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 REPORTdoc

3

June

998

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

4

June

998

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A supplement to this report in the form of a technical addendum includes the supporting
hydraulic and hydrologic data and calculations which may further be used in engineering
studies in the location and design of roads, bridges, and channel modifications.

Water Resources Consultants, Inco prepared a report titled "Floodplain Information, Flood
Control and Floodplain Management Plan for St. Vrain Creek at Longmont, Colorado", dated
April, 1981, under the auspices of the City of Longmont, Boulder County, and the Colorado

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Flood elevations and the corresponding floodplain are often altered by road
construction, floodplain development, flood control improvements, or natural processeso
to utilization of this report for planning or design purposes, the user is advised to contact
Boulder County to determine if the information in this report has been amendedo

A Flood Insurance Study for Lyons, Colorado, (Reference 3) was prepared by the Federal
Insurance Administration in February, 19800 This study contained North and South St. Vrain
Creek flood profiles through the town of Lyons. The hydrologic and hydraulic analyses used in
the study were completed in October, 1977, by the firm of Howard, Needles, Tammen and
Bergendoffo Generally, the hydrologic data used was that generated by the Dorps of Engineers
for the1972 studies of the Upper and Lower St. Vraino

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and bridge
Prior

The most significant model findings relate to the reach of river above the newly constructed
pedestrian bridge between Stao 152 to Stao 1570 Refinements to the base HEC-2 model resulted
in a rise of about 3.0 feet in the general areas. Additionally, the bridge abutments were
constructed in the floodway thereby creating an additional rise of about 1.6 ft. The adjacent
wastewater treatment plant historically resided in the floodplain and is protected by earthen
berms to elevation 5317 feet.

A report titled "Floodplain Information, Upper St. Vrain Creek, Volume IV," September, 1972,
(Reference 2) was prepared by the u.s. Army Corps of Engineers, Omaha District, for the
Denver District and Boulder Countyo This report included floodplain information for
approximately five (5) miles of St. Vrain Creek downstream of Lyons and approximately four
(4) miles of North St. Vrain Creek upstream of Lyonso

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Conversion of the HEC-2 model
input and results

Data input at
and distances

into

the more current

HEC-RAS model

to better view data

A report titled "Floodplain Information, Lower St. Vrain Creek, Volume III," June, 1972.
(Reference 1) was prepared by the U.So Army Corps of Engineers, Omaha District for the
Denver Regional Council of Governments Urban Drainage and Flood Control District, Boulder
County, and the City of Longmont. The Corps report included floodplain information on
approximately 1.5 miles of the St. Vrain immediately upstream of Airport Road (85th Street),
which is the downstream limit of this current report.

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bridges was

The orientation of cross-sections near the wastewater treatment plant and confluence of the
north and south St. Vrain Creeks needed reconfiguring to simulate streamflow and river
current conditionso

modified to more accurately model

ineffective areas, bridge decks

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Hydrology for the study was compiled by Boulder County based on information obtained from
existing hydrologic studies of the St. Vrain.

Related Flood Studies

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Addition of cross sections at the pedestrian bridge and wastewater treatment plant.

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Modeling of ineffective flow areas required adjustment. A number of irrigation ditches,
gravel pit ponds, and broad floodplain areas were included in the computation of base flood
elevation that were blocked out to more accurately reflect conveyance capacitieso

Aerial photography and contour mapping was prepared by Northern Colorado
Photogrammetrics of Greeley, Colorado, December, 1981, for Boulder County in cooperation
with the Colorado Water Conservation Board. The contour interval on all mapping was two-
feet and the horizontal scale was one-inch equal to 100-feet.

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This 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. Subsequent floodplain studies provided an enhanced delineation of the
floodplain; however, this re-analysis found the following model modifications to be necessary:

U.s. Department of the Army, Omaha District Corps of Engineers
Colorado Water Conservation Board
Colorado Division of Water Resources
Boulder County Public Works Department

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Preparation of a report describing characteristics of the study area, flood history of the
North St. Vrain and Sto Vrain Creeks, various hazards associated with areas in or near
the 100-year floodplain, tabulations indicating pertinent floodplain information, and
reduced copies of floodplain maps and flood profileso

The assistance and cooperation of the below listed agencies and organizations in addition to
numerous private citizens in supplying information and data to the base study are appreciated
and gratefully acknowledges:

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4.

North St. Vrain & St.
Floodplain Update

V rain Creeks

Town of Lyons

1.4

North St. Vrain & St.
Floodplain Update

Acknowled

ements

Vrain Creeks

Town of Lyons