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1 ' S.H. 93 Split Flow - FUTURE FLOODS - Another area where a special floodplain analysis was completed included the area Flood Frequency and Discharge between U.S. Highway 36 and State Highway 93. A split flow condition exists along South Boulder Creek over the 2.2 mile reach which encompasses these road Determination of the 100-year flood event is based on analysis of recorded crossings. At both of these roadway crossings the existing bridges are high in historical precipitation and other valid data. Statistically, the 100-year flood ' relation to the western road embankment leading up to the bridge structure. has a one-percent chance of being equalled or exceeded in any given year. Floods Existing elevations along the western roadways leading up to each bridge are up of this magnitude or greater have occurred on South Boulder Creek in the past and to 10 feet below the road elevations at the bridges. As a result, a split flow will occur in the future. ' condition occurs at both road crossings. The bridge structures and a portion of the roadway adjacent to each bridge are not inundated during the 100-year flood. The 100-year flood is identified by the Urban Drainage and Flood Control As a result, there exist two separate, fairly distinct flow paths between State District, the Colorado Water Conservation Board and the Federal Emergency Highway 93 and U.S. Highway 36. Management Agency as the flood magnitude used for the regulation of floodplains and for the design of flood control improvements. In Colorado, the 100-year Flow that splits from the South Boulder Creek main channel at State Highway 93 is floodplain represents an area of state interest as defined in H.B. 1041. Due to ' conveyed in a separate channel to U.S. Highway 36. Hydraulic analyses indicates widespread utilization of the 100-year flood as a basis for floodplain that approximately 3850 cfs crosses the roadway at State Highway 93 and is regulation, it is commonly referred to as the base flood, regulatory flood , ' conveyed west of the main channel whereas 2100 cfs passes through the bridge and and/or intermediate regional flood. is conveyed in the South Boulder Creek main channel . These split flow portions of the 100-year flood are conveyed to a point approximately 3000 feet upstream of Discharge-probability profiles shown in Figure 2 graphically present the relative ' U.S. Highway 36. At this point the capacity in the west overbank area decreases magnitudes of the 10- , 50- , 100- , and 500-year flood events. This discharge as the split flows (left overbank and main stream channel ) converge. An area of information serves as an aid for regulating future floodplain development and shallow flooding connects the two split flow areas with flow returning from the also for planning and engineering of channel improvements and flood protection west overbank channel into the main channel of South Boulder Creek. The capacity measures. in the overbank area decreases to the point where approximately 1660 cfs of the total 6160 cfs (100-year) will overtop the highway near its intersection with Flooded Areas ' State Highway 57 . The remaining 4500 cfs passes through the bridge opening. Flood Hazard Area Delineation maps showing the limits of the 100-year floodplain The split flow returns to the main channel just downstream of U.S. Highway 36 are contained in the Appendix of this report (Sheets 1 to 21). Sheets 22 to 31 ' near South Boulder Road. present water surface profiles corresponding to the 10- , 50-, 100- , and 500-year flood events. Tabulations of discharges and computed water surface elevations ' A separate flood profile for the split flow reach between U.S. Highway 36 and for the 10- , 50- , and 500-year events are presented in Table 3. Table 4, State Highway 93 is plotted on Sheets 30 and 31. This overbank split flow reach entitled "100-Year Floodplain and Floodway Reference Data" presents information ' is hereafter referenced as the State Highway 93 split flow. Limits of the State pertinent to regulation of the 100-year floodplain. Table 4 also includes Highway 93 split flow floodplain are presented on Sheets 9 through 15. information related to flood discharges, channel thalweg elevations , and other descriptive data. 9