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IV. Site Conditions 8� Analyses <br />A. Existing Site Conditions & Existing Drainage <br />Site topography consists of land sloping at approximately 7% west to east across <br />the site. <br />Storm drainage control will be provided by piping of roof runoff and by overland <br />drainage of lawns to yard drains. Flows are then conveyed through storm drains <br />to a detention facility. There are some portions of the property that are too low in <br />elevation to be drained to the detention facilities, and it is proposed that those <br />areas be bypassed. Please n�te that all bypasse� arsas have been calculated, <br />and the storm water runoff volumes from the bypassed areas have been added <br />into the detention volume quantities as compensation for the bypass. <br />For the case where detention occurs in vaults, storm water is discharged through <br />restrictors and then conveyed through a stormdrain pipe to bioswales. For the <br />case where the detention pipe benF;ath the street is used for detention (used for <br />Basin C"), the water is also released through a restrictor, but water quality is <br />�� <br />provided by two feet of dead storage in a 6' diameter pipe. <br />Existing vegetation consists of the site being covered by second-growth forest. <br />Wetland Critical areas exist along the center corridor of the site, specifically, <br />along an existing Type IV stream. There are no indications of erosion problems. <br />A slight risk of erosion will exist in localized construction areas when land is <br />disturbed to build driveways and homes, but any eroded soils should remain in <br />the immediate construction area because slopes are moderate. Best <br />Management Practices, including interception ditches, hydro-seeding, and <br />mulching will be utilized in construction areas. <br />Site soils are described in the Snohomish County Soil Survey as Aldervvood — <br />Urban Land Complex, 2-8% slopes. These soils are moderately deep over a <br />hardpan and are moderately well-drained. Alderwood soils belong to Hydrologic <br />Group Class C. <br />8. Developed Site Conditions, Proposed Drainage, 8 Water Quality <br />The site is to be developed wit ., existing single-family dwelling and ten new <br />duplex housing units. The topo�raphy of the developed site will remain similar to <br />that of ihe undeveloped site with disturbance mainly for placing the duplex <br />structures, dri��eway, and utilities. Some minor grading may be done on each lot <br />to help achieve a level building site and to provide for a driveway. <br />Drainage calculations are based upon the SBUH Method. Detention vaults (with <br />no lids) are proposed for each of tiie two large lots that will have duplexes. The <br />roadway frontage runoff area alon� Rosewood Avenue (Detenion Basin "C") will <br />� <br />