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Fooodplain Delineation Report Hogue Pit Pace 2 of 3 The flow rate of 8,930 cfs for the 100-year flood (a flood event with a probability of occurring or being exceeded in any given year of one percent, or p=0.01) was obtained from FEMA's Flood Insurance Study'. This flaw rate was calculated by FEMA for flows in the Yampa below the confluence of the Yampa and Slate Creek. No other major tributaries flow into the Yampa between the confluence of the Slate and the aggregate mine. Thus the flow rate was considered valid for this location. STUDY METHODOLOGY The flood-plain and flood-way were delineated using methodology used by the U.S. Army Corp of Engineers (USACOE) and FEMA. Specifically, software developed by USACOE, HEC-RAS v. 2.2, was used to model the river. Input parameters were those required by the program. These included cross-section data, reach lengths, friction factors for the left overbank, the main channel and the right overbank, and flow information. The floodway was determined by using FEMA's standard method of assuming a theoretical blockage or encroachment within the floodplain. The encroachment is assumed to equally fill the left and right flood-plain area until it forces a maximum of one (1.0) feet of rise in the water surface elevation. In interpreting the results, the users of this report should bear in mind that flood-plain and Rood-way delineations represent freld conditions at the time survey data was collected. While most changes in channel configuration generally do not have a significant impact on the elevation of the floodplain, such changes can have an impact on the location of the flood- way. RESULTS Complete results are included in the attached Figures 1-9. Results show that the floodway along this reach is fully contained within the riparian corridor of the Yampa River. Based on 'Flood Insurance Study, Routt County, Colorado, Unincorporated Areas, FEMA, Sept. 29, 1989, Community Number - 080156 w~aaau~cs owma.*wr~s,uc G-1