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I <br /> 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 /> II Classification. <br /> • 3:1 Side slopes. <br /> • Six inches of freeboard over the overflow structure. <br /> I • One foot of ponding depth. <br /> • Eighteen inches minimum soil depth — void space utilized as storage in WWHM <br /> model with 40% porosity. <br /> I • 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 /> IInfiltration Rate <br /> The infiltration rate of the bioretention area can be calculated using the USDA Soil <br /> Iclassification. <br /> Table 4 — Onsite Soils <br /> IMap unit symbol Map unit name Rating <br /> 5 Alderwood-Urban gravelly <br /> land complex sandy loam <br /> I Table 3.7 --Recommended Infiltration Rates <br /> based on USDA Soil Textural Classification. <br /> Estimated Long- <br /> I *Short-Term Term(Design) <br /> infiltration Correction infiltration Rate <br /> Rate(in.ihr) Factor,CF (in.ihr) <br /> ' Clean sandy gravels and 20 10 <br /> gravelly sands(Le.,90%of <br /> the total soil sample is <br /> Iretained in the#10 sieve) <br /> Sand 8 4 2*** <br /> ILoamy Sand 2 4 0.5 <br /> Sandy Loam 1 4 0.25 <br /> ILoam 0.5 4 0.13 <br /> *From WEF/ASCE, 1998. <br /> I **Not recommended for treatment <br /> ***Refer to SSC--4 and SSC-b for treatment acceptability criteria <br /> IThe 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 /> IPACLAND Job# 50183001 Page 1 3 <br />