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Flooding & Rainfall Patterns

Rainfall events that generate flooding are occurring more often throughout Iowa.  This seems to be the projected trend for the future.  Communities need to develop more resilient strategies to address flooding issues.  A starting point is to clarify the common misperception about probabilities of rainfall events.

Types of Flooding

A floodplain is the channel of a river and adjacent land areas that are reserved or protected from development. Some communities are required to regulate development in these floodways. Regulation is meant to ensure that there are no increases in upstream flood elevations.

Floodplains are nature’s storage areas for heavy rains that help reduce the likelihood or extent of downstream flooding.  Every waterbody has a floodplain, from the smallest creek to the largest river.  When flooding occurs rain saturates the urban drainage system, which causes flooding.  There are two main types of flooding to consider with respect to stormwater.

Flash Floods are caused by an extreme, localized storm where a lot of ain falls in a short amount of time.  It typically affects a small area within the watershed.  Floods of this type are particularly dangerous because of the suddenness and speed with which they occur. Consequently, they are often difficult to predict.  Flash flooding in urban areas is an increasingly serious problem. It can be due to removal of vegetation and the increase in impervious surface area. It is also a factor of how of drainage systems are designed to simply convey runoff as quickly as possible.

Extreme Floods are caused by large-scale weather systems that generate prolonged rainfall over wide watershed areas.  Floods of this nature are still dangerous, however they are easier to predict.  In fact, the Iowa Flood Center, is working hard to help communities with mapping, monitoring and responding to potential floods.  Starting in the early 90’s, most of Iowa’s urban communities began requiring flood control practices, such as detention and retention basins, to address downstream flooding. 

This “one-size fits all” approach may not be all that is needed to reduce flooding caused by urban development, especially in watersheds that have agriculture development.

Flood boundaries diagram floodway floodplain
Source: WA.gov
Flooding extreme Cedar Rapids floodplain

Flood Frequency

Rainfall events can be estimated through recurrence intervals. The interval expresses the likelihood (as a percentage) that a storm of the same duration and volume will be exceeded in one year. This is known as the Annual Exceedance Probability. A flood event with the duration and volume of a “2-year storm”, which is 2.91 inches of rain over 24 hours, has a 50 percent chance of ocurring in any given year. Likewise, a “100-year storm”, which is equivalent to 6.61 inches of rain over a 24-hour period, has the likelihood of a 1 percent chance annually.

The “100-year storm” should not be mistaken with a storm that would only occur once in 100 years. Rather the phrase is used to illustrate storms of the 100-year magnitude and have a 1 percent chance of occuring in any given year.

Time PeriodAnnual Exceedance Probability
2 years50%
5 years20%
10 years10%
25 years4%
50 years2%
100 years1%
500 years0.20%

Other Resources

Additionally, the Iowa Flood Center has developed many tools for communities in Iowa to assess and predict the impacts of flooding. Check out IFC’s helpful tools for more information.