1997-06-13_REPORT - M1981302 (16)

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' Condition 2 - 100-Year Floodalain Model - FEMA Certified Flood Protection Levee: ' Under Condition 2, the 100-Year Floodplain Model - FEMA Certified Flood Protection Levee Condition, the 100-year flood flows along South Boulder Creek were confined to ' the floodplain areas south and east of the levee. At U.S. 36, downstream of the levee, the 100-year floodplain expands to the west and eventually overflows onto U.S. 36. The ' discharge for this overflow was calculated using a split flow analysis between Cross Sections 22010 and 33020 in the main channel hydraulic model. The analysis assumed 1 the traffic median in the center of U.S. 36 acts as a barrier to the passage of flood flows. The HEC-2 hydraulic model for the certified flood levee condition is found in the Technical Appendix of this report. ' To identify the hydraulic feasibility of the overflow and to determine the extent of the floodplain south of U.S. 36 on the Flatiron Property, the flow path along U.S. 36 was defined and modeled using HEC-2. Two distinct flow paths were identified. One path, called the U.S. 36 Overflow, follows U.S. 36 to the west. The second, called the U.S. 36 ' South Overflow, flows south of the on-ramp at U.S. 36, into the existing ponds on the Flatiron Property, and eventually rejoins the U.S. 36 Overflow. Two HEC-2 models ' were developed to model these two separate flow paths and are included in the Technical Appendix to this Report. Both overflow models are hydraulically connected downstream at Cross Sections 90, ' 100, and 110, and upstream at Cross Sections 150 and 160. The flow regime at Cross Section 90 was assumed to flow at critical because of the steep drop in elevation ' immediately downstream. ' The flow paths along both overflow channels were examined for potential off-site spill areas. The discharge along the U.S. 36 South Overflow transported the overflow ' discharge with no offsite spill areas. However, discharge along the U.S. 36 Overflow 1 ' -17-