Laserfiche WebLink
<br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br />(DIIAFT) <br /> <br />Generally speaking, the Master Plan values are accepted by the Town of Breckenridge, and they <br />were used by WWE for the hydraulic analyses for Sawmill Gulch. Specifically, the 100-year <br />flood peak for Sawmill Gulch in the vicinity of Four O'Clock Road is considered to be 248 cfs. <br /> <br />FLOODPLAIN ANALYSES <br /> <br />Methodology <br /> <br />The floodplain delineation work for the Tyra Tract D site and the upper Sawmill Gulch area were <br /> <br /> <br />conducted using a combination of computer modeling and hand calculations. Computer <br /> <br /> <br />modeling utilized resources available from the Hydrologic Engineering Center (HEC): HEC-2, <br /> <br /> <br />the water surface profile model. (HEC-2 has recently been superseded by the model HEC-RAS, <br /> <br /> <br />which was used to supplement the HEC-2 data.) HEC-2 utilizes hydraulic energy concepts to <br /> <br />provide an estimate for water surface elevations given data inputs of channel cross-sections, <br /> <br /> <br />channel characteristics such as roughness, and flow amounts. Many computer runs were utilized <br /> <br /> <br />to perfonn the evaluation, which ultimately were consolidated into two model runs: existing <br /> <br /> <br />conditions and proposed conditions. <br /> <br />The floodplain assessment for Sawmill Gulch above Four O'Clock Road includes nine sets of <br /> <br />field-surveyed cross-sectional data for the 450-foot reach of stream through Tract D. For upper <br /> <br /> <br />Sawmill Gulch, extending to the Town border, twenty additional cross-sections were utilized for <br /> <br /> <br />the hydraulic modeling encompassing a distance of approximately 4,800 feet. These sections <br /> <br />were taken from the topographic mapping provided by the Town. <br /> <br />Hand calculations were used to supplement the model in two general areas. The first <br />(Attachment A) was to calculate the capacity of the existing Sawmill Gulch culvert at Four <br />O'Clock Road to verify that its capacity at out-of-bank flooding conditions is 130 cfs. Secondly, <br />(Attachment B) hand calculations were perfonned to provide quantity estimates of the bank <br />overflow where the HEC modeling indicated that channel capacity was inadequate to carry the <br />I DO-year flood. This comprises what we have tenned the split flow analysis, which is discussed <br />in more detail later in this report. <br /> <br />971-055,010 <br /> <br />Wright Water Engineers, Inc, <br /> <br />Page 6 <br />