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<br />North St. Vrain & St. Vrain Creeks
<br />Floodplain Update
<br />
<br />Town of Lyons
<br />
<br />North St. Vrain & St. Vrain Creeks
<br />Floodplain Update
<br />
<br />Town of Lyons
<br />
<br />4, Preparation of a report describing characteristics of the study area, flood history of the
<br />North 51. Vrain and St. Vrain Creeks, various hazards associated with areas in or near
<br />the 100-year floodplain, tabulations indicating pertinent floodplain information, and
<br />reduced copies of floodplain maps and flood profiles,
<br />
<br />1.4 Acknowledgements
<br />
<br />The assistance and cooperation of the below listed agencies and organizations in addition to
<br />numerous private citizens in supplying information and data to the base study are appreciated
<br />and gratefully acknowledges:
<br />
<br />This study found that the currently adopted 1980 flood insurance rate maps (FIRM) provide a
<br />reasonable generalized estimate of the floodplain boundaries within the St. Vrain Creek along
<br />the Town of Lyons. Subsequent floodplain studies provided an enhanced delineation of the
<br />floodplain; however. this re-analysis found the following model modifications to be necessary:
<br />
<br />U.s, Department of the Army, Omaha District Corps of Engineers
<br />Colorado Water Conservation Board
<br />Colorado Division of Water Resources
<br />Boulder County Public Works Department
<br />
<br />. Modeling of ineffective flow areas required adjustment. A number of irrigation ditches,
<br />gravel pit ponds, and broad floodplain areas were included in the computation of base flood
<br />elevation that were blocked out to more accurately reflect conveyance capacities.
<br />
<br />Aerial photography and contour mapping was prepared by Northern Colorado
<br />Photogrammetrics of Greeley, Colorado, December, 1981, for Boulder County in cooperation
<br />with the Colorado Water Conservation Board, The contour interval on all mapping was two-
<br />feet and the horizontal scale was one-inch equal to 100-feet.
<br />
<br />. Addition of cross sections at the pedestrian bridge and wastewater treatment plant.
<br />
<br />. The orientation of cross-sections near the wastewater treatment plant and confluence of the
<br />north and south St. Vrain Creeks needed reconfiguring to simulate streamflow and river
<br />current conditions.
<br />
<br />Hydrology for the study was compiled by Boulder County based on information obtained from
<br />existing hydrologic studies of the St, Vrain.
<br />
<br />1.5 Related Flood Studies
<br />
<br />. Data input at bridges was modified to more accurately model ineffective areas, bridge decks
<br />and distances,
<br />
<br />A report titled "Floodplain Information, Lower St. Vrain Creek. Volume Ill," June, 1972,
<br />(Reference 1) was prepared by the U.s, Army Corps of Engineers, Omaha District for the
<br />Denver Regional Council of Governments Urban Drainage and Flood Control District, Boulder
<br />County, and the City of Longmont. The Corps report included floodplain information on
<br />approximately 1.5 miles of the St, Vrain immediately upstream of Airport Road (85th Street),
<br />which is the downstream limit of this current report.
<br />
<br />. Conversion of the HEC-2 model into the more current HEC-RAS model to better view data
<br />input and results,
<br />
<br />The most significant model findings relate to the reach of river above the newly constructed
<br />pedestrian bridge between Sta. 152 to Sta, 157, Refinements to the base HEC-2 model resulted
<br />in a rise of about 3,0 feet in the general areas, Additionally, the bridge abutments were
<br />constructed in the floodway thereby creating an additional rise of about 1,6 ft. The adjacent
<br />wastewater treatment plant historically resided in the floodplain and is protected by earthen
<br />berms to elevation 5317 feet.
<br />
<br />A report titled "Floodplain Information, Upper St. Vrain Creek, Volume IV," September, 1972,
<br />(Reference 2) was prepared by the U,S. Army Corps of Engineers, Omaha District. for the
<br />Denver District and Boulder County, This report included floodplain information for
<br />approximately five (5) miles of St. Vrain Creek downstream of Lyons and approximately four
<br />(4) miles of North St. Vrain Creek upstream of Lyons.
<br />
<br />Flood elevations and the corresponding floodplain are often altered by road and bridge
<br />construction. floodplain development, flood control improvements, or natural processes. Prior
<br />to utilization of this report for planning or design purposes, the user is advised to contact
<br />Boulder County to determine if the information in this report has been amended,
<br />
<br />A Flood Insurance Study for Lyons, Colorado. (Reference 3) was prepared by the Federal
<br />Insurance Administration in February. 1980, This study contained North and South St. Vrain
<br />Creek flood profiles through the town of Lyons. The hydrologic and hydraulic analyses used in
<br />the study were completed in October, 1977, by the firm of Howard, Needles, Tammen and
<br />Bergendoff, Generally, the hydrologic data used was that generated by the Dorps of Engineers
<br />for the1972 studies of the Upper and Lower St. Vrain.
<br />
<br />A supplement to this report in the form of a technical addendum includes the supporting
<br />hydraulic and hydrologic data and calculations which may further be used in engineering
<br />studies in the location and design of roads, bridges, and channel modifications.
<br />
<br />Water Resources Consultants, Inc. prepared a report titled "Floodplain Information, Flood
<br />Control and Floodplain Management Plan for St, Vrain Creek at Longmont, Colorado", dated
<br />April, 1981, under the auspices of the City of Longmont, Boulder County, and the Colorado
<br />
<br />Floodplain Information Report 3
<br />K:1356001004\FLOODPLAIN INFORMATION REPORTdoc
<br />
<br />June 1998
<br />
<br />Floodplain Informatiol! Report 4
<br />K:\356001004\FLOODPLAIN INFORMATION REPORT,doc
<br />
<br />June 1998
<br />
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