2018-03-12-minutes-public-works-study-session

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Summary The modeled Zone A approach has promise if applied carefully and if adequate time is provided to complete each task. Unfortunately, numerous mistakes have been identified and need to be addressed. The most significant are: 1. StreamStats must be applied with caution within Kktitas County because it frequently underestimates discharges. Where it does, the proposed floodplain inundation boundaries are likely too narrow. 2. Cross sections are frequently aligned incorrectly with the most common problem being that the ends of the cross sections are turned up -valley creating a fictitious confined floodplain. Where this occurs, the proposed flood inundation boundaries are likely much too narrow. 3. Omitting bridge and culvert crossings simplifies the modeling effort but care must be taken to estimate inundation limits in the vicinity of these crossings, something that has yet to be done in the delineations provided. 4. Bank stations are frequently incorrectly positioned which leads to significant errors in definition of the Manning's n values and therefore water surface elevation computations. The Yakima River Zone AE model appears well constructed. The following significant issues were identified: 1. A significant reduction in the 100 -year discharge is proposed over the value used in the effective study. This reduction may be valid, but WSE would like to review the hydrology calculations for the proposed flows. 2. A number of cross sections are misaligned or contain date errors. 3. Ineffective flow areas need to be reviewed for continuity of down valley flow. Referent FEMA, 1981. "Flood Insurance Study: City of Cie Elum, City of South Cie Elum, City of Ellensburg, Town of Kittitas, and Kittitas County, Washington". WSE, 2015. "draft Corridor Plan: Yakima River Jeffries Levee to Yakima Canyon Habitat Enhancement and Flood Risk Management Plan" 5