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<br />. <br /> <br />. <br /> <br />. <br /> <br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br /> <br />All elevations presented In this report are datum NGVD 1929, unless specifically noted <br />otherwise. <br /> <br />Existing Conditions Model <br /> <br />The starting point for modeling is at elevation 9,716.0 (NGVD 1929) at the upstream side of <br />Four O'Clock Road. This figure is consistent with FEC's modeling for lower Sawmill Gulch. <br />This was provided to the HEC program as a boundary condition based on the hand calculations <br />of Attachment A. <br /> <br />The HEC-2 program was utilized for existing conditions for Sawmill Gulch upstream of Four <br />O'Clock Road. The HEC-2 model output is included as Appendix A. Mannings "n" values of <br />0.080 (channel) and 0.120 (overbank) were used to reflect the channel roughness conditions. <br />These values are consistent with typical engineering practices, and they are appropriate for the <br />Sawmill Gulch channel characteristics: highly variable, some rock, and significant vegetation, <br />particularly when overbank flows occur. There were nine cross-sections utilized over <br />approximately 450 feet of Sawmill Gulch, beginning at the upstream face of the culvert under <br />Four O'Clock Road. The floodplain represents significant overbank flooding which escapes the <br />main channel of Sawmill Gulch. As discussed in the following section, Split Flow Assessment, <br />substantial quantities of the 100-year flow were calculated to exceed the capacity of the channel, <br />resulting in flooding over the right bank of the channel. At the upstream limit of the Tract D <br />parcel, the channel flow is 248 cfs. Overbank flooding begins to occur at Cross-Section No. 58 <br />and continues up to Cross-Section No. 55. Between Cross-Sections No. 58 and No. 57, WWE <br />estimated that 36 cfs would leave the main channel of Sawmill Gulch, resulting in a main <br />channel flow of 212 cfs. This 212 cfs is contained in the Sawmill Gulch channel between <br />stations No. 56 and No. 57. Additional out-of-bank flooding then occurs between stations No. 56 <br />and No. 55, releasing an additional 32 cfs from the main channel flow. This leaves 180 cfs as the <br />main channel flow from Cross-Section No. 55 downstream until Four O'Clock Road is reached. <br />At Four O'Clock Road, ponding occurs due to the restriction of the existing Four O'Clock Road <br />culvert. An amount of 130 cfs passes through the culvert and continues on into Sawmill Gulch, <br /> <br />971-055.010 <br /> <br />Wright Water Enginee~, Inc. <br /> <br />Page 8 <br />