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e e e FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado Upper Collection System The upper collection system is shown in plan view on Sheet 2 and is profiled in sections on Sheets 3 and 4. At first glance, the upper collection system may seem complicated, but its functionality is actually relatively simple. Overbank flooding is predicted to occur between Sawmill Gulch Cross-Sections No. 58 and No. 57 (36 cfs) and also between Cross-Sections No. 56 and No. 55 (32 cfs). These combined flows, totaling 68 cfs, will be directed to a culvert inlet via a combination of minor regrading, concrete retaining walls, and at-grade retaining walls. Detailed calculations for this system are presented as Attachment C. The flooding from the area of Cross-Sections No. 58 to No. 57 will flow generally northward until redirected by a low retaining wall and minor regrading to channel the water slightly to the west. This water will spill over an at-grade retaining wall into the concrete-walled channel that leads to the 48-inch culvert. The 32 cfs of flooding anticipated to occur from the vicinity of Sawmill Gulch Cross-Sections No. 56 to No. 55 will be stopped from the natural flow path by a concrete retaining wall/flood wall adjacent to the proposed building. The base of the wall is at approximately elevation 9,722, while the top of the wall reaches elevation 9,728. The channel banks adjacent to the downstream limit of this wall are approximately at elevation 9,725. It is anticipated the water will flow over the stream bank, drop downward following the existing, natural topography, until reaching the wall. Water will pond to a depth of no more than elevation 9,724.5, and water will follow the hydraulic gradient southeasterly to the 48-inch diameter culvert. This water joins the flood waters spilling into this concrete channel from the area of Cross-Sections No. 58 to No. 57. The basic hydraulic control for the upper collection system is the inlet of the 48-inch diameter culvert. This structure is placed so that a design flow of 118 cfs, a 50 cfs safety factor above the predicted flow of 68 cfs, will result in a headwater elevation on the culvert of approximately 9,724.5. The culvert will be controlled by inlet conditions, as the pipe slope is approximately 3.6 percent, which is quite steep hydraulically. The top of the flood wall, which forms the retaining wall for the upper collection structure and provides direct protection to the proposed building, is 971-055.010 Wright Water Enginee~, Inc. Page 15