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

I 6. Collect qualitative data on features such as land use, impervious surfaces, topography, and soil types. • Data reviewed during resource review was confirmed during the field inspection. 7. Collect information on pipe sizes, channel characteristics, drainage structures, and relevant critical areas • The system is documented in the City's GIS system. Refer to Appendix A-4 for a printout from the City's GIS. 8. Verify tributary basins delineated in Task 1. • Tributary basins were confirmed from Task 1. 9. Contact neighboring property owners or residents in the area about past or existing drainage problems, and describe these in the report. • No drainage reports were recorded by the City. 10. Note the date and weather conditions at the time of inspection. • Noted above. 3.4 Task 4—Drainage System Description and Existing/Predicted Problems Because the City has a well-documented drainage system via their GIS and the system 1 discharges to a combined storm/sewer system, a field investigation was not performed. Refer to Appendix A-4 for the Downstream Study Map, which is a printout from the City's GIS. 4.0 Permanent Stormwater Control Plan This project meets MRs 5 and 7 by providing a Permanent Stormwater Control Plan, which includes Onsite Stormwater Management(OSM) BMP and Flow Control per Interim Policy 2-11. The stormwater modeling software, MGSFlood (the model), was utilized to size facilities and confirm compliance with the City Manual. 4.1 Site Hydrology The site hydrology is determined by the type of land coverage and soil type. According to the Geotechnical Report, the site is underlain with typically dense to very dense, brown/grey, damp, gravelly sand interpreted as native glacial till. The geotechnical engineer does not consider the subsurface conditions to be generally suitable for infiltration of stormwater. Site soils are modeled as Type C soils because there is no infiltration potential. Existing landscape was modeled as"Till Grass,"while proposed landscape areas were modeled as"Till Pasture." This is due to existing landscaping assumed to not meet BMP 12.20, Post-Construction Soil Quality and Depth, while proposed landscaping will be required to do so. According to the DOE 2019 Stormwater Management Manual for Western Washington (SMMWW), Appendix C, page C-12, areas meeting this BMP shall be entered as pasture in the model. Per Interim Policy 2-11, the predeveloped conditions shall be modeled as the existing conditions. Refer to Appendix A-2 for the Existing Conditions Map and Appendix A-3 for the Developed Conditions Map. The table below provides a summary of areas that were entered into the model: Stormwater Site Plan Report North Sound Behavioral Health Treatment Ciel00 AHBL No.2170857.10