Board of Adjustment Agenda and Packet 2003 04 16

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SH 42/96th Street Connection Drainage Report for Coal Creek Bridge Precipitation and Flood History for Coal Creek Flooding along Coal Creek in the area of Louisville is mainly caused by snowmelt combined with heavy rainfall. However rainfall alone, especially heavy thunderstorms can cause flooding. Flooding in the area usually occurs during the period of May through September. Floods normally peak within a few hours of the heavy rainfall. Significant flooding has been recorded along Coal Creek in 1876, 1891, 1896, 1921, 1935, 1938, 1949, 1957, and 1969, with the maximum of record in the valley near Louisville occurring in June of 1896. One person was killed in an accident on the flooded highway east of Louisville during the September 1938 flood, when the State Highway 42 bridge was on the verge of failure. Regulatory Floodnlain The City of Louisville participates in the National Flood Insurance Program administered by the Federal Emergency Management Agency (FEMA). As part of the program, the City has adopted a floodplain ordinance that provides for floodplain management through restricted uses, development, and construction within flood regulatory districts. A flood plain development permit must be obtained from the City before construction or development begins with the flood regulatory district. The flood regulatory district is the floodplain for the FEMA base flood, which is the 100-year return frequency flood. This ordinance complies with FEMA floodplain management regulations. Existing floodplain information for Coal Creek is contained in the FEMA Flood Insurance Study, Boulder, County, Colorado (FIS), and Flood Insurance Rate Map, Boulder County, Colorado (Map Numbers 08013C0557G and 08013C0576H) (FIRM) revised October 4, 2002. The maps show the floodplain at the project site is in Zone AE, indicating a detailed flood study with base flood elevations determined has been done, but a floodway has not been delineated. In this flood regulatory district, the City ordinance provides that proposed development cannot increase the water surface elevation of the base flood more than one foot at any point. The engineering study that developed the detailed hydrology and hydraulic analyses for the FIS and FIRM was Flood Hazard Analyses for Coal and Rock Creek prepared by the U.S. Department of Agriculture, Soil Conservation Service in October 1976. This study was based on 1974 aerial photography mapping. Proposed 96th Street Extension / Connection Coal Creek Bridge Improvements The new 96th Street bridge over Coal Creek will be 2-span concrete bridge about 247 feet long, and 87 feet wide. The bottom width between the abutment slopes will be about 125 feet. The bridge opening will be skewed about 26 degrees with the roadway to fit the mass of the flood flows. The clearance above the channel bottom will vary between 16 feet on the north and 23 feet on the south. Concrete stub abutments will be constructed at the tops of the 2 to 1 riprap abutment slopes of the bridge opening. The single pier will be about 3 feet wide with approximately triangular shaped ends. The abutments and pier will have foundations extending below potential scour depths into bedrock. The existing low flow channel will be realigned from about 100 feet upstream to 50 feet downstream of the bridge, to flow under the southerly span and to fit the existing irrigation diversion structure downstream. The sides of the low flow channel will be lined with riprap. The channel bottom will be graded at about 1 percent to the toe of the abutment slopes. The Coal Creek Trail will be realigned to pass under the northerly span on the abutment slope above the base flood elevation. All wetland and riparian disturbance caused by the project will be mitigated with wetland and riparian topsoil stockpiling and replacement, and vegetation replanting. 'W1LSO1V 2 &COMPANY EslensoNConneclon