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I Goldfinch Brothers Drainage Study August 27,2008 6. UPSTREAM—DOWNSTREAM ANALYSIS UPSTREAM CONDITIONS The Project Surveyor described a ridgeline just off the property to the north which he thought might represent a barrier to upstream flow contributions to the wetland. However, this could not be substantiated by field examination nor from the topography maps. It appears that there are 3.5 to 5 acres of upstream property to the north, which is immediately tributary to the site wetland system. Water can be expected to arrive at the wetland as surface flow or groundwater flow during the wet season. In addition, with Everett staff assistance, we have realized that there is a drainage pond sited on the "Sears property"just off the NW corner of the Goldfinch property. This pond releases to a 65LF infiltration trench parallel to the Goldfinch property line. There is a 12" storm stub shown on the 1997 Drainage Plan by Kegel & Associates, which reaches to the property line representing an overflow from the infiltration system provided for future downstream connection. This represents the engineered outlet for 1.5 acres of land to the northwest that is tributary to to the existing Goldfinch site. Since there is no evidence of surface flows from the northwest, this SEARS runoff is presumed to arrive at the wetland as a subsurface flow rather that overland. These facts combined with the Alderwood soil conditions led themselves to the following speculative conclusions: 1. The development of the Site Plan will remove only a small percentage of the land tributary to the wetland thus reducing the impacts of the development. 2. Alderwood soils are typically 30" to 36" deep to a definitive hard pan layer. Significant sub-surface flows through the project in conjunction with saturated conditions in winter would prevent pervious pavements from succeeding. 3. If the soils are verified to be as described above, the development of previous pavements underlain by large pockets of gravel would not perform satisfactorily as the void spaces are likely to be continuously occupied by groundwater creating a very undesirable condition for the pervious pavement. Therefore, pending the further analysis of sub-surface conditions and based upon cost considerations, perforated pavement designs have been tabled for this project. In order to address the Sears pipe at the property line, we have proposed to connect to the Sears overflow pipe and to provide emergency overflow capacity to the edge of the wetland, bypassing the proposed new development. To minimize the risk of future erosion, we propose to provide a 20-foot level spreader at the edge of the wetland buffer so that any future overflows from the 65-foot infiltration trench can reach the wetland as a sheet flow. Additional concerns about new wetlands located north of the northwest corner of Goldfinch have been directed to the Project Biologist for evaluation. I 1 11