3618 W MUKILTEO BLVD SITE WORK FOR HARBORS EDGE TOWNHOMES 2025-10-27

SAGE HOMES NORTHWEST, LLC HARBORS EDGE DRAINAGE REPORT AUGUST 2022 Downspout Dispersion Not Feasible — Most of the project was previously cleared of (BMP T5.1013) native vegetation, and there are no suitable dispersal flowpaths meeting the applicable requirements. Perforated Stub -out Not Feasible —The underlying soil conditions are not conducive to Connections infiltration. . (BMP T5.10C) Other Hard Surfaces -Roadways, Driveways, Sidewalks and Stairs Full Dispersion Not Feasible — Most of the project was previously cleared of (BMP T5.30) native vegetation, and there are no suitable dispersal flowpaths meeting the applicable requirements. Permeable Pavement Not Feasible —The underlying soil conditions are not conducive to (BMP T5.15) infiltration. Permeable pavement is considered infeasible if the geotechnical evaluation recommends infiltration not be used due to reasonable concerns of slope failure. Bioretention Not Feasible —The underlying soil conditions are not conducive to (Vol. T7.30) infiltration. A bioretention facility is considered infeasible if the geotechnical evaluation recommends infiltration not be used due to reasonable concerns of slope failure. Flow Dispersion Not Feasible — Most of the project was previously cleared of (BMPs T5.11 /T5.12) native vegetation, and there are no suitable dispersal flowpaths meeting the applicable requirements. 4.4 FLOW CONTROL The project is required to satisfy the flow control requirement outlined in Minimum Requirement #7 of the SWMMWW. This involves evaluating the site using a continuous runoff simulation model, and verifying that the developed discharge durations match the historic discharge durations for the range of pre -developed discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow. For this analysis, the historic runoff conditions should be assumed to have a forested land cover. Existing and developed basin maps are provided in Appendix C of this report. The historic and anticipated land cover conditions are tabulated on those figures and those tabulated values are used as input for the hydrologic modeling for the project discussed below. The project is only required to provide flow control for the improvements proposed by the project itself. In the existing condition, the project site is modeled as being in a forested condition, to satisfy the onsite flow control requirement as discussed in the previous paragraph. There is a small area on Mukilteo Boulevard that will bypass the detention facility due to the grades. Design calculations for the stormwater management facility are provided in Appendix D of this report. The point of compliance shows that the facility, in combination with the bypass area, meets the flow control requirements. PACE) PAGE 17