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North St. Vrain & St. Vrain Creeks Floodplain Update Town of Lyons North St. Vrain & St. Vrain Creeks Floodplain Update Town of Lyons SECTION 1- INTRODUCTION floodplain. The hydrologic data utilized in the 1987 FIR was taken form 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-2 computer program and 2- foot contour topographic mapping (based on 1981 aerial photography). 1.1 Authorization Boulder County originally authorized this report in 1987 in joint sponsorship with the Colorado Water Conservation Board. The first detailed floodplain analysis was prepared by Engineering Professionals, Inc. with the results published in the Floodplain Information Report. North St. Vrain and St. Vrain Creeks. dated December 1987 (1987 FIR). In 1992, Love & Associates, Inc. completed a re-analysis using the HEC-2 model from the 1987 FIR as a base model. The re- analysis focused on correcting errors in the original study and updating the study to include current bridge and culvert configurations. In 1998, the Town of Lyons authorized BRW, Inc. to update and re-analyze the floodplain within the town. 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. This re-analysis was never designated by the Colorado Water Conservation Board. 1.3 Purpose and Scope The Colorado Water Conservation Board's power and duty is to devise and formulate methods, means and plans for bringing about the greater utilization of the waters of the state and the prevention of flood damages therefrom and to designate and approve storm or floodwater runoff channels or basins, and to make such designations available to legislative bodies of cities and incorporated towns, to county planning commissions, and to boards of adjustment of cities, incorporated towns, and counties of this state as stated in Section 37-60-10S(1)(c) of the Colorado Revised Statutes 1973. 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: The cities, incorporated towns, and counties within the study area may provide zoning regulations to establish, regulate, restrict, and limit such uses on or along any stream or floodwater runoff channel or basin, as such storm or floodwater runoff channel or basin has been designated and approved by the Colorado Water Conservation Board, in order to lessen or avoid the hazards to persons an damage to property resulting from the accumulation of storm or flood waters, as stated in Sections 30-28-111 and 31-23-201 of the Colorado Revised Statutes, 1975. Upon official approval of this report by the Colorado Water Conservation Board, the areas described as being inundated by the 100-year flood may be designated as flood hazard areas and their use regulated accordingly by local governmental entities. . 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 1.2 Background . Provide the framework for comprehensive planning and related parks and recreational planning along the St. Vrain Creek riparian corridor. The currently regulated floodplains of South St. Vrain, North St. Vrain and St. Vrain Creeks within the corporate limits of Lyons were determined by the Flood Insurance Study for Lyons, Colorado, prepared by the Federal Insurance Administration (February 1980). Howard, Needles, Tammen and Bergendoff (HNTB, October 1977, Reference 3) completed the hydraulic and hydrologic analyses used in the 1980 study. 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 (lOO-year flow of 10,200 cfs at the confluence of the North and South St. Vrain Creeks). This report was prepared to guide local officials in planning the use and regulation of the floodplain areas such that hazards and future damages are eliminated or minimized. This report includes information on historical floods, existing factors which influence the flood hazard, and the nature and extent of probable future floods. The following is a brief summary of the scope of work for this project. The hydraulic methods, hydrologic data and topographic information used in the 1987 FIR are considered to be more current, however, the 1980 Flood Insurance Study is still currently used by regulatory agencies, lending institutions, and planners for administration of the St. Vrain 1. Calculation of water surface profiles for the 10-,50-,100-, and SOO-year flood events; 2. Delineation of the 100-year and SOO-year floodplain boundaries; 3. Development of discharge profiles indicating the relative magnitudes of the 10-,50-, 100-, and SOO-year flood events; Floodplain Information Report I K:\35600\OO4\FLOODPLAIN INFORMATION REPORT.doc June 1998 Floodplain Information Report 2 K:\35600\004IFLOODPLAIN INFORMATION REPORT.doc June 1998