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CITY OF EVERETT STORMWATER MANAGEMENT MANUAL <br /> C. Moderately high runoff potential. Soils having slow infiltration rates when thoroughly wetted, <br /> and consisting chiefly of soils with a layer that impedes downward movement of water or soils <br /> with moderately fine to fine textures. These soils have a slow rate of water transmission. <br /> D. High runoff potential. Soils having very slow infiltration rates when thoroughly wetted and <br /> consisting chiefly of clay soils with a high swelling potential.soils with a permanent high water <br /> table, soils with a hardpan or clay layer at or near the surface,and shallow soils over nearly <br /> impervious material. These soils.have a very slow rate of water transmission. <br /> The following table shows the hydrologic group of soils present in the Everett area: <br /> Table 5.5 Hydrologic Groups(HG)for Soils in the City of Everett <br /> Soil Type HG Soil Type HG <br /> Alderwood C Mukilteo D <br /> Bellingham C Norma D <br /> Cathcart C Pastik C <br /> Custer C Puget C <br /> Everett A Puyallup B <br /> Indianola A Snohomish D <br /> K itsap C • Sumus C <br /> Lynnwood A Tokul C <br /> McKenna D Winston A <br /> The Soil Conservation Service(SCS)also developed"curve number"(CN)values based on soil type and <br /> land use.Table 5.6 shows the CN values, by land use description, for the four hydrologic soil groups. <br /> These numbers are for a 24-hour duration storm and typical antecedent soil moisture conditions preceding <br /> 24-hour storms in western Washington. <br /> The following are important criterialconsiderations for selection of CN values: <br /> I. Separate CN values must be selected for the pervious and impervious areas of an urban basin or <br /> subbasin. <br /> For single family residential areas,the percent impervious given in Table 5.6 may be used to <br /> compute the respective pervious and impervious areas. For other development(commercial, <br /> multi-family), the percent impervious must be computed from the site plan. <br /> 2. The pervious area CN value is generally a weighted average of all the pervious area CNs within <br /> the subbasin. CN values can be area-weighted when they apply to pervious areas of similar <br /> CNs(within 20 CN points). Where pervious areas have high and low CNs.separate <br /> hydrographs should he generated and summed to form one hydrograph, unless the low CN <br /> areas are less than 15 percent of the subbasin. <br /> Volume i- Minimum Technical Requirements <br /> 5-5 <br />