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4.0 HYDRAULIC MODELING AND ANALYSIS <br />WSE developed and applied one-dimensional (113) steady state HEC -RAS (v5.0.7) computer models to <br />simulate flooding along Cedar Gulch Creek and Crystal Creek upstream from Cie Elum. A separate 1D/213 <br />pseudo steady state HEC -RAS model was used to simulate flooding along the lower portion of Crystal <br />Creek including Cie Elum. The lower model encompassed the entire reach of Crystal Creek that was <br />previously studied using detailed methods. Model layout and extents for the three (3) models are shown <br />in Figure 2. <br />1D model reaches were developed by: <br />1. Developing model cross section layout and geometry data within ArcGIS 10.5 <br />2. Cutting initial cross section elevation data from 2011 LiDAR (FEMA, 2011) <br />3. Incorporating in -channel survey, where available, based on project survey collected by the County <br />4. Incorporating bridge and culvert data based on County survgV ind observations from WSE's July <br />22, 2020 site inspection. <br />5. Refining the model by updating bank stations, ineffecth*/ flow4�s, and Manning's n roughness <br />values based on site observations. `• ; <br />6. Applying peak flow (steady state) hydrology d inedthrough regional regression analysis to <br />simulate peak flood elevations and flooding extero"IL .' . <br />Development of the Crystal Creek 1D/2D modek0*"Wn Cie Elut `included the following additional steps: <br />1. Adding lateral structures to represent hio'growi the left (east) bank of Crystal Creek <br />2. Defining a 2D area within Cle EL*'lWed old .*11 LiDAR elevation data to route shallow overland <br />flow leaving the main chanp�F <br />3. Overlaying a computatiorh(meshA*WZ"Yea using a nominal grid size of 50 feet by 50 feet. <br />4. Refining the mesh "forcilr reaklines to increase model resolution near key flow controls. <br />5. Applying a detalw0nd' r Ia4 to the 2D area with Manning's `n' roughness values based on <br />land use in 204 aerial phot*aphy: <br />6. Applying unstead*e low hyckology with a constant inflow (pseudo steady state) to simulate peak <br />flood elevations and d*extents. <br />4.1 MODEL DEVELOPMENT <br />The UDAR data used in model development was collected in 2011 to support FEMA flood map updates in <br />Kittitas County (FEMA, 2011). Stream channel survey was collected by the County from October 1St to <br />November 26th, 2019 to support the current study. Survey was completed at 108 cross sections and 26 <br />water crossing structures including bridges and culverts. WSE additionally traveled to the study reaches <br />on July 22, 2020 to examine hydraulic conditions, observe crossing structures, and record structure <br />geometry. <br />IS4 WATERSHED P a g e 13 <br />$CWNCE & ENGiNEEHiNG <br />