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<br />. <br /> <br />e <br /> <br />e <br /> <br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br /> <br />existing top-of-bank. Our predicted water surface at this location is elevation 9,724, some 0.6 <br />feet below the top-of-bank. From this point eastward, the existing ground topography drops <br />away, while the top-of-wall rises gradually to its ultimate elevation of 9,728. Thus, at a location <br />only 5 feet eastward of the top of the right bank, the height of wall has increased from ground <br />elevation of about 9,723 to the top-of-wall at about 9,726.5. <br /> <br />Lower Collection System <br /> <br />The lower collection system consists of an intake structure, a pipeline and an outlet structure, all <br />adjacent to Four O'Clock Road. The design capacity of this system is 50 cfs, the difference <br />between the culvert capacity of Pour O'Clock Road, 130 cfs, and the predicted instream flow rate <br />of 180 cfs, which assumes out-of-channel flooding has occurred in the vicinity of the upper <br />collection system. The purpose of the lower collection system is to capture Sawmill Gulch <br />flooding and transport it so that it does not interfere with or endanger the proposed building or <br />appurtenant facilities. Should flooding exceed the 50 cfs design flow, overland surface flows <br />would pass adjacent to the shoulder of Four O'Clock Road. <br /> <br />Hydraulic calculations for the lower collection system are provided in Attachment D to this <br />report for those readers who wish to pursue the details of the structure capacity. Here, however, <br />we provide an overview of the structure operation. <br /> <br />It is anticipated that the Four O'Clock Road culvert will cause ponding to reach an elevation of <br />9,716 (NGVD 1929). At that elevation, an estimated 130 cfs will pass through the Four O'Clock <br />Road culvert, and higher flows will begin to overtop the right bank area adjacent to the culvert <br />entrance. Thus, elevation 9,716 was used as the design water surface for the lower collection <br />system intake. As shown on Sheets 2, 3 and 4, the proposed design specifies a rectangular intake <br />structure 20 feet long by 4 feet wide. This structure will have an "open" grating cover to allow <br />water to drop vertically into the concrete box. The structure's sill adjacent to Sawmill Gulch has <br />been set at elevation 9,715.2 to allow for approximately 0.8 feet of depth of weir flow to occur <br /> <br />971-055.010 <br /> <br />Wright Water Enginee~, Inc. <br /> <br />Page 17 <br />