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3.2.1 Fourmile Canyon Creek The regulatory model is located in Appendix 3. The regulatory analysis used HEC-2 as the hydraulic model. The current LOMR supercedes all earlier submittals. The LOMR uses HEC-RAS version 3.1.1 (USACE, 2003) to identify flood profiles and to delineate the floodplains. The Fourmile Canyon Creek LOMR ties upstream of the City limits to Cross Section 505 in the regulatory model (Tie Model). Flood peak discharges for Cross Section 505 match those in the regulatory model. At the downstream end of the LOMR, as noted later in this section, the Creek follows two distinct flow paths to Boulder Creek. The main channel (south flow path) of the LOMR ties downstream to Boulder Creek at Cross Section 10, obtained from 1995-1996 FIS Profile 31P Station 934+75 (FEMA, 1995). Please note the starting water surface in the FIS is set at the vertical datum NOVO 1929, and 3.133 feet is added to the water surface elevation to convert to the vertical datum of the LOMR NA YO 1988. The left overbank flow (east flow path) terminates in a lake close to Boulder Creek, and its initial cross section begins at critical depth. As noted in the previous section, flood peak estimates have been lowered significantly due to the spill from Fourmile Canyon Creek to Wonderland Creek. As a result, water surface elevations in the LOMR cannot be compared directly to regulatory water surface elevations. HEC-RAS models developed for Fourmile Canyon Creek are found in Appendix 3. In addition, a HEC-RAS model of the spill area was developed and is also presented in Appendix 3. Floodplain profiles and the work maps used to plot the plan view of the 100- and 500-year floodplains are found in the set of 24" x 36" drawings. 3.2.1.1 Spills Near Broadway Street The south bank of Fourmile Canyon Creek cannot contain flood peaks in the channel during the 50-, 100-, and 500-year event in the vicinity of Broadway Street. Flood flows which leave the main channel, flow to the south/southeast, eventually leaving the Fourmile Canyon Creek Catchment and reaching Wonderland Creek. It is unlikely, in the immediate future, channelization or bridge improvements will be made on this section of Fourmile Canyon Creek to contain these events. As a result, after consultation with the City, UOFCD, and FEMA representatives, it was decided to recognize the spills as part of the flood situation within and between the two Creeks and adjust flood peaks in both Creeks to reflect the spills. Spill discharges were modeled using lateral weirs in HEC-RAS. Four lateral weirs, included two upstream of Broadway Street Bridge (labeled 466.09 and 481.59 in HEC- RAS model) and two located downstream of Broadway Street Bridge (labeled 437.99 and 441.09 in HEC-RAS model) were used to simulate overbank flows. Table 5 summarizes the overbank flow discharge, indicating the majority of the flows leave the channel upstream of Broadway. -13-