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Lazy Glen Trailer Court, located upstream of the Highway 82 bridge, is on a historic floodplain. Fill and small dikes have been placed to control flooding. Detailed analysis shows that the smaller floods such as the 5 or lO-year flood, would not flood any portion of Lazy Glen but the 100-year flood would overflow into the area at two points. Only one ranch building occupies the floodplain in the reach above the Lazy Glen Trailer Court. The upstream reach of the study area is largely confined within well-defined natural banks. ..~'1 ~~~.. 'V-.-- -- Page 14 July 24,2000 .~tiIi.' s Mmixn.,;,...r~_ " ...............~I.::L~ -- The flood profiles may be used in areas where controversy arises over the 100-year flood boundary on the flooded area sheets. Since the flood profile plates give the elevation and distance or stationing from a known point, the high water elevations can be surveyed on the ground to alleviate any discrepancies on he base map. Page 15 July 24, 2000 The flooded area maps, which show area is shown on the flooded area maps the boundaries of the by cross-hatching. 100-year and 500-year floods. The low hazard The flood frequency elevation and discharge data table, Table 3, lists the 10, 50, 100, and 500-year flood elevations at selected reference points. The flooded area sheets give the plan view of the flooded area on a contour base map, and the high water elevations for the 100-year flood can be interpolated from this. The flood profile plates show the streambed elevation and the high water elevations for all four frequency floods. Also, the cross section figures shown a graphical representation of the high water elevations at typical valley cross sections throughout the study reach. The hydraulic analyses the profiles are, thus, and do not fail for this study were based on unobstructed flow. considered valid only if hydraulic The flood elevations shown on structures remain unobstructed, operate properly, 5.2 Flood Elevations Starting water-surface elevations for Roaring Fork River calculated using critical depth at the beginning and end of the study. This is a reasonable assumption due to the steep gradient. The 500-year flood event flood can and do occur. the floodplain. is important in making the public aware that floods larger than the The 500-year flood event can also be used for regulating developments 100-year within The water surface elevations for floods of the selected recurrence intervals were computed through use of the Corps of Engineers' HEC-RAS backwater computer program. A total of 112 cross sections were analyzed for the hydraulic analysis of the Roaring Fork River and were secured from topographic mapping and field surveys. The locations of these cross sections are shown by reference point on the flooded area maps and the flood profiles in the back of this report. The 10, 50, 100, and 500-year flood elevations and discharges are listed in Table 3. Channel roughness factors (Manning's n) for these computations were assigned on the basis of field inspection of the floodplain areas The 10, 50, 100, and 500-year flood events were used as the flood frequencies for this floodplain analysis. Thus, the data developed in this report will be compatible not only for regulation purposes and State of Colorado H. B. 1041 designations, but are also for FEMA flood insurance rate studies. 5 1 and Dischar e SECTION 5 INTERPRETATION AND USE OF REPORT DATA 4.2 Hydraulic Anal SIS ort Town of Basalt, Ea le & Pitkin Counties, Colorado ort Town 0 le & Pitkin Counties, Colorado