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system (BMP T5.12). Pipe drains will collect the roof drain downspout runoff from the new duplex and connect the runoff to an onsite dispersion trench system (BMP T5.10B).. Overflow from both dispersion systems will follow existing natural (pre development) conditions and to the existing conveyance system on Holly Drive. Runoff from both the existing and developed conditions will be collected along the same system (existing storm system on Holly Drive). The proposed drainage design does not create a basin diversion and the entire project lies within a single threshold discharge area. This project will not have any significant impacts on the existing downstream drainage system. The added flows are less than 0.15 cfs (0.0585 cfs) over the existing 100 year condition. In addition, the necessary erosion control measures will be provided to protect the exposed soil during construction and once final grading is completed. Existing and developed basin maps are attached, see Figure2. MINIMUM REQUIREMENT NO. 5 — On-site Stormwater Management The total new plus replaced hard impervious surface is 6,779 sq. ft. The total effective, pollution-generating impervious surface (PGIS) is 3,254 sq. ft. Due to the project proposing more than 5,000 sf. of new impervious surfaces, Minimum Requirements#1 through #9 must be met. Because the project is within the City, the project must meet either the Low Impact Development Performance Standard or List#2 in table I-3.2 of Volume I of the SWMMWW. This project has chosen to fulfill the requirements of List#2. Refer to the feasibility list below for appropriate BMP selection or infeasibility description: For each surface, considered the BMPs in the order listed for that type of surface. Used the first BMP that is considered feasible. Feasibility of the BMPs were determined by evaluation against design criteria, limitations, and infeasibility criteria identified for each BMP in the SWMMWW; and Competing Needs Criteria listed in Chapter 5 of Volume V of the SWMMWW. Lawn and landscaped areas: Implementation of"Post-Construction Soils Quality and Depth" in accordance with BMP T5.13 in Chapter 5 of Volume V of the SV/MMWW is required for all disturbed areas subject to clearing and grading that have not been covered by impervious surface. T5.13 is implemented on this project—The T5.13 BMP Post-Construction Soil Quality and Depth is achieved by implementing best management practice. Soils that have been disturbed, will be amended according to the following procedures: a. Scarify subsoil to a depth of one foot. Drainage Report—9227 4t"Ave W Everett,WA 98204 Nov. 10,2022 Page 14