Laserfiche WebLink
<br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br />..DAFT) <br /> <br />The 32 cfs of flooding anticipated to occur from the vicinity of Sawmill Gulch Cross-Sections <br />No. 56 to No, 55 will be stopped from the natural flow path by a concrete retaining wall/flood <br />wall adjacent to the proposed building. The base of the wall is at approximately elevation 9,722, <br />while the top of the wall reaches elevation 9,728. The channel banks adjacent to the downstream <br />limit of this wall are approximately at elevation 9,725. It is anticipated the water will flow over <br />the stream bank, drop downward following the existing, natural topography, until reaching the <br />wall. Water will pond to a depth of no more than elevation 9,724.5, and water will follow the <br />hydraulic gradient southeasterly to the 48-inch diameter culvert. This water joins the flood <br />waters spilling into this concrete channel from the area of Cross-Sections No. 58 to No. 57, <br /> <br />The basic hydraulic control for the upper collection system is the inlet of the 48-inch diameter <br /> <br /> <br />culvert. This structure is placed so that a design flow of 118 cfs, a 50 cfs safety factor above the <br /> <br /> <br />predicted flow of 68 cfs, will result in a headwater elevation on the culvert of approximately <br /> <br />9,724.5. The culvert will be controlled by inlet conditions, as the pipe slope is approximately 3.6 <br /> <br /> <br />percent, which is quite steep hydraulically. The top of the flood wall, which forms the retaining <br /> <br /> <br />wall for the upper collection structure and provides direct protection to the proposed building, is <br /> <br />set at an elevation of 9,728. This provides for slightly more than the FEMA-required 3 feet of <br /> <br /> <br />freeboard for a flood wall and is also consistent as a safety wall for the plaza area adjacent to the <br /> <br />building. <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. <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 />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 />971-055_010 <br /> <br />Wright Water Engineers, Inc, <br /> <br />Page 15 <br />