11