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<br />. <br /> <br />e <br /> <br />e <br /> <br />FLOODPLAIN REPORT - TyrB Tract D and Sawmill Gulch, Breckenridge, Colorado <br /> <br />set at an elevation of 9,728. This provides for slightly more than the FEMA-required 3 feet of <br />freeboard for a flood wall and is also consistent as a safety wall for the plaza area adjacent to the <br />building. The "plaza" elevation on the other side of the wall is to be 9,725:t, slightly above the <br />predicted water surface upstream. Thus, the wall is not a levee. <br /> <br />Downstream of the 48-inch culvert intake, the hydraulically steep culvert will pass the <br />conservative design flow of 118 cfs (again, a safety factor of 50 cfs) in open channel flow <br />conditions with a normal depth of2.6 feet. (Some variations occur due to slope changes.) <br /> <br /> <br />Manholes are specified at alignment changes in the 48-inch diameter culvert and these manholes <br /> <br /> <br />are specified to have shaped inverts to provide for hydraulically efficient flow conditions. <br /> <br /> <br />Generally, there is sufficient cover, however, to provide for a re-establishment of culvert inlet <br /> <br /> <br />flow conditions even if such shaped inverts were not provided. <br /> <br />Just upstream of the culvert outlet, minor inflows from local drainage systems for the Tract D <br /> <br /> <br />property and its easterly neighbor are directed to the 48-inch diameter culvert at a manhole <br /> <br /> <br />junction. These flows are anticipated to be on the order of less than 10 cfs, which have no <br /> <br /> <br />significant affect hydraulically on the design flow of 118 cfs. <br /> <br /> <br />At the culvert outlet, an energy dissipation structure is specified per a Bureau of Reclamation <br />design. This type of structure operates effectively with essentially no tailwater, which is <br />appropriate for this situation. Immediately below the outlet structure, the water will be forced to <br />spread out to approximate the pre-project conditions via a small (2 feet high or less) berm that <br />extends for 30 to 50 feet. Flow will pass over this grassed berm in a shallow flow condition and <br />be released in its historic flow path adjacent to Four O'Clock Road. <br /> <br />The flood wall elevation generally has a top-of-wall at or exceeding elevation 9,728. The only <br />exception is where this wall forms a cut-off between the building and Sawmill Gulch proper. <br />Due to environmental and aesthetic considerations, the wall height starts at elevation 9,726 <br />where it begins atop the right bank of Sawmill Gulch. The existing top-of-bank elevation is <br />slightly less than elevation 9,725. Thus, this minimal height wall is approximately 1.2 feet above <br /> <br />971-055.010 <br /> <br />Page 16 <br /> <br />Wright Water Enginee~, Inc. <br />