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2025 Hazard Mitigation Plan Kittitas Gounty, Washington ,ffit_%d Considerable Used rarely, when there are indications flash flooding capable of unusual severity or impact is imminent or ongoing and urgent action is needed to protect lives and property. Willtriqger the Wireless Emerqency Alert system. Catastrophic Used exceedingly rarely, when a flash flood threat to life and catastrophic damage is occurring or is imminent, and floodwaters have risen or will rise to levels rarely if ever seen. Will triqqer the Wireless Emerqencv Alert svstem. DefinitionType When a river reaches flood stage, NWS River Forecast Centers convey the magnitude of observed forecast flooding using flood severity categories based on property damage and public threat. The three (3) flood severity categories are listed in Table 4-67.-155 Table 4-67. NWS River Flood Severity Categories 4.6,5.4. Probability and Frequency The probability of occurrence for flooding in Kittitas County is medium because significant occurrences of this hazard have occasionally impacted the County and will likely occur again within 25 years. Kittitas County experiences episodes of riverine flooding almost every winter and significant floods that can cause property damage typically to occur every three (3) to seven (7) years. However, nuisance flooding (i.e., urban flooding) related to drainage issues is experienced annually in urban portions of the County. lsolated and lower-impact events occur with recurrent regularity. The number of years between floods of any given severity significantly varies due to a naturally changing climate. According to USGS, the reoccurrence interval is based on the probability that the given event will be equaled or exceeded in any given year based on past occurrences. The concept of reoccurrence intervals and probabilities is outlined in Table 4-68.156 Table 4-68. Recurrence lntervals and Probabilities of Occurrences 155 National Oceanic and Atmospheric Administration, National Severe Storms Laboratory. (n.d.). Severe Weather 101 : Frequently Asked Questions About Floods. Retrieved from https://www.nssl. noaa.gov/education/svrwxl 01 /floods/faq/. 156 United States Geological Surve, New Jersey Water Science Center. (n.d.). Floods: Recurrence lntervals and 1 O0-Year Floods. Retrieved from https://www.usqs.qov/centers/new-i recurrence-intervals-and-1 00-vear-floods#data Minor Flooding Minimal or no property damage, but possibly some public threat or inconvenience Moderate Flooding Some inundation of structures and roads near streams. Some evacuations of people andior transfer of propertv to hiqher elevations are necessary. Major Flooding Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of propertv to hiqher elevations. Type Definition 100 1 in 100 1 1in50 250 4251in25 10 1in10 10 5 1in5 20 Probability of OccurrenceRecurrence lnterval Occurrence in An Given YearlnaGiven Year Percent Chance of Chapter 4: Hazard ldentification and Risk Assessment ersev-water-science-cente r/f loods- 130