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2025 Hazard Mitigation Plan <br />Kittitas County, Washington <br /> <br /> <br />Chapter 4: Hazard Identification and Risk Assessment 130 <br />Type Definition <br />Considerable <br />Used rarely, when there are indications flash flooding capable of unusual severity or <br />impact is imminent or ongoing and urgent action is needed to protect lives and property. <br />Will trigger the Wireless Emergency Alert system. <br />Catastrophic <br />Used exceedingly rarely, when a flash flood threat to life and catastrophic damage is <br />occurring or is imminent, and floodwaters have risen or will rise to levels rarely if ever <br />seen. Will trigger the Wireless Emergency Alert system. <br /> <br />When a river reaches flood stage, NWS River Forecast Centers convey the magnitude of observed <br />forecast flooding using flood severity categories based on property damage and public threat. The three <br />(3) flood severity categories are listed in Table 4-67.155F <br />155 <br />Table 4-67. NWS River Flood Severity Categories <br />Type Definition <br />Minor Flooding Minimal or no property damage, but possibly some public threat or inconvenience. <br />Moderate Flooding Some inundation of structures and roads near streams. Some evacuations of people <br />and/or transfer of property to higher elevations are necessary. <br />Major Flooding Extensive inundation of structures and roads. Significant evacuations of people and/or <br />transfer of property to higher elevations. <br />4.6.5.4. Probability and Frequency <br />The probability of occurrence for flooding in Kittitas County is medium because significant occurrences <br />of this hazard have occasionally impacted the County and will likely occur again within 25 years. Kittitas <br />County experiences episodes of riverine flooding almost every winter and significant floods that can <br />cause property damage typically to occur every three (3) to seven (7) years. However, nuisance flooding <br />(i.e., urban flooding) related to drainage issues is experienced annually in urban portions of the County. <br /> <br />Isolated and lower-impact events occur with recurrent regularity. The number of years between floods of <br />any given severity significantly varies due to a naturally changing climate. According to USGS, the <br />reoccurrence interval is based on the probability that the given event will be equaled or exceeded in any <br />given year based on past occurrences. The concept of reoccurrence intervals and probabilities is outlined <br />in Table 4-68.156F <br />156 <br />Table 4-68. Recurrence Intervals and Probabilities of Occurrences <br />Recurrence Interval <br />(years) <br />Probability of Occurrence <br />In any Given Year <br />Percent Chance of <br />Occurrence in Any Given Year <br />100 1 in 100 1 <br />50 1 in 50 2 <br />25 1 in 25 4 <br />10 1 in 10 10 <br />5 1 in 5 20 <br /> <br />155 National Oceanic and Atmospheric Administration, National Severe Storms Laboratory. (n.d.). Severe Weather <br />101: Frequently Asked Questions About Floods. Retrieved from <br />https://www.nssl.noaa.gov/education/svrwx101/floods/faq/. <br />156 United States Geological Surve, New Jersey Water Science Center. (n.d.). Floods: Recurrence Intervals and <br />100-Year Floods. Retrieved from https://www.usgs.gov/centers/new-jersey-water-science-center/floods- <br />recurrence-intervals-and-100-year-floods#data.