![]() The X-hour period represents the time period over which the precipitation level occurs and accounts for the duration of a design precipitation event. As structure become more critical, the X-year increases, decreasing the likelihood the structure will fail due to precipitation. a 25 year storm, is a different way of presenting a probability and should not be interpreted as when another one is expected.ĭifferent structures are assigned a specific “X-year” return period design storm criterion when designed and constructed, based on specific government requirements, the importance of the structure, and the discretion of the engineer. ![]() A higher value for X-year should be used for designing structures when significant undesired impacts are expected if the structure was to fail. A lower value for X-year can be used for designing structures where only minor impacts are expected if failure occurs. For example, a structure using a 25-year design storm criterion is expected to fail only one time in 25 years or, stated another way, has a 4% probability of failure. Another way of interpreting the return period can be thought of as the “severity of failure” or how frequently the design is expected to fail. The X-year is formally defined as the return period in years and accounts for frequency of occurrence. ![]() Each map is divided into counties and shaded as various precipitation threat levels arise. Interpreting Design Storm Mapsįive maps are used to display precipitation depth estimates for specific design storm criteria. Joseph Zulovich, agricultural engineer with the University of Missouri, defines a design storm as the amount of rain for which a structure was designed. Drainage features and other conditions are considered when designing structures for specific storm conditions. Design storm criteria are created to accommodate a specific level of weather severity based on rainfall intensity, duration and statistical frequency of recurrence. What is a Design Storm?Ī structure designed and built to account for rain storms uses design storm criteria. The rainfall data required for design purposes is based on the rainfall characteristics of intensity, duration and frequency of occurrence. You should never drive into flooded areas, as it only takes 18 inches of water to float a typical vehicle.Rainfall data is required to design structures that manage rainfall runoff like storm water diversions or store water collected from precipitation like ponds and uncovered manure storages. Flooding has killed an average of 87 people per year, according to NOAA, making it the second most deadly weather-related hazard over the last 30 years, just behind heat.Īlmost half of all flash-flood deaths occur in motor vehicles. Heavy rainfall and flooding should be taken seriously. Additionally, these extremes generally took place during the summer months or early in the fall. Most of the Northeast has 24-hour rainfall extremes between 10 and 15 inches, with the exception being Massachusetts at 18.2 inches. This is because these areas are well-removed from ocean influences, including tropical storms and hurricanes, and have a rather dry climate overall. ![]() ![]() For example, the most extreme 24-hour rainfall total on record in Utah is only 5.1 inches, the lowest of all 50 states. Some of the lowest extreme rainfall totals are in the Northern Plains, Rockies and Great Basin. The exception there is South Carolina with a 24-hour extreme of 14.8 inches, observed at Myrtle Beach and associated with Hurricane Floyd on Sept. In the Southeast, the 24-hour rainfall extremes are mainly between 20 and 25 inches. ![]()
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