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

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4.2 HYDRAULIC ANALYSIS The Cedar Gulch HEC -RAS model was configured and run for the 100 -year flows listed in Table 3. The upper and lower Crystal Creek models were run for the five flow quantiles (10-, 25-, 50-, 100-, 500 -year events) shown in Table 3. Flows were specified at the upstream end of each reach, confluence locations, and downstream of major tributary inflows. Normal depth boundary conditions were applied at the downstream end of Cedar Gulch Creek and the lower Crystal Creek model reach, and at the downstream (east) boundary of the 2D area in Cie Elum. Friction slopes were determined based on average channel/basin slope at each location. The upper Crystal Creek steady ID model used a fixed water surface boundary condition that was based on the steady state water surface elevation output from the lower Crystal Creek model. Model simulations for Cedar Gulch Creek and Upper Crystal Creek were performed using the subcritical t flow regime option. Model simulations for the lower Crystal Creek ],* D model were performed using the subcritical unsteady flow option with constant inflows (pseudy state). For model stability, flows in the lower Crystal Creek model were ramped up to the V staff ipflow shown in Table 3 over approximately 48 hours. 4.2.2 MODEL RESULTS Model results for the 100 -year event on Cedar QWch Creek, fpr Crystal Creek, and Lower Crystal Creek are included in Appendix A. Flood profiles forth$Z]-; 50-, 1•, and 500 -year event on Crystal Creek are provided in Appendix B as 01P through 20P — mj-oed`ac�3rding to FEMA FIS profile guidelines and specifications. Mapping was completed"date'l replace the effective FEMA floodplain within the 5 ' study area. 4.2.3 FLOODWAY MODELING The "regulatory floodwa means chAgi*1 of the creek and the adjacent land that must be reserved in order to discharge* 100 -year fIciaod without increasing the water surface elevation more than a designation height (typic�N'"1_foot).SE defined a floodway for the entire Crystal Creek study reach. An effective floodway currently e4"r the lower 9900 feet of Crystal Creek but was defined as "coincident with the 100 -year boundary" (FtMA, 1981). WSE determined an updated floodway boundary through an encroachment analysis by the following steps: Within the lower model: Applied the effective floodway boundary to the updated model using blocked obstructions. The boundary had to be adjusted in several areas where the effective floodway boundary was outside the current 100 -year boundary. • Ran the model to simulate the 100 -year flood, with floodway enforced. Compared floodway water surface elevations to BFEs to determine if floodway rise was less than 1 foot at all cross sections. WATERSHED P a g e 19 SCIENCE 4 ENGINLERING