# AgEBB-MU CAFNR Extension

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. The rainfall data required for design purposes is based on the rainfall characteristics of intensity, duration and frequency of occurrence.

## What is a Design Storm?

A structure designed and built to account for rain storms uses design storm criteria. Design storm criteria are created to accommodate a specific level of weather severity based on rainfall intensity, duration and statistical frequency of recurrence. Drainage features and other conditions are considered when designing structures for specific storm conditions. Dr. Joseph Zulovich, agricultural engineer with the University of Missouri, defines a design storm as the amount of rain for which a structure was designed.

## Interpreting Design Storm Maps

Five maps are used to display precipitation depth estimates for specific design storm criteria. Each map is divided into counties and shaded as various precipitation threat levels arise. Each map consists of two numbers:

• X-year/X-hour period

The X-year is formally defined as the return period in years and accounts for frequency of occurrence. 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. 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. A lower value for X-year can be used for designing structures where only minor impacts are expected if failure occurs. A higher value for X-year should be used for designing structures when significant undesired impacts are expected if the structure was to fail. Remember, the year value, i.e. a 25 year storm, is a different way of presenting a probability and should not be interpreted as when another one is expected.

Different 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. As structure become more critical, the X-year increases, decreasing the likelihood the structure will fail due to precipitation.

The X-hour period represents the time period over which the precipitation level occurs and accounts for the duration of a design precipitation event. The length of this time period depends upon the purpose a structure and the length of time over which precipitation occurs. 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. 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.

The combined rainfall design criteria of design precipitation depth, storm return frequency, and storm duration are combined in the various maps. 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 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. Similarly, 10-year/90-day storm has a 10% chance of occurring producing a given amount of rain over a continuous 90 day period. The 10-year/90-day storm criterion is exceeded when more precipitation occurs in a 90 day period than stated on the 10-year/90-day design storm map.

## How County Statuses Are Measured

DSAS was developed to track and identify extreme precipitation accumulations using data from the National Weather Service’s daily gridded radar precipitation analysis and the National Oceanic and Atmospheric Administration’s (NOAA) Climate Atlas 14, Volume 8. The radar analyzes approximately 11,000 points across Missouri to select the highest precipitation estimate per county for 24-hour, 10-day, 90-day, 180-day, and 365-day periods. These totals are then compared to precipitation frequency estimates derived from the climate atlas to calculate the status of each county, thus the design storm maps.

### Precipitation Frequency Estimates

DSAS collects the highest precipitation recorded, out of multiple data points, per county. Each county will register as one of the following on each map:

 Clear (white) No watch or alert status Watch (yellow) ≥90% of design storm criteria met Alert (blue) ≥100% of design storm criteria met

The county statuses are designed to serve in a similar way as storm watches and storm warnings. Watches indicate conditions are favorable for severe weather to occur; whereas warnings indicate severe weather is present or coming. Regarding design storms, a “watch” status indicates criteria is close to being met (currently at or over 90% of criteria); and the “alert” status indicates the design storm has reached or exceeded 100% of its criteria.