2018-12-26_REVISION - M2008078 (28)

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study. Flood events of the magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50, 100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long term average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1 percent chance of annual occurrence) in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported here reflect flooding potentials based on conditions existing in the county at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community. As part of the new detailed studies along the Arkansas River in at the City of Lamar, the Town of Granada, and Town of Holly, ICON Engineering, Inc. determined the 10-, 2-, 1-, and 0.2%-annual-chance peak discharges. The methodology for determining flows included both statistical analysis of gage data and regional regression equations. The methodology for statistical regression used the U.S. Army Corps of Engineers Hydrologic Engineering Center's Statistical Software Package (HEC- SSP vl.0, Reference 13). This program calculates peak discharges from stream gage data using the Bulletin 17B methodology (reference 14). In order to maintain homogeneous hydrologic conditions, the gage record data were limited to water years after construction of the John Martin Reservoir in 1942. For the reach of the Arkansas River in Prowers County, statistical analysis of stream gage data with HEC-SSP was used to determine peak flows at the City of Lamar. The Arkansas River gage record at Lamar includes 62 years from 1944 to 2008. The gage record used in the initial analysis was limited to the years 1944 to 1982 to match the data used in the FIS. The HEC-SSP results were then calibrated to the FIS results by using a weighted skew factor of -0.12 and a regional skew MSE of 0.302. This regional skew factor is in agreement with the value indicated by the map for generalized skew coefficients found in Bulletin 17B. The gage record was then updated by adding the record for the years 1982- 2008 to the HEC-SSP model. Results of the analysis are shown in Table 4. The additional record data had the effect of significantly reducing flow values for the all storm frequencies in the updated model flows. Unlike the Lamar site, the Arkansas River stream gage records for Granada and 7