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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />11;1 Hazard Assessment: RAINFALL, EXTREME <br />4.3.6.1. Definition <br />Extreme rainfall leads to flash <br />flooding, infrastructural and <br />property damage, and even loss <br />of life. Although the definition <br />varies by application, extreme <br />rainfall events are generally <br />understood to have rates that <br />meet or exceed a given <br />threshold, often tied to storage <br />or drainage capacity. <br />In forecasting applications, extreme rainfall drives the issuance of National Weather Service flash -flood <br />products based on "flash -flood guidance," which is a changing, location -dependent value that utilizes <br />pre-existing soil moisture and land cover conditions. Unsaturated soils and ample vegetation require <br />higher precipitation rates to trigger flash -flooding than saturated soils, denuded vegetation, or impervious <br />surfaces. <br />Extreme rainfall also is critical to hydrologic design of roads, trails, culverts, retention and detention ponds, <br />dams, and other types of infrastructure. Engineers and planners design these facilities to withstand all but <br />some small percentage of all heavy rainfall events. For instance, many non -critical features like small roads <br />and trails are designed to withstand a storm that has a 10% probability in any given year (also known as <br />the 10-year storm). More critical features will often be designed for 100-year rainfall events --those that <br />have a 1% probability in any given year. NOAA Atlas 14 contains the most recent scientific estimates of <br />rainfall amounts for durations from 5 minutes to 60 days, and with recurrence intervals of 1 through <br />500-years. <br />117 <br />