FLOOD04288

• County: Boulder
• Community: Lyons
• Stream Name: St. Vrain and North St. Vrain
• Basin: South Platte
• Title: North St. Vrain and St. Vrain Floodplain Update
• Date: 6/1/1998
• Prepared For: Lyons
• Prepared By: BRW, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

North St. Vrain & St. Vrain Creeks
Floodplain Update

Town of Lyons

Water Conservation Board (Reference 4). The limits of this study were St. Vrain Creek at
Airport Road (85th Street) to St. Vrain Creek at 119th Street.

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). The Camp, Dresser and McKee report included information on the
South and Middle St. Vrain Creek floodplains from Lyons to Peaceful Valley, Colorado.

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.

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

In 1992, Love & Associates, Inc. completed a re-analysis using the HEC-2 model from the 1987
FIR as a base model (Reference 16). 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.

In this study, BRW, Inc. updated the HEC-2 models previously prepared by Engineering
Professionals Inc. 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 efforts.

Floodplain Informal/on Report 5
KoI35600\OO4\FLOODPLAIN INFORMATION REPORTdoc

June 1998