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FLOODPLAIN REPORT - Tyro Tract D and Sawmill Gulch, Breckenridge, Colorado (D.IAFT) The profile data for the existing conditions model, Tables 1 and 2, applies to the proposed conditions as well. HYDRAULIC STRUCTURE ANALYSES This section provides a description of the proposed facilities that are intended to capture out-of- bank flooding from Sawmill Gulch. One set of facilities captures flooding in the vicinity of Sawmill Gulch Cross-Sections No. 55 through No. 58; this is referred to the "upper collection system," and provides for the estimated out-of-bank flooding peak flow rate of 68 cfs with a safety factor of an extra 50 cfs. An additional 50 cfs is predicted as out-of-bank flooding at Four O'Clock Road. The facilities proposed to capture this flow are referred to herein as the "lower collection system." This captures flows that are forced out of Sawmill Gulch due to the existing culvert at Four O'Clock Road. Each system is discussed below. Upper Collection System The upper collection system is shown in plan view on Drawing 2 and is profiled in sections on Drawings 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. 971-055_010 Wright Water Engineers, Inc. Page 14