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<br />. <br /> <br />e <br /> <br />. <br /> <br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br /> <br />while 50 cfs is forced over the right bank of the channel. This flow then follows Four O'Clock <br />Road and soon joins with the overbank flooding that occurs from the upstream areas between <br />Cross-Sections No. 55 and No. 56, and Cross Sections No. 57 to No. 58. <br /> <br />The flood releases from the main channel of Sawmill Gulch from Cross Sections No. 57 to No. <br />58 and Cross-Sections No. 55 and No. 56 combine together and flow through a historic <br />secondary channel roughly parallel to Sawmill Gulch. The combined flow is 68 cfs, which <br />results in shallow flooding through the secondary channel. This continues in a northerly to <br />northeasterly direction until it meets the area adjacent to Four O'Clock Road. At this location, <br />the overbank flow of 68 cfs joins the 50 cfs that is forced out of the Sawmill Gulch channel by <br />the Four O'Clock Road culvert, resulting in 118 cfs of flow that parallels Four O'Clock Road. <br />Discussions with the Town of Breckenridge indicate that they are developing plans for a storm <br />sewer system to capture this potential 100-year overflow, and convey it in a storm sewer system <br />parallel to Four O'Clock Road until reaching the Blue River. <br /> <br />Split Flow Assessment <br /> <br />The existing conditions model run for Sawmill Gulch was done under several assumptions of <br />channel flow rates prior to the final model run. Initially, the fulllOO-year flood peak of 248 cfs <br />was used. The initial model run revealed significant locations where water surface profiles <br />exceeded the top of the right bank elevations. This indicates that out-of-bank flooding will <br />occur, and created the need for further analysis, <br /> <br />Our out-of-bank flooding scenario is consistent with the previous worked performed by FLO <br />Engineering. WWE made a model run at the low end of flows that are anticipated to be carried <br />by this system: 130 cfs, which is the capacity of the existing culvert under Four O'Clock Road. <br />At a flow of 130 cfs, our model run did not indicate that overflows occur, and therefore the <br />channel is capable of handling 130 cfs without significant flooding. <br /> <br />WWE then looked more closely at the areas where overbank flooding is suspected to occur when <br /> <br />flows exceed 130 cfs. Hand calculations are in Attachment B. Starting at the upstream limit of <br /> <br />971-055.010 <br /> <br />Wright Water Engineers, Inc. <br /> <br />Page 9 <br />