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<br />North St. Vrain & 51. Vrain Creeks
<br />Floodplain Update
<br />
<br />Town of Lyons
<br />
<br />North St. Vrain & St. Vrain Creeks
<br />Floodplain Update
<br />
<br />Town of Lyons
<br />
<br />TABLE 7
<br />NORTH ST. VRAIN - ST. VRAIN CREEK BENCHMARK INFORMATION
<br />
<br />Boulder County. Bureau of Reclamation (USBR), US Geological Survey (USGS), US Coast &
<br />Geodetic Survey (USC&GS), and National Geodetic Survey (NGS), The six were labeled '"SVC
<br />9'", '"SVC 10'", '"RM1'", '"CLIFF', ooHEADGATE'" and '"A329'", and are shown on the following
<br />Benchmark Location Map,
<br />
<br />POINT ELEVATION GENERAL LOCATION
<br />BM SVC 1 5738.49 Brass Cap Monument (BCM) North Side of Co, Rd. 80, approximately 0,9 mi.
<br /> West Hwy, 36 and Co, Rd, 80 Intersection
<br />BM SVC 2 5847,21 BCM South Side Highway 36 approximately, ,45 mj, West of Mile Marker 16
<br />BM SVC 3 5757,38 BCM South Side Highway 36 approximately, 0.1 mi. West of Mile Marker 16
<br />BM SVC 4 5614.61 BCM l' West of Highway Right-of-Way Marker, approximately 0,2 mj, West
<br /> of Mile Marker 17
<br />BM SVC 5 5544.18 Steel Axle in Boulder, South Side of Hwy, 36, approximately 0,7 mi, West of
<br /> Mile Marker 18
<br />BM SVC 6 5507.67 BCM North Side Highway 36. West of Co, Rd, 71, West of Box Culvert under
<br /> Hwy, 36
<br />BM SVC 7 5462,15 BCM West Side Highway 36, approximately 0,36 mj, West of Miles Marker 19
<br />BM SVC 8 5448.45 HeM Southwest Comer Intersection of Applevalley Road and Private Drive,
<br /> approximately 0.72 rni. West of Hwy. 36 and ApplevalIey Road Intersection
<br />BM SVC 11 5313.69 HeM near Southeast Fence corner of Sewage Treatment Plant
<br />BM SVC 12 5264,18 HeM Near Fence Comer Southwest of Hwy. 66 and Hwy. 36 Intersection
<br />BM SVC 13 5239,83 HeM near Southeast Corner Bridge, Co. Rd. 49, approximately .05 mi. South
<br /> of Hwy, 66
<br />BM SVC 14 5222.24 BCM North Side Railroad, approximately 0,1 mi, West of Mile Marker 30.
<br /> Hwy, 66
<br />BM SVC 15 5159.46 HeM Northwest Corner Intersection of Co. Rd. 41 and Burlington Northern
<br /> Railroad
<br />BM SVC 16 5138,29 BCM East Side Co, Rd, 41 approximately 0.2 mi, North of Hygiene Road
<br />BM SVC 17 5126.81 BCM l' East of Northwest Corner, Section 3S-3N-70W, South of Hygiene
<br /> Road, approximately 0.5 mi. West of Bridge over St. Vrain Creek
<br />BM SVC 18 5094.11 BCM East Side of Co, Rd, 3S, approximately 0,4 mi. East of Hygiene Road
<br />BM SVC 19 5066,19 BCM South Side Co, Rd. 35, approximately 0,4 mi. East of Hygiene Road
<br />BM SVC 20 503S,36 BCM near Irrigation Box, North Side Co, Rd, 35, approximately 0,43 mi, West
<br /> of Airport Road
<br />USC&GS J 320 5025,90 BCM Northeast Leg of Tower
<br />
<br />BRW searched for the three bendunarks, numbered RMl, RM2, and RM3, as shown on the
<br />current Flood Insurance Rate Maps (FIRM) Number 08013C0232 F and 080l3C0234 F published
<br />by FEMA. Only benchmark RMl was located and subsequently used in the level loop survey.
<br />Benchmarks RM2 and RM3 have either been destroyed or were not properly described, and
<br />therefore were not included.
<br />
<br />BRW also located two of the five Boulder County benchmarks within or near the Town limits.
<br />Benchmarks SVC9 and SVClO were located and used in the level loop survey, Benchmarks
<br />SVCll, SVC12, and SVC13 were in areas where recent construction has been performed, They
<br />could not be located and are believed to have been destroyed,
<br />
<br />Two other published benchmarks were located and included in the survey. The benchmarks
<br />are designated "Cliff'" and "A329" and were set by NGS and USC&GS, respectively. Another
<br />two benchmarks were included in the level loop survey for reference and are designated
<br />"Gauging Station" and "Z410".
<br />
<br />A brass cap benchmark set by the USBR on the wingwall of an irrigation canal headgate was
<br />also located and was stamped "Bureau of Reclamation BM 7 5303,3241936", However, NVGD
<br />published the elevation for benchmark BM7 as 5305.56, an apparent discrepancy of about 2.2
<br />feet. This benchmark, referred to as '"Headgate" by BRW, was included in the level loop survey
<br />and was found to be at elevation 5305.56 feet (same elevation as published BM7). The brass cap
<br />set by the USBR with the incorrect stamped elevation has been a source of contention and
<br />confusion with respect to regulated floodplain elevations, BRW has determined that the
<br />elevation stamped on the brass cap is incorrect and should not be used,
<br />
<br />Since the previous floodplain delineation work by Engineering Professionals, Inc, and Love &
<br />Associates, Inc, used the Boulder County benchmarks for the aerial topography, BRW
<br />performed the survey using the County benchmarks as the datum, A survey level loop was
<br />performed establishing and verifying benchmarks through the Town limits, All of the six
<br />published benchmarks located in the BRW survey were within a :to.3-foot tolerance. Four new
<br />temporary benchmarks were also set and are listed as BMl through BM4. A list of all the
<br />benchmarks (permanent and temporary) that were surveyed and included in the level loop is
<br />shown in Table 7 A. A Benchmark Location Map showing their approximate location is
<br />included. These benchmarks should be used for all future construction to determine regulated
<br />floodplain elevations within the Town limits,
<br />
<br />Floodplain Information Report 30
<br />K,\35600\OO4\FLOODPLAIN INFORMATION REPORT,doc
<br />
<br />June 1998
<br />
<br />Floodplain Information Report 31
<br />K\35600\004\FLOODPLAIN INFORMATION REPORT.doc
<br />
<br />June 1998
<br />
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