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

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occurs in the Unnamed Tributary 8 model (Figure 8). There is a large gap in the cross sections and therefore a large gap in the floodplain delineation. 9. Disconnected Floodplain Areas — Pools of water disconnected from the main channel show up in many of the modeled Zone A floodplain delineations. Automated mapping procedures for HEC -RAS often produce such disconnected areas, but these are typically either removed in final mapping or physically connected to the main channel delineation. WSE assumes these problems will be corrected in the final mapping. Zone AE Detailed Study of the Yakima River from Easton to Cie Elum In general, the Zone AE model for the Yakima River from Easton to Cie Elum appears reasonably wall developed; however, Issues have been identified which are described below and presented in Figure 9. Hydrology — STARR has reduced the magnitude of the 100 -year flow significantly which is the primary reason BFEs in the new study are lower and floodplain inundation limits are reduced when compared to the effective study. For example, the new model uses a flow of 24,095 cfs at a location just downstream of the confluence with the Teanaway River (cross section 85147), while the effective model uses 29,100 cfs at the same location (FEMA, 1981). Similarly, just upstream from the confluence the new model uses 19,828 cls while the effective model uses 25,200 cfs. Documentation as to how the new discharge was computed was not provided. WSE would like to review the hydrology calculations developed for this study. 2. Cross Section Orientation and Data — Several cross sections appear to be skewed significantly to the direction of flow or contain data errors. Several examples are highlighted in Figure 9. 3. Definition of Ineffective Areas -- Ineffective flow areas are defined liberally throughout the Yakima River made[, and most appear logical. However, WSE noticed a number of instances where continuity of flow along a floodplain is interrupted by questionable Ineffective flow designations. WSE recommends that STARR conduct a simple review of all ineffective areas to double check for continuity of down valley flow. In several reaches the 500 -year flood is conveyed through the floodplain at one cross section, but not the next. It appears Ineffective elevations were set to accommodate the 100 -year event, but not the 500 -year. Ineffective elevations need to be adjusted or ineffective areas added or deleted such that flow remains either consistently effective or Ineffective along a reach during the 500 -year flood; unless features are present that physically do block flow. 4. Downstream Boundary Condition - Normal depth procedures are used to estimate the downstream boundary condition for each profile; however, the slope applied appears to be too flat when compared to the slope of the river, valley, and energy grade line. 4