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VI-9 I I I I I I I I I I I I I I I I I I I fl oodpl ain 1 imits and water surfacf' profil e in thi s area for the CSDH channel'improvements. This new floodplain should be evaluated and aprroved by the regul ating agencies and by Adans County. Tennyson and Lowell, north of the proposed 1-76 embankment, will be overtopped by the IOO-year flood in excess of the allowable Urban Drainage and Flood Control District's (UD&FCD) criteria. Because of the roor benefit/cost ratio for structural improvements in thi s area, it is recommended that no improvements be made at thi s time north of the proposed 1-76 embankment. However, when the 1 ands north of 1-76 are ready for developnent, the property owners need to recognize the extent of the 100-year floodplain and provide for the needed channels and other necessary improvements for flood danage mitigation. In pl anning future developnents on the north side of 1-76, provisions should al so be made for a grass-lined drainage channel and new culverts to el iminate the overtopping of Tennyson and Lowell. The grass-lined channel \\Ould carry the 100-year flows in this part of the IOO-year floodpl ain, as shown on the Phase B Prel iminary Design Drawings sheets 13 and 14 in Appendix D. In order to reduce the depth of overtopping of Pecos Street to within the UD&FCD's allowable six-inch depth of overtopping for the IOO-year flood. it is recommended that the following improvements be made: addition of a 150- foot long bridge span to the existing bridge at Pecos, construction of a rock-l ined channel transition upstream and downstream of Pecos Street, rai se the existing grade of Pecos Street south of the bridge and Pecos and W. 64th Avenue north of the bridge, and provide a rock protected overflow section downstream of Pecos. These improvements lIOuld not change the 1 imits or water surface profile of the existing 100-year floodpl ain at Pecos Street. The depth of overtopping of Pecos Street will be reduced to six-inches or 1 ess over the crown of the road. The overfl ow wi 11 permit the flood waters downstrean of Pecos Street to flow in the same manner as the 100-year floodpl ain for existing channel conditions. The 100-year floodpl ain should be regul ated in thi s area even after construction of the Pecos Street Bridge, channel, and road improvements. Reach I (South Pl aUe River Confl uence to Huron Street Extended Sta. 0+00 to Sta. 153+00) Reach II A (Huron Street extended to I400-feet Upstream of the D&RGW Rail- road Bridge Sta. 153+00 to Sta. 272+30) Brannan Sand and Gravel company has constructed channel improvements from several hundred feet upstream of the South Pl aUe River to York Street (Sta- tion 4+00 to 29+80). At the present time, an earthen constriction at the downstream end of the channel improvements backs up 1 arge flows causing upstream flooding outside the 1 imits of the improved channel. Construction of 400-feet of rock-lined channel improvements downstream of the Brannan Sand and Gravel channel will el iminate the constriction and wi 11 prevent the 100-year flood waters downstream of York Street from flowing to the north, out of the main channel. The limits of the 100-year floodplain after the improvements are made are shown on sheet 11 of the prel iminary design pl an and profile drawings. This floodrl ain should be approved and regul ated by Ad iIllS County after the improvements are constructed. The addition of 200-feet of new bridge span to the Denver and Rio Grande Western Ra il road bridge, and 3100-feet of rock-l ined channel improvements upstreilll of the rai 1 road bridge to downstream of Federal Bo ul ev ard, wi 11 eliminate the overtopping of Federal Boulevard by the 1.00-year flood. TIle 100-year fl oodpl ain wi 11 be confined to the channel improvements. Between Stations 240+75 and 245+00, the 100-year channel transitions to an unimprov- ed reach and to an unmodified floodpl ain. Brannan Sand and Gravel Company has tentative pl ans for channel improvements downstream of Station 240+75 (i.e., from Station 204+00 to 243+00). How- ever, at the time of this study information was not avail able from Brannan Sand and Gravel Company on their proposed channel improvements. Unti 1 these pl ans have been approved and the channel constructed, the JOO-year flood- pl ain for the unimproved existing channel conditions should be preserved and regulated in this area.