2021-01-11-minutes-public-works-study-session

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The majority of the floodway was contained within the 11) portion of the model; however, a short section of floodway was extended into the 21) model to prevent surcharges greater than 1 -ft near the downstream model boundary. The floodway boundary in the 2D area was set by raising the elevation of the terrain surface outside of the floodway by a scaler (10 ft). For the upper Crystal Creek model, the downstream model boundary condition was set to the floodway watersurface elevation from the lower model. Floodway boundaries were generally set at the edge of the 100 -year floodplain and resulted in little to no rise. The resulting floodway data tables for Crystal Creek are included in Appendix C. No floodway was determined for Cedar Gulch Creek which will retain the approximate Zone A mapping designation. 5.0 MAPPING WSE completed floodplain mapping for the 100 -year event on Cedat Gulch Creek and the 100- and 500 - year events on Crystal Creek based on hydraulic modeling resulo, TXhew mapping will replace current "effective" FEMA mapping of the creeks shown on Fop panelss•,�530096000113, 53009502348, 5302990005A, 5300950231B, and 53009502328. The eW ve study of Cr al Creek and Cedar Gulch Creek used "approximate" methods and therefore was ppe ing the "Zone A" designation. Updated mapping for Cedar Gulch Creek will maintain the Zone A de! ation. Updated Mapping for Crystal Creek uses the Zone AE designation and includes a flo r—Propos rystal Creek mapping ties into effective mapping at the Yakima River in Cie Elum. Analy*. wal, iVtetl�d without consideration of backwater from the Yakima; however, backwate _-QQ-year. ing from the Yakima will control mapped flood elevations in the lower portions of C.46um. \ 5.0.1 Proposed Floodplain Mappr'it Floodplain mapping for C adar Gulch Creek are shown in the floodplain workmap exhibits in Appendix D. Exhlbitseorhd 2 coves 'hhe etre study area at the scale of the effective mapping (1:6000). Exhibit 3 covers Cie Eluns,,Aa finer scale! (1:3000). The automated floodplain mapping feature of HEC -RAS (RASMapper) was used to rate titer surface elevations and initial flood extents based on HEC -RAS model output and the DEM. T"stream reaches of both Crystal and Cedar Gulch Creek are steep (6- 20%) and confined within a ravine, the DEM (which was based on LiDAR) did not contain sufficient channel detail to accurately map in the areas between cross sections. Consequently, this resulted in "dry" creek sections, where the projected water surface elevation was less than the channel bottom elevation in the DEM. In these regions, WSE delineated the floodplain based on channel width at the bounding cross sections. Base Flood Elevation (BFE) contours were then created to represent the final 100 -year water surface elevation. BFEs were generated based on the modeled 100 -year water surface elevation and then trimmed to the edge of the final floodplain polygon. 19_01� WATERSHED 110 SCIENCE & ENGINEERJNG