The design storm criteria for a structure using a 25-year/24-hour design storm is exceeded when the actual precipitation depth over a continuous 24 hour period exceeds the stated depth for a 25-year/24-hour storm. For example, a 25-year/24-hour storm has a 4% chance of occurring producing a given amount of precipitation over a continuous 24 hour period. The combined rainfall design criteria of design precipitation depth, storm return frequency, and storm duration are combined in the various maps. For water impoundment structure like a pond or manure structure that collects precipitation water, this design precipitation period is typically the number of days an uncovered storage is expected to be filled. For a structure that diverts or transfers storm water such as a diversion or storm water drain system, this design precipitation period is typically the length of time rain continuously falls that provides maximum rainfall water runoff. The length of this time period depends upon the purpose a structure and the length of time over which precipitation occurs. 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. 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.

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