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Eagle and Colorado Rivers Floodplain Information Report Eagle and Colorado Rivers Floodvlain Information Report le County, Colorado Ea INTERPRETATION AND USE OF REPORT DATA SECTION 5 Colorado in the back of this report. The Channel roughness inspection of the of the 100 and floodplain analysis. and State of e were used as the flood frequencies for this this report will be compatible not only for regulation purposes tions, but are also for FEMA flood insurance rate studies. is important in making the public aware that floods larger than the lOO-year The 500-year flood event can also be used for regulating developments within The 500-year flood event flood can and do occur. the floodplain 5.2 Flood Elevations The flood frequency elevation and discharge data table, Table 3, lists the 10,50, 100, and 500-year flood elevations at reference points (cross-section locations). Base flood elevation contours are shown on the the lOO-year flood. The flooded area sheets give the plan view of the flooded area on a map, and the high water elevations for the lOO-year flood can be interpolated from this The flood profile plates show the streambed elevation and the high water elevations for all The flood profiles may be used in areas where controversy arises over the lOO-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 the base map. The starting (downstream) water surface for the Eagle River model was the known water surface elevations from the Colorado River model at cross section 44. This appears to be a reasonable and conservative assumption, because a coincidental flood peak in the Colorado River would create backwater upstream on the Eagle River through cross section 5. By viewing the Eagle River profile, it is apparent that either a large deposit of volcanic ash & debris, or channel degradation from the Colorado River propagating up the Eagle River, has caused a relatively steep slope in the Eagle River channel near the confluence (cross sections 1 through 13 show a steep slope verses 13 through 42 show a flat slope). Due to the steep channel slope in this area, a normal depth calculation would produce a lower water surface on the Eagle River, than the backwater elevation caused by a 100-year flood on the Colorado River. The upstream ending water surface on the Eagle River was computed as critical depth near the town limits of Minturn at a county bridge structure, a hydraulic control structure. This is a reasonable assumption due to the steep gradient narrow cham1el geometry, a.Tld limited conveyance capacity of the bridge. rise" floodway. standard of a "one-foot Table 4 shows the floodway data adhering to the national unobstructed flow. The flood elevations shown on structures remain unobstructed, operate properly, for this study were based on considered valid only analyses LHe, thus, ut Colorado River A portion of the Colorado River reach analyzed for this report was studied in detail by Wright Water Engineers, Inc. for Two Rivers Development Company, LLC in February 1998. FEMA approved a CLOMR for the proposed development to fill in the flood fringe near the gravel lakes along the north bank of the Colorado River at Dotsero. The results of the existing conditions analysis from the CLOMR study were used for comparison with this study. In general, the CLOMR study assumed a deeper channel thalweg and steeper channel gradient. However, the CLOMR study also assumed a 100-year peak discharge of 26,000 which is greater than the 23,600 cfs flow determined for this study. Therefore, in general, the water surface elevations shown in this report are lower than the approved CLOMR existing conditions model. Grading of the development site was occurring at the time of this study, however, all topographic mapping of the area occurred pre-development. The topographic mapping for the Colorado River was flown October 1998 when the flow was approximately 1300 to 1400 cfs. Although this is a relatively low flow for the Colorado River, it represents approximately 5.8% of the 100-year flow. On average, the average depth of water in the channel during the mapping was approximately 2 to 3 feet. Normal depth calculations were computed for variable channel widths, a channel gradient of 0.001 and composite roughness of 0.040. The resulting August 22, 2003 Page 12 M1 MlIixU.~~.. ~tJ--"-- -- 22,2003 August Page 11 iH1 MlIixD..~~IL 'U -,-''--- -'- shown by reference point on the flooded area maps and the flood profiles 10, 50, 100, and 500-year flood elevations and discharges are listed in Table 3 factors (Manning's n) for these computations were assigned on the basis of field floodplain areas. The attached flood hazard area delineation maps show the boundaries 500-year floods, as well as the floodway delineation. Ea and Discha The 10, 50, 100, and 500-year flood events Thus, the data developed in Colorado H. B. 1041 designa Flood Fre 5.1 total of four models are included with this report: 1. Colorado River Floodplain Model (10, 50 100, and 500- Y ear Profiles) 2. Colorado River Floodway Model (100-Year and Floodway Profiles) 3. Eagle River Floodplain Model (10, 50 100, and 500-Year Profiles) L1 Pt),olo D~"Ar Plr\l"'\rhu""" ""If",,,.10.1 (1 fV) V 1;'0......._ ...............:1 Dl............An......... D___.c.:l ~~ \ '. .&...J\.40......... ..........,......... ..I.. ..LvvunuJ U.1.VU\,.I.J. \.J.VV-.1. val. auu .I. ".l.VVuvvay .llUJ.l1c;;:)} A Starting water-surface elevations for the Colorado River were calculated using normal depth at the beginning of the study with a gradient of 0.00954 feet/feet. The upstream ending water surface was also computed as normal depth downstream of the 1-70 bridges at a gradient of 0.00244 feet/feet. Mapping indicates the channel gradient is greater upstream of the confluence with the Eagle River. mapping for contour base information. four frequency floods 5.3 if hydraulic