9425 18TH AVE W 2018-01-01 MF Import

CMI Homehuilder's Inc. WeUand Delineation and The City requested in their Revised Addendum to Final Mitigated Deterrninaticn on Non- Significance SEPA #50-98, Rev: #18-98 dated April 17, 2003, documentation that the wetland north of the property would not be drained due to construction of the created wetland up against the north property line. Wetland A extends onto the property to the north of the project site as mentioned in Section 4.2. This off-site wetland boundary was visually verified in the field at the same time as Wetland A. This wetland has very similar characteristics as Wetland A. It too is situated at the base of a genile west-facing slope, within a broad swale. The boundary for Wetland A and the wetland noRh of the property both border around the 500-foot contour interval. This contour extends northward and then siightly eastward approximately 40-50 feet beyond the property line. The rock wall shown a�ong the north property line (Sheet W2.0) was required in order to pull back the existing slopes to create wetland. The created wetland boundary borders around the 502 contour interval. The topography just north of the created wetland, on the adjacent property, is at the 505 contour interval, wetl abov2 the existing wetland boundary. In addition, the created wetland area was designed to be hydrologically supported from collected clean rooftop runoff dispersed along the outer edge of the buffer (see Section 7.1 below). This hydrological support will more than likely provide additional hydrology to the existing wetland north of the property line as interflow. Therefore, construction of the created wetland along the north property line should not inadvertently drai� the existing wetland north of the property lirie due to ;he existing slope in this area, and the fact that the off-site wetland is at the toe of this existing slope. Since the boundary of the off-site wetland extends slightiy eastward approximately 40-50 fee! north of the property line, creation of wetland in this area will result in no additional wetland buffers imposed on the prope;ty to the north of the site. The proposed project would provide a 37.5-foot enhanced buffer (50% reduction of standard 75- foot buffer) in the northern and southern portions of Wetland A and the created wetland area, and an averaged 75-foot buffer for the remaining buffers on the site. Enhancement measures will include the removal of non-native species in the area of buffer reduction and the instaliation of native plants and nabitat features (down logs, stumps) for increased wildlife habitat value (Sheet W3.0). Approximately three (3) feet of the outer edge of a portion of the averaged buffer in the centrai area of the site will be temporarily disturbed due to a 10-foot construction easement for the sanitary sewer installation. In addition to this, a dispersion trench will be constructed along the edge of Wetland A for the release of treated stormwater from the site. All disturbed buffer areas will be restored with a variety of native trees and shrubs and a split-rail fence and informative signage will be installed along the entire perimeter of the buffer to reduce human entry and disturbance to the existing wetiand and buffer ecosystem. 7.1 Hydrological Support and Grading Following construction, the created wetland and Wetland A will be hydrologicaily supported through the collection and dispersal of clean rooftop runoff as well as treated stormwater. The created wetland and Wetland A will receive collected clean rooftop runoff from the three developed basins on the site. This clean rooftop water wifl be routed to a series of infiltrationldispersion trenches located just outside the buffer boundary (Sheet W1.0). This form of support is intended to augment the wetland hydrology through the slow release of water along the outer edge of the wetland buffer. The infiltration trenches are designed to infilirate the 2-year storm event, which will provide hydrology to the wetland as interflow. Runoff in excess of the ground's capacity to accept the water (i.e., storm events cbove the 2-year event) will sheet flow through the wetland buffer, reaching the wetland areas as surface flow. This hydrological 20 May 2003 Taiasaea Consuttan�s, LLC 364GMi1Rpt4(20May03).doc PageS I�\