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

D. Groundwater Recharge: Although the hydrogeologist has determined that the existing wetland has minimal groundwater recharge capability, proposed mitigation will consist of the design of a perforated pipe bedded in free -draining material under the proposed Home Base building to provide groundwater recharge with treated storm drainage runoff equal to or exceeding existing conditions. (Please refer to the Wetlands Report by Wetland Resources, Inc., for additional information.) E. Wildlife Habitat Value: Off -site mitigation on the Spane site, located downstream in the Wood Creek Basin, will provide enhanced wildlife habitat value to replace on -site wetlands habitat value. 6.0 DETENTION/WATER QUALITY TREATMENT ANALYSIS We have identified five major runoff sources that contribute drainage to the existing wetland. The five runoff sources consist of: (1) the 16th Avenue S.E. right-of-way; (2) the Family Tree aparunent complex; (3) the Interstate 5 corridor; (4) the Silver Lake Professional Center; and (5) the Home Base site. Each of these sources will be described, including existing and proposed conditions, in the following paragraphs. 16th Avenue S.E.: The 16th Avenue S.E. drainage system receives runoff from three areas in addition to the area within the 16th Avenue S.E. right-of-way. The northwest cornet of the Costco property and the 102nd Place S.E. Street improvements total 4 acres of recently developed area that is tributary to 16th Avenue S.E. Both areas contain detention and water quality treatment systems that are in accordance with Wood Creek Basin standards. A third source of drainage to the 16th Avenue S.E. right-of-way is the mini -storage warehouse and north half of 102nd Place S.E., located immediately north of the Costco property. Drainage from these areas does not contain any detention or water quality treatment prior to entering the 16th Avenue S.E. system. The north end of 16th Avenue S.E., adjacent to 100th Street S.E., contains a detention pipe, which provides approximately 2,000 cubic feet of storage. This storage is exclusively for the area within the 16th Avenue S.E. right-of-way and does not include any upstream drainage, such as the mini -storage warehouse. All of the drainage currently flowing through the 16th Avenue S.E. storm drainage system will be directed into the off -site pond that is being reconstructed for water quality improvements. This pond is currently the detention pond for the Family Tree apartment complex and is required to store approximately 45,000 cubic feet of live volume. There is very little dead storage, approximately 6 inches at the bottom of the pond in the existing conditions. The live storage of this pond, as well as the live storage of the detention pipe located in 16th Avenue S.E., will be replaced in their entirety by the reconstructed pond. The 16th Avenue S.E. detention pipe will be abandoned in place in a dry condition, with each end plugged at existing catch basins. The 3.1 total acres, including 16th Avenue S.E. and the mini -storage warehouse, previously did not receive any water quality treatment. The rerouting of the 16th Avenue S.E. system into the Family Tree apartment complex pond will provide both deep cell and wetland treatment cell water quality treatment for this runoff. Family Tree Apartment Complex: The second source of runoff to the wetland is from the 6.9-acre parcel containing the Family Tree apartment complex. This runoff currently receives minimal treatment from an under -designed biofiltration swale located in the upper end of the Family Tree apartment complex detention pond. The reconstructed pond will provide water quality treatment in accordance with the Wood Creek Basin standards in the City of Everett manual. Additional dead storage will be provided in this pond to compensate for dead storage eliminated by filling of wetlands. The total live volume of storage required for the Family Tree apartment complex and 16th Avenue S.E. right-of-way, 5947.001 [KLrss/tTl