Laserfiche WebLink
<br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br />(8DAFT) <br /> <br />Split Flow Assessment <br /> <br />The existing conditions model run for Sawmill Gulch was done under several assumptions of <br /> <br />channel flow rates prior to the final model run. Initially, the full 100-year flood peak of 248 cfs <br /> <br /> <br />was used. The initial model run revealed significant locations where water surface profiles <br /> <br /> <br />exceeded the top of the right bank elevations. This indicates that out-of-bank flooding will <br /> <br /> <br />occur, and created the need for further analysis. <br /> <br />Our out-of-bank flooding scenario is consistent with the previous worked performed by FLO <br />Engineering. We made a model run at the low end of flows that are anticipated to be carried by <br />this system: 130 cfs, which is the capacity of the existing culvert under Four O'Clock Road. At <br />a flow of 130 cfs, our model run did not indicate that overflows occur, and therefore the channel <br />is capable of handling 130 cfs without significant flooding. <br /> <br />We then looked more closely at the areas where overbank flooding is suspected to occur when <br /> <br /> <br />flows exceed 130 cfs. Our hand calculations are in Attachment B. Starting at the upstream limit <br /> <br /> <br />of Tyra Tract D, with the water surface profile associated with the maximum flow limit of 248 <br /> <br /> <br />cfs, we compared the resulting estimating water surface with the top-of-bank elevations. The <br /> <br /> <br />water surface at 248 cfs extends beyond a topographic divide, where water would not return to <br /> <br />the Gulch. Using the average velocity and the flow area, we estimated that 36 cfs would leave <br /> <br /> <br />the channel. Therefore, we reran the model and reduced the channel flow at Cross-Section No. <br /> <br /> <br />57 accordingly, leaving 212 cfs in the main channel. We found the channel capacity between <br /> <br />Cross-Sections No. 57 and No. 58 to be sufficient to carry this flow value without additional out- <br /> <br />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 />212 cfs, the water surface profile exceeds the right bank elevation. Using the weir equation with <br />a C coefficient of 2.64 (Q = CLH'-') to estimate the amount of flow reasonably expected to spill <br />out of Sawmill Gulch between channel Cross-Section No. 56 and No. 55, our calculations reveal <br />this to be 32 cfs. This leaves 180 cfs in the main channel downstream of Cross-Section No. 55. <br />We reran the model with a change in flow reflecting this case, and found that the channel <br /> <br />971-055_010 <br /> <br />Wright Water Engineers, Inc, <br /> <br />Page 9 <br />