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Flooding & Rainfall Patterns

Rainfall Frequency

More frequent 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 the probabilities in any given year for a significant rainfall event to occur.

In older areas of most cities, developed prior to the 1980’s, there are no flood management practices in place.  In many communities, in newer developments, there are ordinance requirements to install detention and retention basins that are used to temporarily impound stormwater and release it at slower rates that minimize flooding to local streams.

Other methods for managing floods include floodway and floodplain management.  A floodway according to FEMA, is the channel of a river or other watercourse and adjacent land areas that are reserved or protected from development in order to discharge flood waters without increasing the water surface elevation more than a designated height. Communities are required to regulate development in these floodways to ensure that there are no increases in upstream flood elevations. 


Floodplains & Heavy Rains

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.  The excess water cannot be absorbed by soil and all of the surrounding impervious surfaces so it overflows and submerges land that is typically not under water.  While there are many causes for flooding, the focus here will be on two types of rain events that cause two different types of flood events:

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, they are often difficult to predict.  Flash flooding in urban areas is an increasingly serious problem due to removal of vegetation, paving and replacement of ground cover by impervious surfaces that increase runoff, and construction of drainage systems that increase the conveyance and delivery of runoff.

Source: WA.gov
Source: WA.gov
Time Period (Years) Annual Exceedance Probability
2 50%
5 20%
10 10%
25 4%
50 2%
100 1%
500 0.20%

Extreme Floods are caused by large-scale weather systems generating 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 assist 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 as well.


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 have a 1 percent chance of occuring in any given year.