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SI. Vrain Creek Floodplain Update March 5, 1998 4.0 UPDATED SURVEY INFORMATION COMPILED To resolve elevation differences in previous floodplain studies and provide the basis for future elevation certifications, BRW investigated the established benchmarks located within and near the Town of Lyons corporate limits. Six existing benchmarks set by Boulder County, Bureau of Reclamation (USBR), US Geological Survey (USGS), US Coast & Geodetic Survey (USC&GS), and National Geodetic Survey (NGS) were located. BRW searched for the three benchmarks, numbered RM1, RM2, and RM3, as shown on the current Flood Insurance Rate Maps (FIRM) Number 08013C0232 F and 08013C0234 F published by FEMA. Only benchmark RMl was located and subsequently used in the level loop survey. Benchmarks RM2 and RM3 have either been destroyed or were not properly described, and therefore were not included. BRW also located two of the five Boulder County benchmarks within or near the Town limits. Benchmarks SVC9 and SVC10 were located and used in the level loop survey. Benchmarks SVCll, SVC12, and SVC13 were located in areas where recent construction has been performed and are believed to have been destroyed. A benchmark set by the USBR was also located. The brass cap was stamped "Bureau of Reclamation BM 7 5303.324 1936". However, NVGD published the elevation for this benchmark as 5305.56, an apparent discrepancy of about 2.2 feet. This benchmark was included in the BRW level loop survey. Two other published benchmarks were located and included in the survey. The benchmarks are designated "Cliff" and "A329" and were set by NGS and USC&GS, respectively. Another two benchmarks were included in the level loop survey for reference and are designated "Gauging Station" and "2410". Since the previous floodplain delineation work by EPI and LA used the Boulder County benchmarks for the aerial topography, BRW performed the survey using the County benchmarks as the datum. A survey level loop was performed establishing and verifying benchmarks through the Town limits. Four new temporary benchmarks were also set and are listed as BM1 through BM4. A list of all the benchmarks (permanent and temporary) that were surveyed and included in the level loop and map showing their approximate location is included. These benchmarks should be used for all future construction to determine regulated floodplain elevations within the Town limits. All of the six published benchmarks located in the survey were within a :to.3-foot tolerance. The brass cap set by the USBR with the incorrect stamped elevation has apparently been a source of contention and confusion with respect to regulated floodplain elevations. BRW has determined that the elevation stamped on the brass cap is incorrect and should not be used. 5.0 FINDINGS The study found that the currently adopted 1980 flood insurance rate maps (FIRM) provide a reasonable generalized estimate of the floodplain boundaries within the St. Vrain Creek along the Town of Lyons. The EPI and LA studies provided an enhanced \\blW _den2\Admin\KIMBERL Y\35600report.doc 3