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11400 SE 8th Street P 425-453-9501 <br /> Suite 345 F 425-453-8208 L A ND <br /> Bellevue,WA 98004 WWW.PACLAND.COM <br /> Memorandum <br /> It was assumed that the contributing areas would be developed to a maximum of 90% <br /> impervious area, or 18.81 acres, which resulted in 90,000 cubic feet of storage. Additionally, a <br /> 0.46 correction factor was applied for a total storage volume of 130,500 cubic feet. <br /> Per the current site build-out for Lots 3C, 3D, 3E and 23rd Drive W., including the proposed <br /> canopies, the total impervious area draining to the large pond is approximately 12.4 acres. This <br /> is an impervious coverage of 59%, which is well below the 90% coverage the pond was <br /> designed for. Therefore, the playground canopies comply with the previously approved and <br /> constructed stormwater site plan. See the attached Basin Exhibit for a delineation of the <br /> contributing basins. <br /> East Basin <br /> The area within the east basin is detained in the smaller pond located in the northeast portion <br /> of Lot 3D. This pond was sized in 2008 by PAC LAND and includes the following areas: <br /> Contributing Basin Area(acres) <br /> Eastern Lot 3C 3.33 <br /> Eastern Lot 3D 4.21 <br /> Total 7.54 <br /> In sizing this pond, it was assumed that the 4.21 acres from Lot 3D would be developed to a <br /> maximum of 90% impervious area and the 3.33 acres from Lot 3C would be 77% impervious, <br /> for total impervious area of 6.34 acres. The resultant pond design volume is 34,053 cubic feet, <br /> and after applying a correction factor of 0.44, the total storage volume of this pond is 49,036 <br /> cubic feet. <br /> As Lot 3C is fully developed and per the site build-out on Eastern Lot 3D, the current total <br /> impervious area draining to the east pond is 3.13 acres. This results in an impervious coverage <br /> of 41.5%, which is well below the 84% coverage the pond was designed for. See the attached <br /> Basin Exhibit for a delineation of the contributing basins. <br /> Basin Breaks <br /> It appears that the predeveloped basin breaks were maintained with the current site grading <br /> and conveyance systems. However, detailed field studies would be necessary to confirm the <br /> exact location of the basin break in the developed conditions. <br /> 2/A— <br />