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 rain 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.
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 Management Is Critical to Community Safety