Precipitation (Rainfall) Intensity and Frequency
by Edwin Friedrichs, PE, PTP
Conversations about precipitation events often include discussion about the amount of precipitation and duration of the event, but how do these factors relate to the percent chance of a specific precipitation event occurring? Note the term “event” is used here as opposed to the term “flood” which is normally reported. A ‘flood’ is described by FEMA as a complete or partial inundation of two or more acres of land. As you can see in Table 1 below there are precipitation events that would be described as a 1% chance event with relatively small total precipitation depths such as a 3.81” for a 30-minute event duration. It is possible that some 1% chance precipitation events would not inundate, or flood, two acres of land depending on the size of the event and the geography of the area.
A precipitation frequency estimate (PFE) is defined as the depth of precipitation at a specific location for a specific duration that has a certain probability of occurring in any given year. Depth of precipitation is typically measured in inches. PFE is always referred to with an associated duration and location. For example, the PFE for a 24-hour precipitation amount in Harris County, Texas with a 1% chance of occurring in any given year at a specific location is 18 inches. (See Table 1)
PFEs are defined in terms of an Average Recurrence Interval (ARI) or Annual Exceedance Probability (AEP) expressed in a period of time. AEPs are often translated into a percent chance of occurring in any given year to alleviate confusion; such as a 100-year flood being described as a 1% annual chance flood. (More about Annual Exceedance Probability and Average Recurrence Interval Terms)
The depth of precipitation varies with the duration of the event. Table 1 shows the depth of precipitation for different specific durations and specific occurrence probability. Note that a precipitation event that produces 7.64 inches of precipitation in a 2-hour period in Region 3 of Harris County, Texas has a 1% chance of occurring in any given year at a specific location. Similarly, a precipitation event that produces 12.5 inches of precipitation in a 6-hour period in Region 3 of Harris County, Texas also has a 1% chance of occurring in any given year at a specific location. Therefore, there are many precipitation events that can be called a 1% chance event depending on the amount of precipitation produced over a certain time period. The resulting flooding condition will vary with the amount of precipitation, existing drainage system capacity, density of development, and area topography.
The percent chance of precipitation varies for similar depths of precipitation based on duration of the event. As shown in Table 2, a 12.3-12.6-inch depth of precipitation has a varying percent chance of occurring depending on the duration. Information regarding precipitation intensity and frequency at a particular location is typically available from the local governmental agency.
Related Content: What does 100-Year Flood Really Mean?