2019-10-28-minutes-public-works-study-session

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nhc conditions model best replicates the flood inundation that is experienced by the property owners, the hydraulic model can then be used to compare this baseline condition to the proposed project condition. 4.2 Model Results The following sections provide a summary of the flood rise analysis hydraulic results for the existing and proposed conditions along Lick Creek. The County requested water surface elevations during the 2- and 100 -year flow to allow WDFW to assess potential impacts due to the project. The 2- and 100 -year USGS regression equation peak flows are provided in Appendix A. All results are in the NAVD88 datum. 4.2.1 2 -Year Flow Figures A-1 and A-2 in Appendix A provide water surface elevations during the estimated 2 -year peak flow (70 cfs) for the existing and proposed conditions, respectively. Figure A-3 provides a comparison of water surface elevations between the proposed and existing conditions to show how the proposed project may effect water surface elevations along Lick Creek. In general, there is minimal to no change in water surface elevations outside of the County right of way, through the private properties, as shown by the white color in Figure A-3. There is a reduction in water surface elevations upstream of the existing NF Teanaway Road culverts due to their removal as well as the additional conveyance provided by the proposed 40 -foot bridge. Immediately downstream, and through the realigned portion (approximately 325 feet downstream of the proposed crossing) of Lick Creek, there is an increase in water surface elevations (shown as red in Figure A-3) due to the proposed alignment currently being dry in the existing conditions. This area, based on historic aerial photos, is one of the historic alignments of Lick Creek. 4.2.2 100 -Year Flow Figures A-4 and A-5 in Appendix A provide water surface elevations during the estimated 100 -year peak flow (270 cfs) for the existing and proposed conditions, respectively. Figure A-6 provides a comparison of water surface elevations between the proposed and existing conditions to show how the proposed project may effect water surface elevations along Lick Creek. During existing conditions (Figure A-4 in Appendix A), the two NF Teanaway Road culverts do not have sufficient capacity to pass the estimated 100 -year peak flow without having backwater. This backwater, located upstream of the culverts, results in the forest service road on the southwest bank of Lick Creek to overtop (Figure 4-1). The 2D hydraulic model estimates approximately 44 cfs overtopping the forest service road, with the remainder of flow being conveyed through the two existing NF Teanaway Road culverts. As the flow overtops the forest service road, it follows a ditch along the west side of NF Teanaway Road, crossing several private driveways and overtops the NF Teanaway Road in two locations (Figure A-4). For modeling purposes, the model domain ended just south of culvert 9 in Figure 2-3 as the topography south of this area increases in elevation, thus directing the water to the north and overtopping the NF Teanaway Road back into Lick Creek. These locations of overtopping were confirmed based on discussions with Kittitas County staff, property owners, and evidence of erosion on the east side of the NF Teanaway Road embankment during the April 12th, 2019 site visit (Figure 4-2). North Fork Teanaway Road - Lick Creek Fish Passage Enhancement Project Flood Rise Analysis — Maintaining Creek Flows Along Teanaway Road