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Federal Emergency Management Agency - Letter of Map Revision Two-Mile Canyon Creek - Boulder, Colorado Page 3 3.0 ENGINEERING METHODS 3.1 HydrolOlnc Analysis The hydrologic analysis for this study wall taken from the existing Flood Insurance Study (FIS) for the City of Boulder, Boulder Colorado, Community Number 080024 dated May 3, 1990 (Reference 2). Peak discharge-drainage area relationships for this n:ach are shown in Table 1. 3.2 Hydraulic Analysis Water surface elevations of the selected recurrence interv;als were computed using the USACE HEC-2 step-backwater computer program (Reference 3). Cross sections used in the backwater analyses for the study were developed using aerial photography compiled in October 1981 (Reference 4) and supple:mented with on-site field surveying at selected locations. Cross sections from the original flood insw:ance sUldy were used, where applicable. In most cases, the original cross sections were rotated to reflect the actual field conditions based on the aerial topography. Additional cross sections were added at the roadways where needed to depict roadway overtopping conditions. A split flow analysis has been utilized between 13th Street and 8th Street. Overtopping of the right overbank is present at the 90 degree bend in the channel. The HEC-2 analysis indicated that approximately 25% of the 100-year flow escapes the main channel at this location and continues along 8th Street and Hawthorne Avenue until rejoining the main channel flows at 13th Street (please see "Hawthorne Street Multiple Profile Run, Appendix). Location of selected cross sections used in the hydraulic analyses are shown on the Flood Boundary and Floodway Maps (Exhibit 1). Roughness factors (Manning's "n") used in the hydraulic computations wl~re taken from the existing study and confirmed by :field observation. Roughness values for this study range from 0.02 to 0.10. Starting water surface elevations develop(:d for the previous study were utilized in this re-analysis. The hydraulic analyses for this study is based on fully obstructed culverts, as determined by the City of Boulder through field inspections. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used for this study are shown on the Flood Boundary and Floodway Maps (Exhibit 1).