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Verizon Communications-Stormwater Site Plan Everett,WA <br /> • Infiltration rate was calculated using Table 3.7 in the 2005 DOE and the USDA Soil <br /> Classification. <br /> • 3:1 Side slopes. <br /> • Six inches of freeboard over the overflow structure. <br /> • One foot of ponding depth. <br /> • Eighteen inches minimum soil depth —void space utilized as storage in WWHM <br /> model with 40% porosity. <br /> • To create a more aesthetically pleasing landscape feature, the actual dimensions of <br /> the bioretention area do not exactly match the WWHM model design. However, <br /> the proposed bottom area is equal to or greater than used in the WWHM model. <br /> Infiltration Rate <br /> The infiltration rate of the bioretention area can be calculated using the USDA Soil <br /> classification. <br /> Table 4— Onsite Soils <br /> Map unit Map unit name Rating <br /> symbol <br /> 5 Alderwood-Urban gravelly <br /> land complex sandy loam <br /> Table 3.7--Recommended Infiltration Rates <br /> based on USDA Soil Textural Classification. <br /> Estimated Long- <br /> *Short-Term Term(Design) <br /> Infiltration Correction Infiltration Rate <br /> Rate(in./hr) Factor,CF (hull) <br /> Clean sandy gravels and 20 210` <br /> gravelly sands(i.e_,90%of <br /> the total soil sample is <br /> retained in the#10 sieve) <br /> Sand 8 4 2*** <br /> Loamy Sand 2 4 0.5 <br /> Sandy Loans 1 4 05 <br /> Loam 0.5 4 0.13 <br /> *From WEF-IASCE. 1998. <br /> **Not recommended for treatment <br /> * `*Refer to SSC-4 and SSC-6 for treatment acceptability criteria <br /> The Estimated Long-Term (Design) Infiltration Rate (in/hr) is used to model the bioretention <br /> area in WWHM. Since a correction factor is already incorporated into the infiltration rate, <br /> as shown in the table above, the "infiltration safety factor" in WWHM was set to 1. <br /> PACLAND Job#50183001 Page 13 <br />