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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 />The floodplain assessment for Sawmill Gulch above Four O'Clock Road includes nine sets of <br />field-surveyed cross-sectional data for the 450-foot reach of stream through Tract D. For upper <br />Sawmill Gulch, extending to the Town border, twenty additional cross-sections were utilized for <br />the hydraulic modeling encompassing a distance of approximately 4,800 feet. These sections <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 about 130 cfs. <br />Secondly, (Attachment B) hand calculations were performed to provide quantity estimates of the <br />bank overflow where the HEC modeling indicated that channel capacity was inadequate to carry <br />the 100-year flood. This comprises what we have termed the split flow analysis, which is <br />discussed in more detail later in this report, <br /> <br />Survey Datum <br /> <br />There are two separate sets of survey data and mapping, each of which uses a different datum. <br />The basic topography information for the detailed assessment of the Tyra Tract D parcel is based <br />upon field surveyed cross-sections utilizing an original 1984 survey, The survey was updated <br />and revised in 1989, 1996 and 1997. This survey is based on the NGVD 1929 datum, which is <br />also the USGS Mean Sea Level datum. <br /> <br />The upper Sawmill Gulch base mapping, extending from the upstream boundary of Tyra Tract to <br />the Town boundary, was provided by the Town of Breckenridge and uses the NA VD 1988 for its <br />vertical datum. The NAVD 1988 datum is 5.91 feet higher in elevation (1.8 meters) than the <br />NGVD 1929 datum. <br /> <br />Thus, the two sets of floodplain mapping contained in this report, one for the Tract D parcel and <br />one for upper Sawmill Gulch, each have a different survey datum. The survey datum for upper <br />Sawmill Gulch is 5.91 feet higher in elevation than the survey datum of the Tract D mapping. <br /> <br />971-055.010 <br /> <br />Wright Water Enginee~, Inc. <br /> <br />Page 7 <br />