2013-10-16_REVISION - C1992080 (3)

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1 appears that the rip rap may be larger than designed, however, it is difficult to determine the actual D50 without a proper geotechnical analysis. From TR -11, the permanent diversion was designed to convey the 100 year peak flow of 346 cfs with 0.3 feet of freeboard. A channel roughness coefficient of 0.044 was used. As part of this technical revision, a HEC -RAS analysis was performed to determine the ability of the as -built channel to convey the same 100 year design event. The as -built survey topography was used to generate cross sections and channel slope. A channel roughness coefficient of 0.047 was used to simulate the coarser rip rap in the as -built channel. It was determined that the 100 year peak flow would be contained within the as -built channel. The HEC -RAS cross sections with 100 year water surface elevations are shown in Appendix A. ' A capacity analysis for the as -built twin 66" CMP culverts was not conducted as part of this technical revision since this was done by Goff Engineering in May 2007. That analysis concluded that the twin 66" ' culvert would pass the 100 year design event. The analysis is included in the appendix. IUpper Carbon Junction Channel (Abatement Step #2) ' The Channel Improvement Plan, Exhibit C2.0, shows the extents of the reestablishment of the Upper Carbon Junction Channel, This includes the reclamation /removal of the Carbon Junction Temporary Diversion, realignment of approximately 150 LF of the un -named tributary near the historic confluence ' with the Upper Carbon Junction Channel and reestablishment of the Upper Carbon Junction channel. The Carbon Junction Temporary Diversion runs along the easterly side of the Upper Carbon Junction ' Channel approximately 1400 LF from Sta. 26 +70 to approximate Sta. 36 +00. Reclamation of the temporary diversion consists of removing approximately 630 LF of the existing channel and berm from its current confluence with the Upper Carbon Junction Channel (Sta. 26 +70) upstream to the un -named ' tributary (Sta. 33 +00). The existing diversion berm from the un -named tributary upstream to the end of the berm ( =Sta. 36 +00) will be reclaimed /removed as well. ' The un -named tributary will be re- routed to its near historic confluence with Upper Carbon Junction Channel at approximate Sta. 31 +60. The new channel is approximately 150 feet long, 3' bottom width and 3' deep. It will be armored with 12" nominal diameter rip rap to prevent erosion. A HydroCAD ' analysis was completed to determine the 10 year design flow generated from the un -named tributary basin (Basin 2) and the capacity of the channel re -route (Appendix Q. The 10 year design flow of 23.58 cfs will be contained within the proposed channel cross section. Discharge velocities at the discharge of the re- routed un -named tributary to the Upper Carbon Junction channel are 4.6 fps and should not require additional rip rap at the confluence. At the current confluence of the existing Carbon Junction Temporary Diversion and the Upper Carbon Junction Channel an area of sediment deposition has formed creating a sump condition on the upstream side of the confluence. This portion of the Upper Carbon Junction Channel, approximately 100 LF, will ' be regraded to remove the sediment deposition and sump condition, and provide positive drainage along the channel. The regrading work should match the existing channel at the upstream and downstream ends, reestablishing the channel to its' near historic condition. 1