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

5

June

998

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In this study, BRW, Inco updated the HEC-2 models previously prepared by Engineering
Professionals Inco and Love & Associates, Inc. into a HEC-RAS model. BRW's work included
encoding additional sections to reflect recent construction of a pedestrian bridge by the Town of
Lyons, updating cross sectional data at several sections, and making modifications and
corrections to the previous modeling effortso

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In 1992, Love & Associates, Inc. completed a re-analysis using the HEC-2 model from the 1987
FIR as a base model (Reference 16)0 The re-analysis focused on correcting errors in the original
study and updating the study to include current bridge and culvert configurations. The HEC-2
modifications generally included corrections to cross-sectional coding to agree with the
topography and current bridge or culvert information, re-orientation of cross sections to be
perpendicular to the direction of flood flows, removal and! or addition of cross-sections to
improve the definition of the floodplain, identification of ineffective flow areas, and a split flow
analysis for St. Vrain Creek. The manning's roughness coefficients were kept the same in the re-
analysis as in the 1987 FIR; however, the expansion and contraction coefficients were modified
as appropriate.

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Engineering Professionals, Inco analyzed the North St. Vrain and St. Vrain Creeks
County, Colorado and published the results in the "Floodplain Information Report,
Vrain and St. Vrain Creeks", dated December 1987 (1987 FIR) (Reference 15)

in Boulder
North St.

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Gingery Associates, Inc., prepared a flood insurance study for the City of Longmont in January,
1977 (Reference 6). The study included St. Vrain Creek from Airport Road to 119th Street.

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A report titled "Floodplain Information Report, St. Vrain Canyon Upstream of Lyons, Boulder
County, Colorado," was prepared by Camp, Dresser and McKee, Inc., of Denver, Colorado,
October, 1978 (Reference 5)0 The Camp, Dresser and McKee report included information on the
South and Middle St. Vrain Creek floodplains from Lyons to Peaceful Valley, Colorado.

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Water Conservation Board
Airport Road (85th Street)

(Reference 4). The limits of
to St. Vrain Creek at 119th Street.

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North St. Vrain & St.
Floodplain Update

Vrain Creeks

this study were St.

Vrain Creek at

Town of Lyons