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1 <br /> ' Addendum to Storm Drainage Report <br /> ' adjustments to the water/sanitary services. It is noted now for location <br /> of as-built condition and consideration of new design. <br /> ' Infiltration rather than detention was used on this site due to the <br /> characteristics of the soil. The low allowable release rates required an <br /> extremely large size of structural detention vault. For this reason, the <br /> ' porous pavement /infiltration was the original proposed remedy to <br /> provide flow control (detention) from the site. Space limitation is an <br /> additional problem in providing a structural detention vault. <br /> Four locations were considered for the location of the proposed flow <br /> control infiltration facility, the northwest corner below the rockery in <br /> the NGPA lot,the SW corner below the rockery in the landscaped area, <br /> ' the parking lot area in front of and south of the building,and the chosen <br /> location in the parking area north of the building. The NW and SW <br /> locations were not chosen due to accessibility and construction of a <br /> ' facility with steep side slopes causing a dangerous shoring problem. <br /> The two locations SW and South of the building were not chosen <br /> because of the existing sanitary main running along the south property <br /> line. Infiltration in the SW and South areas could impact the existing <br /> ' sanitary trench bedding, causing failure of the sanitary main. The <br /> location at the NW corner below the rockery was not chosen because of <br /> steep side slopes, accessibility for future maintenance, and difficulty in <br /> construction. The parking area north of the building was chosen <br /> because the infiltration trench would be constructed in a cut area within <br /> the native soil,has good accessibility,and ease of construction. <br /> The following outlines the methodology of storm drain design and the <br /> proposed remediation: <br /> ' I. Infiltration Facility: Sizing of infiltration facility by Western <br /> Washington Hydrology Manual (WWHM). A 32 feet wide, 45 <br /> feet long, 5 feet deep infiltration facility was found to release <br /> 98%of the continuous series of storms calculated by WWHM. <br /> ' An emergency overflow pipe is required to be provided for any <br /> situation,even for a full 100%infiltration system. <br /> 2. Easement Acquisition from the adjacent property owned by <br /> City of Everett is_needed for the overflow pipe andrel- <br /> 3. Storm Filter: Al impervious sr ces, rooftop, sidewalk, and <br /> pavement areas will be directed to a storm filter upstream of <br /> the infiltration to help filter suspended solids that could plug <br /> the infiltration facilities. <br /> ' 4. Pre-Settling Tank:A combination of 48"diameter pipe and 72" <br /> diameter manholes provide the volume required upstream of <br /> the storm filter to catch the larger solids prior to entering the <br /> storm filter or the infiltration tank. <br /> Seaway Center Lot 2A—Storm Drainage Addendum P;.&offc Tool Welding10351231DesfgnIRepor1s135123-storm-RDDENDUAf doc <br /> TVHPacific,Inc. December 2,2009/Page 2 <br />