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1 I I 1 1 I I I I I 1 1 1 1 I 1 1 1 ',1 North St. Vrain & St. Vrain Creeks Floodplain Updale North St. Vrain & St. Vrain Creeks Floodplain Update Town of Lyons Town of Lyons 4. Preparation of a report describing characteristics of the study area, flood history of the North St. Vrain and St. 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 profiles. 1.4 Acknowled!:ements 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: 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 . 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 capacities. 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 lOa-feet. . Addition of cross sections at the pedestrian bridge and wastewater treatment plant. . 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 conditions. Hydrology for the study was compiled by Boulder County based on information obtained from existing hydrologic studies of the St. Vrain. 1.5 Related Flood Studies . Data input at bridges was modified to more accurately model ineffective areas, bridge decks and distances. A report titled "Floodplain Information, Lower St. Vrain Creek, Volume III," June, 1972. (Reference 1) was prepared by the u.s. 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. . Conversion of the HEC-Z model into the more current HEC-RAS model to better view data input and results. The most significant model findings relate to the reach of river above the newly constructed pedestrian bridge between Sta. 152 to Sta. 157. 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 flood way 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 County. 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 Lyons. Flood elevations and the corresponding floodplain are often altered by road and bridge construction, floodplain development, flood control improvements, or natural processes. Prior 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 amended. A Flood Insurance Study for Lyons, Colorado, (Reference 3) was prepared by the Federal Insurance Administration in February, 1980. 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, N~mmen Imd Bergendoff. Generally, the hydrologic data used was that generated by t e Dorps f Engineers for the1972 studies of the Upper and Lower St. Vrain. 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, Inc. 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 Floodplain Information Report 3 K:1356001004\FLOODPLAIN INFORMATION REPORT.doc Floodplain IlIfonlla/ion Report 4 K:\3S600\OO4\FLOODPLAIN INFORMATION REPORT. doc June 1998 June 1998