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<br />e <br /> <br />e <br /> <br />e <br /> <br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br /> <br />Tyra Tract D, with the water surface profile associated with the maximum flow limit of 248 cfs, <br />WWE compared the resulting estimating water surface with the top-of-bank elevations. The <br />water surface at 248 cfs extends beyond a topographic divide, where water would not return to <br />the Gulch. Using the average velocity and the flow area, we estimated that 36 cfs would leave <br />the channel. Therefore, WWE reran the model and reduced the channel flow at Cross-Section <br />No. 57 accordingly, leaving 212 cfs in the main channel. WWE found the channel capacity <br />between Cross-Sections No. 57 and No. 58 to be sufficient to carry this flow value without <br />additional out-of-bank flooding. <br /> <br />This is not the case between Cross-Sections No. 56 and No. 55, however. At the flow level of <br /> <br /> <br />212 cfs, the water surface profile exceeds the right bank elevation. Using the weir equation with <br /> <br /> <br />a C coefficient of 2.64 (Q = CLH15) to estimate the amount of flow reasonably expected to spill <br /> <br /> <br />out of Sawmill Gulch between channel Cross-Section No. 56 and No. 55, our calculations reveal <br /> <br /> <br />this to be 32 cfs. This leaves 180 cfs in the main channel downstream of Cross-Section No. 55. <br /> <br /> <br />We reran the model with a change in flow reflecting this case, and found that the channel <br /> <br /> <br />capacity was sufficient to carry 180 cfs downstream of Cross-Section No. 55 until reaching Four <br /> <br /> <br />O'Clock Road. Thus, in general, no additional overbank flooding occurs through this reach. <br /> <br />However, the flow restriction of the Four O'Clock Road culvert (an existing 5.5-foot wide x 4.5- <br />foot high corrugated metal pipe, 63 feet long at a slope of about 4.8 percent) allows only about <br />130 cfs to pass underneath Four O'Clock Road. Thus, ponding occurs immediately adjacent to <br />Four O'Clock Road. Flows in excess of 130 cfs will flow over the right bank (at elevation <br />9,716) and follow the right shoulder of Four O'Clock Road to the east. Under our calculated <br />scenario (Attachment A), this means that 50 cfs, out of the channel flow of 180 cfs, will flood <br />over the right bank of Sawmill Gulch at Four O'Clock Road. This flow will continue eastward <br />for approximately 80 to 100 feet where it will be joined by the flows that overtop the right bank <br />further upstream. These combined flood overflows comprise the split flow of Sawmill Gulch, <br />and they total 118 cfs. <br /> <br />971-055.010 <br /> <br />Wright Water Enginee~, Inc. <br /> <br />Page 10 <br />