Resolution No. 4814 - Laserfiche WebLink

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3. Zone AH, flood depths of 1 to 3 feet, usually in areas of ponding, with flood elevation data, 4. Zone A0, flood depths of 1 to 3 feet, usually sheet flow on sloping terrain, with average depths determined. For areas of alluvial fan floodin9, flow velocities also determined. 5. Zone X (shaded), areas within 500-year floodplain, areas of 100-year flood with depth less than one foot, or with drainage areas less than one square mile, or areas protected by levees from 100-year flood. 6. Zone X, areas determined to be outside the 500-year floodplain. For reference, the Eugene and Springfield flood plain maps also show the Urban Growth Boundary and the Eugene/Springfield Metro Area Plan boundary, which encompasses a somewhat larger region than the Urban Growth Boundary. 6,3.2 Flood Hazard Data For mapped floodplain areas, the flood hazard data included in the Flood Insurance Study (FIS) allow quantitative calculation of the frequency and severity of flooding for any property within the floodplain. Such calculations are very important for mitigation planning, because they allow the level of flood risk for any structure to be evaluated quantitatively. The example below illustrates these concepts. For example, for Willamette River at State Highway 126 in Springfield, the FEMA FIS includes the following data: Figure 6,1 Flood Hazard Data Willamette River at the State Highway 126 in Springfield Flood Frequency Discharge Elevation (yea rs) (cfs) (feet) 10 40,000 435.3 50 59,000 437,8 lO0 71,000 439.4 500 111,000 443.4 The stream discharge data shown above are from the table on page 28 of the FEMA Flood Insurance Study (FIS) for Lane County. Stream discharge means the volume of water flowing down the river and is typically measured in cubic feet of water per second (cfs). The flood elevation data are from the Flood Profile Graph 193P at the end of the FIS. Flood elevation data vary with location along the reach of the river and thus separate flood elevation data points must be read from the graph at each location along the river. Quantitative flood hazard data, such as shown above, are very important for mitigation planning purposes because they allow quantitative determination of the frequency and severity (i.e., depth) of flooding for any building or other facility (e.g., road or water treatment plant) for which elevation data exist. Such quantitative flood hazard data Public Review Draft: August 6, 2004 6-3