902 PINE ST NORTH SOUND BEHAVIORAL HEALTH TREATMENT CENTER 2025-11-04

I Table 1 —Land Cover Summary I C, Lawn* FC— FC— FC— Bypass Bypass C, Bio- Total (ac) C, Pasture* Impery Non-target Impery Pasture retention** (ac) (ac) (ac) Impery(ac) (ac) (ac) (ac) Existing 0.30 - 0.54 0.19 - - - 1.03 Developed - 0.25 0.28 0.19 0.18 0.12 0.01 1.03 *Bioretention cell area is excluded from Sub-Basin area. **Bioretention is represented by"Area at Riser Crest El"in the model. 4.2 Onsite Stormwater Management(OSM) I The OSM BMPs are described in Volume Ill, Chapter 12 of the City Manual. Refer to Table 2 for a summary of each BMP considered for feasibility review. Table 2—OSM BMP Feasibility Review BMP Infeasibility Justification BMP 12.10 Vegetation Protection Areas BMP 12.20 is provided. Impervious areas of the project area have been reduced. BMP 12.20 Soil Amendments, Quality, and Depth BMP 12.20 is provided for all proposed landscaped areas. BMP 12.30 Onsite Infiltration Areas BMP 12.30 is not feasible because the soil onsite has no infiltration potential. BMP 12.40 Raingardens Bioretention cell is provided. BMP 12.40 Downspout Dispersion Systems Splash blocks area provided to spread roof runoff over bioretention cell. BMP 12.50 Concentrated Flow Dispersion BMP 12.50 is not feasible due to site constrains. I BMP 12.60 Sheet Flow Dispersion BMP 12.60 is provided for all impervious or pervious surfaces that are graded to avoid concentrating flows. The bioretention area provided for meeting OSM is sized per the SMMWW, Volume Ill, BMP T5.14. The design guidelines recommend that the ponding area be sized as 5 percent of the total impervious surface area and 2 percent of the total pervious surface area draining to it. Because of the limited available area, the bioretention cell is upsized to the maximum extent and then back calculated to an equivalent mitigated impervious area. Refer to Appendix A-3 for area delineation and Table 3 below for a summary of the provided ponding surface area and the equivalent impervious area draining to the bioretention cell. Table 3—OSM Sizing Provided Ponding Equivalent Mitigated OSM - Surface Area Impervious Surface Area (sf) (ac) Bioretention 564 0.23 4.3 Water Quality Treatment The project site is exempted from Runoff Treatment according to Interim Policy 2-11, as discussed in Section 2.0. Stormwater Site Plan Report I North Sound Behavioral Health Treatment OW : Q AHBL No.2170857.10