1802 75TH ST SW 2021-12-30 - Laserfiche WebLink

Targeted Drainage Report— Twentieth Ave Substation WO# 100026553 2.0 MINIMUM REQUIREMENTS According to the SMMWW, this project is subject to the Minimum Requirements (MR's) for new development and redevelopment in accordance volume I chapter 2 figures 1-2.4.1 and 1-2.4.2. The figures are available for review in Appendix A. Figure 1-2.4.1 The site has more 59.3% existing impervious coverage exceeding the 35% coverage which triggers use of figure 1-2.4.2. Figure 1-2.4.2 • The proposal includes roughly 11,000 square feet of disturbed area. • The proposal does not add 5,000 square feet,of new hard surfaces, convert % of an acre or more of vegetation to lawn or landscaped areas or convert 2.5 acres or more of native vegetation to pasture. • The project is not a road related project. • The project does not exceed 50% of the assessed value of the existing site improvements. The substation yard rock and underlaying CSBC layer do not meet the definition of a "hard surface" or an "impervious surface" per EMC 14.28.20. o The substation surface is not a permeable pavement or a vegetated roof and does not prevent or retard the entry of water into the soil mantle as described within EMC 14.28.020. The station yard (the area inside the station security fence) and a 3-ft to 5-ft wide gravel perimeter outside the security fence will be surfaced with substation rock. The rock surface provides a layer of resistance to help reduce the risk of step and touch potential; minimize weed growth; provide a clean and reasonably dry surface during wet periods; and dissipates rainfall impact preventing erosion. Substation rock is a poorly graded mix of crushed rock ranging from 1 inch to 3/8 inch with fines content of less than 1.5%. Station rock is placed 4 inches deep across the surface of the station yard and the perimeter. In place, station rock has a minimum void ratio of 0.30. The new substation rock will be underlain with 8 inches of Crushed Surfacing Base Course (CSBC) meeting the gradation and quality criteria in WSDOT Standard . Specification 9-09.9(3). ZGA tested samples of CSBC from Iron Mountain in Granite Falls and CEMEX in Everett for permeability and void ratio at 95% of the modified Proctor maximum dry density. Void ratio ranged from 0.44 (Iron Mtn.) to 0.235 (CEMEX) and permeability ranged from 58.3 in/hr (Iron Mtn.) to 47.1 in/hr (CEMEX). Refer Appendix B for testing data. Page 5