2019-12-09-minutes-public-works-study-session

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�CENTURY Xk E N G t N E E R I N • Wetland mitigation design drawings. 3.2. FLOOD MITIGATION Issue: The preferred alternative identified by initial flood mitigation alternatives analysis requires construction of drainage improvements outside of the direct study area and within the limits of the 100 - year floodplain. This was determined to be unavoidable as all other on-site improvements that were evaluated resulted in a worsening of downstream flooding. Due to unexpected direct impacts outside the direct study area, additional cultural resources and wetland delineation efforts will need to be expanded to encompass the limits of flood mitigation improvements. Hydraulic modelling of the proposed flood mitigation indicates that the improvements will result in a reduction in downstream flooding between Whiskey and Mercer Creeks by adding additional flood storage infrastructure on the northwest side of the airfield. The added upstream storage and subsequent downstream flooding reduction are likely to affect the current 100 -year flood limits of Whiskey and Mercer Creeks. This revision in flood limits will require additional permitting and coordination with the Federal Emergency Management Agency (FEMA), which was not originally anticipated in the project scope. The County is also in the midst of revising the existing flood conditions shown in the current Flood Insurance Rate Map (FIRM). The proposal of drainage improvements which will further revise the FIRM will require close coordination between the project team and FEMA. A greater level of design will also be required to accurately perforin the "No Net Loss" and mitigation analysis associated with the County's Floodplain Development Permit application. In addition to the anticipated direct floodplain impacts, the Draft Environmental Inventory Memo has identified the potential for summer run steel head trout within the southwestern study area in Whiskey Creek. The Century West project team is unable to make an effects determination without further development of the flood mitigation and stormwater management design. It is likely that a Biological Assessment (BA) will be required during the Individual Permit process to determine effects to downstream federally -listed fish. Additional effort is required for items such as: FEMA coordination and permitting; Further flood mitigation design; • Site assessment and wetland delineations within the limits of flood mitigation impacts; ■ Cultural resources assessment within flood mitigation impact zone; • Effects to stream flow and downstream fish habitat related to proposed flood attenuation; 0 Further threatened and endangered species investigation and BA efforts as required. Section 4 of this memo, Permit Submittal Requirements, provides a list of the application materials required for the environmental permits, the level of engineering required for those materials and whether the effort required is included in the current scope of services or will require additional effort. The additional effort required to prepare and submit the environmental permits and to complete the NEPA analysis is documented in Section 0 of this memo, The Washington Department of Ecology must Bowers Field Airport Environmental Assessment Page 4 Advanced Design Needs for NEPA Analysis November 27, 2019