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<br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br />(D..AFT) <br /> <br />Survey Datum <br /> <br />Normally, the datum used for survey purposes is a relatively unimportant technical detail. <br />However, it is a relatively important parameter regarding Sawmill Gulch. The reason for this is <br />that there are two separate sets of survey data, each of which uses a different datum. The basic <br />topography information for the detailed assessment of the Tyra Tract D parcel is based upon field <br />surveyed cross-sections utilizing an original 1984 survey. The survey was updated and revised <br />in 1989, 1996 and 1997. This survey is based on the NGVD 1929 datum, which is also <br />sometimes referred to as USGS Mean Sea Level datum. <br /> <br />The upper Sawmill Gulch study, extending from the upstream boundary of Tyra Tract to the <br />Town boundary, utilized base mapping provided by the Town of Breckenridge, which uses the <br />NA VD 1988 for its vertical datum. The NA VD 1988 datum is 5.91 feet higher in elevation (1.8 <br />meters) than the NGVD 1929 datum. <br /> <br />Thus, the two sets of floodplain mapping contained in this report, one for the Tract D parcel and <br /> <br /> <br />one for upper Sawmill Gulch, each have a different survey datum. The survey datum for upper <br /> <br />Sawmill Gulch is 5.91 feet higher in elevation than the survey datum of the Tract D mapping. <br /> <br />All elevations presented in this report are datum NGVD 1929, unless specifically noted <br />otherwise. <br /> <br />Existing Conditions Model <br /> <br />The starting point for modeling is at elevation 9,716.0 (NGVD 1929) at the upstream side of <br />Four O'Clock Road. This figure is consistent with FEC's modeling for lower Sawmill Gulch. <br /> <br />The HEC-2 program was utilized for existing conditions for Sawmill Gulch upstream of Four <br />O'Clock Road. The HEC-2 model output is included as Appendix A. Mannings "n" values of <br />0.080 (channel) and 0.120 (overbank) were used to reflect the channel roughness conditions. <br />These values are consistent with typical engineering practices, and they are appropriate for the <br />Sawmill Gulch channel characteristics: highly variable, some rock, and significant vegetation, <br /> <br />971-055,010 <br /> <br />Wright Water Engineers, Inc. <br /> <br />Page 7 <br />