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

SAGE HOMES NORTHWEST, LLC HARBORS EDGE DRAINAGE REPORT AUGUST 2022 The wetland located around the Edgewater Creek, where the conveyance pipes discharge is classified as a Category III wetland with a habitat rating of 6. See the memo from Wetland Resources for details. Figure 1-3.5 in the SWMMWW shows the required Wetland Protection Levels that apply to the project. The levels are listed below as well as how the project is addressing them. • General Protection The existing wetland around Edgewater Creek will not be disturbed by the construction of Harbor's Edge. The existing outfall from the existing conveyance system does not need to be disturbed. Therefore, this condition will be met. • Protection from Pollutants The project will provide Modular Wetlands to treat the stormwater before entering the detention vault. Since these facilities meet the water quality requirements, they are acceptable. • Wetland Hydroperiod Protection (Method 2) The purpose of this step is to maintain the existing wetlands annual fluctuations in water depth and the timing as closely as possible. This project falls under the category of Method 2; which requires using a site discharge volume model to evaluate hydrologic changes in a wetland, with no additional wetland monitoring requirement. Western Washington Hydrology Model 2012 (WWHM) was used to evaluate the hydrologic changes. The WWHM model compares the proposed site conditions to the existing site conditions and if there is more than a 20% difference of the total volume of water into the wetland per day then it is classified as fail. The site has failed because the volume of water leaving the site is less than the 20% of the existing conditions. The detention vault is designed to historical conditions which means that the predeveloped conditions are modeled as forested. The allowable release rates from the detention vault are 50% of the 2-year through the 50-year. These pre -developed rates are going to be lower than existing conditions because this site is a redevelopment site. Therefore, to match the volume of water leaving the site and entering the wetland the flow control requirement would not be met. The flow control requirement is currently meeting the City of Everett stormwater code, if we meet the wetland hydroperiod analysis, we will not be meeting the flow control requirement. Given that the existing conveyance system discharges into a wetland on a steely sloped ravine and then into Puget Sound, the flow control requirement was deemed to be the most important. See Appendix G for the model results. This site is part of the Edgewater Creek Drainage Basin. The Edgewater Creek Drainage Basin is approximately 229 acres. The project site is 1.63 acres. As this site is 0.7% of the area draining to Edgewater Creek, the changes in flow from the site area are minimal. 10. OPERATION AND MAINTENANCE MANUAL Operations and maintenance recommendations are provided in a separate document. PACE / PAGE 22