FLOOD00588 - Laserfiche WebLink

Home Browse Search

insurance premium rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equalled 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 approxi- mately 40 percent (4 in 10), and, for any 90-year period, the risk in- creases to approximately 60 percent (6 in 10). The analyses reported here reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood eleva- tions 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 floods of the selected recurrence intervals for each stream studied in detail in the community. For Surface Creek, Cedar Run, and Tongue Creek at Orchard City, peak discharges for the 10-, 50-, 100-, and 500-year floods were determined using a regional floodflow frequency analysis (Refer- ence 6). For the regional study, U.S. Geological Survey stream- gage records for 15 gaging stations in hydrologically similar watersheds were analyzed (Reference 7). The length of record at these gaging stations ranges from 9 to 53 years. The station data were separated into rainfall and snowmelt peaks. Frequency analyses were conducted on the rainfall and snowmelt data (Refer- ence 8). The results of the snowmelt and rainfall frequency analysis were combined statistically to give a maximum flow for both events. Regional curves of drainage area versus discharge were computed for the four frequency floods. Peak discharge-drainage area relationships for Surface Creek and Cedar Run are shown in Table 1. Table 1. Summary of Discharges Flooding Source and ,Location Drainage Area (Square Miles) Peak Discharges (Cubic Feet per Second) 10-Year 50-Year 100-Year 500-Year Surface Creek At North Study Limit, Road R75 At South Study Limit, Orchard City Corporate Limits, North End, Section 26 38.6 630 970 1150 1720 44.7 700 1070 1270 1900 6