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

SAGE HOMES NORTHWEST, LLC HARBORS EDGE DRAINAGE REPORT AUGUST 2022 4. FLOW CONTROL AND WATER QUALITY ANALYSIS AND DESIGN 4.1 HYDROLOGIC ANALYSIS 4.1.1 Existing Conditions The site is currently developed as four single family lots. One of the lots has a home on it and one has a business, these will be removed. The vegetation on the lots consists of grass with some trees and gardens. The existing wetland on the south side of the property will be filled in. The ground slopes to the north side of the property and to the existing storm drain system. A map depicting the existing site conditions is provided in Appendix C of this report. 4.1.2 Developed Conditions The project is proposing the construction of 25 townhomes with a loop driveway to provide access to the townhomes. A stormwater conveyance network will be provided beneath the interior driveway to collect and convey roadway runoff to the detention facility. Roof/footing drains will be provided for each building, which will also tie into the roadway drainage network. A map showing the proposed site conditions is provided in Appendix C of this report. 4.1.3 Hydrologic Modeling Hydrologic modeling was performed for the project in order to design the proposed detention and treatment facilities. Hydrologic modeling was performed using MGSFlood°, which is a continuous runoff simulation model approved for use by Ecology. The calculations demonstrate that the project is in compliance with the applicable flow -control regulations, and also calculates the appropriate water quality design parameters. Hydrologic modeling calculations are provided in Appendix D of this report. Input parameters for the hydrologic model are summarized on the basin maps provided in Appendix C. 4.2 WATER QUALITY CALCULATIONS Volume V, Chapter 2 of the SWMMWW provides a step-by-step selection process for determining the required level of treatment for the project site. That step-by-step process is provided below: • Step 1— Determine the receiving waters and pollutants of concern: The project site flows to the existing conveyance system on West Mukilteo Boulevard and then into Edgewater Creek. The online Water Quality Atlas published by Ecology does not identify any specific concerns for this area. • Step 2 — Determine if an oil control facility/device is required: Oil control is required when a site has "high -use" characteristics, such as a commercial/industrial area, heavy vehicle storage, or arterial roadways with a high traffic count. This project does not meet the high -use criteria, so oil control is not required. • Step 3 — Determine if infiltration for pollutant removal is practicable: The project is not suitable for infiltration. • Step 4 — Determine if control of phosphorus is required: Phosphorus control is not required. PACE l PAGE 15