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Sauk County Hazard Mitigation Plan: 2019-2024 Chapter 5. Assessment of Natural Hazards ASSESSMENT OF NATURAL HAZARDS Chapter Contents 1. Chapter Overview 2. Hazard Identification 1. CHAPTER OVERVIEW This chapter evaluates the natural hazards that have or could occur in the 3. History of Weather-Related Events county. Initially, 14 natural hazards were identified. They were prioritized and earthquakes, wildland fire, and land subsidence were dropped from future 4. Presidential Declarations consideration. The remaining natural hazards are described in this chapter. After Dam Failure describing the nature of the hazard, the frequency of occurrence is documented 5. along with its effect on critical facilities, various population groups, and 6. Flooding economic sectors. Estimates of economic loss are included when there is enough empirical data to do so. 7. Dense Fog 8. Tornadoes 9. Hailstorms 2. HAZARD IDENTIFICATION As part of an initial screening process, the steering committee used the 10. Thunderstorms methodology developed by Wisconsin Emergency Management1 to evaluate 14 natural hazards that were initially identified as a potential threat. The members 11. Winter Storms of the steering committee used a group consensus process to assign a numeric Extreme Heat value to the factors listed in Table 5-1 to help determine those hazards that 12. warrant the most attention on a countywide basis. 13. Extreme Cold Table 5-1. Hazard Assessment Criteria 14. Drought Factor Description 15. Summary of Risk by Jurisdiction Historical Hazard Frequency Frequency of past occurrences 16. Summary of Damage Estimates Anticipated Hazard Probability Probability of the hazard occurring again Historical Health and Public Safety Degree of past hazard events causing injuries, sickness, and/or deaths Residential Damage Degree of past hazard events causing damages to homes Business Damage Degree of past hazard events causing damages to businesses Public Costs Amount of local, state, and federal funds expended on past hazard recovery activities Magnitude of Population at Risk Amount of the area’s population still vulnerable to injury, sickness, and/or death Magnitude of Homes at Risk Amount of homes still vulnerable to damage Magnitude of Businesses at Risk Amount of businesses still vulnerable to damage or interruption of business trade Magnitude of Public Infrastructure at Risk Amount of infrastructure that is susceptible to damages Source: Resource Guide to All Hazards Mitigation Planning In Wisconsin, 2003. Wisconsin Emergency Management Table 5-2 shows the results of that exercise. The three highest ranked natural hazards are thunderstorms, winter storms, and tornadoes. Given the distance to a known fault line, earthquakes were judged to be of little concern. In addition, given the topography and soils in the county, land failures, including subsidence and mass movement, were judged to be of little concern. Wildland fire was also 1 Resource Guide to All Hazards Mitigation Planning In Wisconsin, 2003. Wisconsin Emergency Management 5 - 1 Sauk County Hazard Mitigation Plan: 2019-2024 Chapter 5. Assessment of Natural Hazards deemed to be comparatively of low priority. These were removed from further consideration in this plan. Table 5-2. Countywide Comparative Analysis of Natural Hazards; Sauk County: 2019 1 2 3 4 5 6 7 8 9 10 11 Historical Magnitude Magnitude Magnitude Magnitude Historical Anticipated Health and of of of of Public Hazard Hazard Public Residential Business Public Population Homes Businesses Infrastructure Frequency Probability Safety Damage Damage Costs At Risk At Risk At Risk At Risk Overall Hazard (1,2,3) (1,2,3) (1,2,3) (1,2,3) (1,2,3) (1,2,3) (1,2,3) (1,2,3) (1,2,3) (1,2,3) Score Thunderstorm 3 3 1 1 1 1 3 3 2 2 20 Flooding – 2 2 2 1 2 3 1 1 2 2 20 riverine Tornado 1 1 1 1 2 2 3 3 3 2 19 Winter storm 1 1 1 1 2 2 3 2 2 1 19 Flooding – 2 2 1 1 2 3 1 1 1 2 17 stormwater Dense fog 3 3 1 1 1 1 2 1 1 1 15 Hail 2 2 1 2 1 1 1 2 2 1 14 Extreme cold 2 2 2 1 1 1 2 1 1 1 14 Excessive 2 2 2 1 1 1 2 1 1 1 14 heat Drought 1 1 1 1 2 1 3 1 1 1 13 Dam Failure 1 1 1 1 1 1 1 1 1 2 11 Notes: This matrix is based on a qualitative assessment and is intended to identify those hazards posing the greatest concern. A low, medium, or high numerical rating of 1, 2, or 3, respectively, is assigned to each criterion and then the ratings for each hazard are totaled. Column 1 refers to the frequency of past occurrences. Column 2 refers to the probability of the hazard occurring again. Column 3 refers to the degree of past hazard events causing injuries, sickness, and/or deaths. Column 4 refers to the degree of past hazard events causing damages to homes. Column 5 refers to the degree of past hazard events causing damages to businesses. Column 6 refers to the amount of local, state, and federal funds expended on past hazard recovery activities. Column 7 refers to the amount of the area’s population still vulnerable to injury, sickness, and/or death. Column 8 refers to the amount of homes still vulnerable to damage. Column 9 refers to the amount of businesses still vulnerable to damage or interruption of business trade. Column 10 refers to the amount of infrastructure that is susceptible to damage. Column 11 is the raw score for the hazard. The nature of the identified hazards are quite different as shown in Table 5-3. Some of the hazards are characteristically localized occurrences, while others could potentially cover the entire county and the surrounding region. Further, some hazards occur with little advance warning and others, such as riverine flooding on larger rivers, can be forecasted with some degree of accuracy several days in advance of the actual event. Some hazards have the potential to occur often, while others occur rather infrequently. In an effort to keep residents and local officials informed about potential events, the National Weather Service (NWS) issues, outlooks, watches, and warnings for most weather events. NWS mentions the possibility of a hazard in daily message entitled “Hazardous Weather Outlook” (HWO) that is disseminated to the media, posted on its web site, broadcast on NOAA Weather Radio All Hazards, and made available on various computer circuits. An outlook covers possible events seven days out. Confidence factor is about 30 percent for issuance. A message entitled “watch” for most weather hazards is generally issued hours to a couple days in advance of a possible event. Confidence factor is about 60 percent for issuance. It is disseminated to the media, posted on the NWS web site, broadcast on NOAA Weather Radio All Hazards, and made 5 - 2 Sauk County Hazard Mitigation Plan: 2019-2024 Chapter 5. Assessment of Natural Hazards available on various computer circuits. A “warning” message for most weather hazards has a confidence factor of 80 to 100 percent. It is disseminated to the media, posted on the NWS web site, broadcast on NOAA Weather Radio All Hazards, and made available on various computer circuits. Table 5-3. Nature of Natural Hazards; Sauk County Amount of Recurrence Natural Hazard Extent Advance Notification Interval [1] Dam Failure Along affected stream corridor None to weeks N/A [2] Flooding – Riverine Along affected stream corridor and around lakes Several days 0.01 [3] Flooding – Stormwater Poorly drained areas Several days 0.01 [3] Dense Fog Small pockets in low-lying areas to countywide Several days 0.7 Tornado Generally, a linear path up to several miles long Several days 0.7 Hail Storm 1 square mile and larger Several days 0.7 Thunderstorm 10 square miles and larger Several days 0.33 Snow Storm Countywide Several days 1.3 Extreme Heat Countywide Several days 0.4 Extreme Cold Countywide Several days 0.3 Drought - short-lived Countywide Several months 10 Drought - long-lived Countywide Year 75 Notes: 1. Recurrence interval is number of events occurring over a period of time. 2. It is not possible to define a recurrence interval for this type of hazard 3. Based on a 100-year flood Table 5-4 lists each of the hazards and identifies whether they will have an impact on the critical facilities identified in this plan, noncritical buildings and structures, special populations, the general population, and broadly defined economic sectors. 5 - 3 Sauk County Hazard Mitigation Plan: 2019-2024 Chapter 5. Assessment of Natural Hazards Table 5-4. General Effects of Natural Hazards on Facilities, Population Groups, and Economic Sectors; Sauk County: 2019 Dam Flooding - Flooding - Dense Tornado/ Winter Extreme Critical Facility Failure Riverine Stormwater Fog High Wind Hail Thunderstorm Storm Temperature Drought Facility with Hazardous Materials - D - - D - - - - - Infrastructure Bridge D D - - - - - - - - Dam D D - - - - - - - - Communication Tower - - - - D - - - - - Electric Facility – Power Plant - - - - D - - - - - Electric Facility – Substation - - - - D - - - - - Natural Gas Facility - - - - D - - - - - Petroleum Pipeline - - - - - - - - - - Public-Use Airport - - - I D I I I - - Telephone Facility - - - - D - - - - - Utility Offices/Yard - - - - D - - - - - Water Facility [1] - - - - D - - - - I Wastewater Facility - - - - D - - - - - Government Facility Community Center - - - - D D - - - - Library - - - - D D - - - - Municipal Garage - - - - D D - - - - Municipal Office and Other - - - - D D - - - - Post Office - - - - D D - - - - Senior Center - - - - D D - - - - Health Care Facility Health Care Clinic - - - - D D - - - - Hospital - - - - D D - - - - Public Safety Facility EMS Facility - - - - D D - - - - Fire Station