FLOOD10328

• County: Boulder
• Community: Lyons
• Stream Name: North Saint Vrain and Saint Vrain Creeks
• Title: Floodplain Information Report, North Saint Vrain Creek, Saint Vrain Creek, Floodplain Update, Town of Lyons, Boulder County, Colorado
• Date: 6/1/1998
• Prepared For: Lyons
• Prepared By: BRW, Inc
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Floodplain Information Report
K:\35600\004\FLOODPLAIN INFORMATION REPORT.doc

30

June

998

Floodplain Information Report
K:\35600\004\FLOODPLAIN INFORMATION REPORTdoc

3

June

998

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USC&GS J 320

BM SVC 20

BM SVC

19

5025.90

5035.36

5066.19

BCM Northeast

BCM near Irrigation Box, North Side Co. Rd. 35, approximately 0.43 mi. West
of Airport Road

BCM South Side Co. Rd. 35, approximately 0.4 mi. East of

Leg of Tower

Hygiene Road

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BM SVC

18

5094.11

BCM East Side of Co. Rd. 35, approximately 0.4 mi. East of

BCM l' East of Northwest Corner, Section 35-3N-70W, South of Hygiene
Road, approximately 0.5 mi. West of Bridge over St. Vrain Creek

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BM SVC 17

BM SVC

16

Hygiene Road

5126.81

5138.29

BCM East Side Co. Rd. 41 approximately 0.2 mi. North of Hygiene Road

Since the previous floodplain delineation work by Engineering Professionals, Inco and Love &
Associates, Inc. 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 limitso All of the six
published benchmarks located in the BRW survey were within a :!::003-foot tolerance. 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 is
shown in Table 7 Ao A Benchmark Location Map showing their approximate location is
included. These benchmarks should be used for all future construction to determine regulated
floodplain elevations within the Town limitso

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BM SVC 15

5159.46

BCM Northwest
Railroad

BCM North Side Railroad, approximately 0.1
Hwy. 66

Corner Intersection of Co. Rd. 41 and Burlington Northern

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BM SVC 14

5222.24

mi. West of

BCM near Southeast Corner Bridge, Co. Rd. 49, approximately .05 mi. South
of Hwy. 66

:1

BM SVC 13

BM SVC

12

Mile Marker 30,

5239.83

5264.18

BCM Near Fence Corner Southwest of

Hwy. 66 and Hwy. 36 Intersection

A brass cap benchmark set by the USBR on the wingwall of an irrigation canal headgate was
also located and was stamped "Bureau of Reclamation BM 7 530303241936". However, NVGD
published the elevation for benchmark BM7 as 5305056, an apparent discrepancy of about 2.2
feet. This benchmark, referred to as "Headgate" by BRW, was included in the level loop survey
and was found to be at elevation 5305.56 feet (same elevation as published BM7). The brass cap
set by the USBR with the incorrect stamped elevation has 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.

BM SVC 11

5313.69

BCM near Southeast Fence corner of Sewage Treatment Plant

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BM SVC 8

BM SVC 7

5448.45

5462.15

BCM Southwest Corner Intersection of Applevalley Road and Private Drive,
approximately 0.72 mi. West of Hwy. 36 and Applevalley Road Intersection

BCM West Side Highway 36, approximately 0.36 mi. West of Miles

Marker 19

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, respectivelyo Another
two benchmarks were included in the level loop survey for reference and are designated
"Gauging Station" and "2410"

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BM SVC 6

BM SVC 5

5507.67

5544.18

BCM North Side Highway 36, West of Co. Rd. 71
Hwy.36

Steel Axle in Boulder,
Mile Marker 18

South Side of

Hwy. 36, approximately 0.7 mi. West of

West of Box Culvert under

BRW also located two of the five Boulder County benchmarks within or near the Town limits.
Benchmarks SVC9 and SVClO were located and used in the level loop surveyo Benchmarks
SVC11, SVC12, and SVC13 were in areas where recent construction has been performed. They
could not be located and are believed to have been destroyed.

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BM SVC 4

BM SVC 3

BM SVC 2

5614.61

5757.38

5847.21

BCM l' West of Highway Right-of-Way Marker, approximately 0.2 mi. West
of Mile Marker 17

BCM South Side Highway 36 approximately, 0.1

BCM South Side Highway 36 approximately, .45 mi. West of

mi. West of

Mile Marker 16

Mile Marker 16

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 RM1 was located and subsequently used in the level loop surveyo
Benchmarks RM2 and RM3 have either been destroyed or were not properly described, and
therefore were not includedo

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BM svc

POINT

1

NORTH ST. VRAIN

5738.49

ELEVATION

Brass Cap Monument (BCM) North Side of
West Hwy. 36 and Co. Rd. 80 Intersection

GENERAL LOeA TION

TABLE 7
ST. VRAIN CREEK BENCHMARK INFORMATION

Co. Rd. 80, approximately 0.9 mi.

Boulder County, Bureau of Reclamation (USBR), US Geological Survey (USGS), US Coast &
Geodetic Survey (USC&GS), and National Geodetic Survey (NGS). The six were labeled "SVC
9", "SVC 10", "RM1", "CLIFF", "HEADGATE" and "A329", and are shown on the following
Benchmark Location Mapo

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North St. Vrain & St.
Floodplain Update

Vrain Creeks

Town of Lyons

North St. Vrain & St.
Floodplain Update

Vrain Creeks

Town of Lyons