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3.0 UPSTREAM DRAINAGE AREAIANALYSIS <br />For purposes of this report, the upstream drainage area is defined as any area off -site contributing to <br />the existing on -site wetland. All of the upstream area lies to the south or Suuthwat of the site. <br />Approximately 16 acres of commercially developed property m the south and 4 acres of the Immune <br />5 corridor to the southwest are tributary to the existing we0amd from off site. Some of the upstream <br />wmrmcial developments wntain detention and water quality treaunent and come do not. A detailed <br />breakdown ofupsueam areas, including existing syserm and proposed solutions is contained in Section <br />6 of this report. <br />4.0 DOWNSTREAM DRAINAGE ANALYSIS <br />Any overflow from the wetland discharges from the site at the northwest toner and continues in the <br />norm. Runoff flows through additional wetlands located on Washington Sum Depanmaot of <br />Transportation (WSDO) right-of-way in the term before entering a 30-inch concrete storm sewn pipe <br />system, several hundred feel north of the sim. This underground pipe system conveys mnoff <br />approximately 800 feel m the north where it outlets into a detention pond located in the WSDOT right- <br />of-way at the intersection of the Interstate 5 as rump and 19th Avenue S.E. Drainage released from <br />this detention pond commum to the wrm via a tightline system and enters the upper reach of the south <br />fork of Wood Creek. <br />5.0 COMPONENTS OF WL-FLANDIWATER QUALITY SYSTEM <br />The existing 42,800-square foot on -site wetland is primarily a scrub shrub wedand providing several <br />relatively low functions; of water quality lratums. The following paragraphs describe the five primary <br />functions of the wedand and how these functions will be replaced following development. <br />A. Sediment Removal: The main function of the dead storage of a wetland is sediment <br />removal. The dad storage volume of the existing wetland has been calculated to be <br />approximately 1.25 acre foot based an surveyed cross sections at approximately 50-font <br />intervals across the wetland. <br />Following development, the dad storage provided in the groundwater recharge system, <br />two proposed open ponds, and an underground vault will equal or exceed the Sam of the <br />dud storage provided in the existing wetland and the dad storage required for the <br />development of the Home Base site. Baffling will be provided in increase residence time, <br />as required in each pond. <br />B. WetaM Vegetation: The existing wedabe vegetation provides nutrient uptake from mwff <br />passing through the wetland vegetation, as well as an additional mans of sediment <br />removal. Because the existing wetlands consist of mostly scrub sbmb plants, nutrient <br />uptake u estimated m beminimal. Wedand Imment cells are proposed in each of the two <br />ponds, containing 2-foot water depths and wetland vegetation. Increased stem density and <br />baffling will be used to increase contact with wetland planting. (please refer to the Weiland <br />Report by Wetland Resources, Inc., for additional information.) <br />C. Live Storage: Live Storage is the fluctuation in the water level due to storm events. Based <br />on our field observations, the maximum fluctuation in water level within the wetlands is <br />approximately 14 inches. We have proposed to provide a comparable live storage <br />capability in the two ponds and vault in excess of the on -site detention pond volume <br />requirements. <br />59417.00I In1Ajn/ I <br />