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<br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br />(I.I.AFT) <br /> <br />appurtenant facilities. Should flooding exceeds the 50 cfs design flow, surface flows would <br /> <br /> <br />probably pass adjacent to the shoulder of Four O'Clock Road with little resulting damage. <br /> <br />Hydraulic calculations for the lower collection system are provided in Attachment D to this <br /> <br />report for those readers who wish to pursue the details of the structure capacity. Here, however, <br /> <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 /> <br />9,716 (NGVD 1929). At that elevation, an estimated 130 cfs will pass through the Four O'Clock <br /> <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 Drawings 2, 3 and 4, the proposed design specifies a rectangular <br />intake structure 20 feet long by 4 feet wide, This structure will have an "open" grating cover to <br />allow water to drop vertically into the concrete box. The elevation of the structure's sill adjacent <br />to Sawmill Gulch has been set at elevation 9,715.2 to allow for approximately 0.8 feet of depth <br /> <br />of weir flow to occur over the sill, which has a capacity of 50 cfs. The open grating will then act <br /> <br />as a potential orifice intake to the water flow. As is typical of urban designs, we assumed that 50 <br /> <br />percent clogging of the grating may occur due to debris, Even with this assumption, there is still <br /> <br /> <br />capacity for inflow of over 100 cfs into the structure, far more than the anticipated design flow. <br /> <br /> <br />Thus, both the sill and the grating provide for the design flow to enter this structure. <br /> <br />Once in the concrete box, flow is then controlled by the culvert pipe that will convey the water to <br />the east. This pipe is specified as a 30-inch diameter pipe; it is approximately 210 feet long, and <br />the average slope of the pipe is approximately 7 percent. At this steep slope, flow into and <br />through the pipe will typically be controlled by the inlet conditions at the concrete box. Using <br />standard culvert inlet flow nomographs, a 30-inch diameter pipe with an invert elevation of9,71O <br />and a beveled intake will pass 55 cfs at a headwater elevation of 9,716. The plans call for the <br /> <br />971-055_010 <br /> <br />Wright Water Engineers. Inc, <br /> <br />Page 17 <br />