9900 19TH AVE SE 2018-01-02 MF Import

3.0 UPSTREAM DRAINAGE AREA/ANALYSIS For purposes of this report, the upstream drainage area is defined as any area off -site contributing to the existing on -site wetland. All of the upstream area lies to thz south or southwest of the site. Approximately 16 acres of commercially developed property to the south and 4 acres of the Interstate 5 coffidor to the southwest are tributary to the existing wetland from off site. Some of the upstream commercial developments contain detention and water quality treatment and some do not. A detailed breakdown of upstream areas, including existing systems and proposed solutions is contained in Section 6 of this report. 4.0 DOWNSTREAM DRAINAGE ANALYSIS Any overflow from the wetland discharges from the site at the northwest corner and continues to the north. Runoff flows through additional wetlands located on Washington State Department of Transportation (WSDOT) right-of-way to the north before entering a 30-inch concrete storm sewer pipe system, several hundred feet. north of the site. This underground pipe system conveys runoff approximately 800 feet to the north where it outlets into a detention pond located in the WSDOT right - of --way at the intersection of the Interstate 5 access ramp and 19th Avenue S.E. Drainage released from this detention pond continues to the north via a tightline system and enters the upper reach of the south fork of Wood Creek. 5.0 COMPONENTS OF WETLAND/WATER QUALITY SYSTEM The existing 42,800-square foot on -site wetland is primarily a scrub shrub wetland providing several relatively low functions of water quality treatment. The following paragraphs describe the five primary functions of the wetland and how these functions will be replaced following development. A. Sediment Removal: The main function of the dead storage of a wetland is sediment removal. The dead storage volume of the existing wetland has been calculated to be approximately 1.25 acre foot based on surveyed cross sections at approximately 50-foot intervals across the wetland. Following development, the dead storage provided in the groundwater recharge system, two proposed open ponds, and an underground vault will equal or exceed the sum of the dead storage provided in the existing wetland and the dead storage required for the development of the Home Base site. Baffling will be provided to increase residence time, as required in each pond. B. Wetland Vegetation: The existing wetland vegetation provides nutrient uptake from runoff passing through the wetland vegetation., as well as an additional means of sediment removal. Because the existing wetlands consist of mostly scrub shrub plants, nutrient uptake is estimated to be minimal. Weiland treatment cells are proposed in each of the two ponds, containing 2-foot water depths and wetland vegetation. Increased stem density and baffling will be used to increase contact with wetland planting. (Please refer to the Wetland Report by Wetland Resources, Inc., for additional information) C. Live Storage: Live storage is the fluctuation in the water level due to storm events. Based on our field observations, the maximum fluctuation in water level within the wetlands is approximately 14 inches. We have proposed to provide a comparable live storage capability in the two ponds and vault it. excess of the on -site detention pond volume requirements. 5947.00I IKIJjulknl