FLOOD04065

• County: Weld
• Stream Name: St. Vrain Creek
• Basin: South Platte
• Title: St. Vrain Floodplain Update
• Date: 4/2/1998
• Prepared For: Weld County Unincorporated
• Prepared By: consultants
• Floodplain - Doc Type: Community File

St. Vrain Creek Floodplain Update

March 5, 1998

1.0 BACKGROUND

Currently, the regulated floodplains of South St. Vrain, North St. Vrain and St. Vrain
Creeks within the corporate limits of Lyons are determined by the Flood Insurance
Study for Lyons, Colorado, prepared by the Federal Insurance Administration (February
1980). The hydraulic and hydrologic analyses used in this study were completed by
Howard, Needles, Tammen and Bergendoff (HNTB, October, 1977). Generally, the
hydrologic data used by HNTB was that generated by the Army Corps of Engineers
(ACOE) for the 1972 studies of the St. Vrain Creek basin (100-year flow of 10,200 cfs at
the confluence of the North and South St. Vrain Creeks). The 1980 Flood Insurance
Study is currently used by regulatory agencies, lending institutions, and planners for
administration of the St. Vrain floodplain.

The hydraulic methods, hydrologic data and topographic information used in the 1987
FIR are considered to be more current. The hydrologic data utilized in the 1987 FIR was
taken from a 1981 report by the Corps of Engineers, "St. Vrain Creek Hydrology,
Boulder and Weld Counties". The 100-year flow at the confluence of the North and
South St. Vrain Creeks was reduced to 8,880 cfs based on additional years of stream
gauge data and revised hydrologic analysis methods. Floodplain characteristics were
determined using the Corps of Engineers HEC-RAS computer program and 2-foot
contour topographic mapping (based on 1981 aerial photography). The 1992 revisions
to the 1987 revisions to the 1987 FIR completed by Love & Associates focused on
correcting hydraulic modeling errors in the original study and updating the study to
include current bridge and culvert information.

2.0 PURPOSE & OBJECTIVES

Updating the floodplain maps for the St. Vrain Creek through Lyons is important
because of the anticipated land use changes and urbanization expected to occur in the
area over the next several years. These maps will form the basis for administration of
activities in the floodplain and will be used by residents, town and county officials,
CWCB, and FEMA for regulating activities and administering the National Flood
Insurance Program (NFIP). Objectives of the updated plan include:

. Reflect changes posed by the newly adopted ACOE hydrologic study, which
reduced discharges by approximately 1,000 cfs.

. Model river hydraulics associated with man-made and natural channel
modifications occurring since 1987.

. Provide the basis for subsequent modeling of floodplain encroachments and
floodplain activities within such as construction of the McConnell Drive Bridge, and

. Provide the framework for comprehensive planning and related parks and
recreational planning along the St. Vrain Creek riparian corridor.

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