Why are cities using retention basins and rain gardens?

Longford Pond in Fitchburg, WI, retains sediments flowing between Dunn’s Marsh and nearby Lake Waubesa. Credit: Susan Fisk

Q: As I watch various commercial or residential real estate development projects in my community, I observe more features related to storm water management as compared to older developments–retention basins, rain gardens, concrete lined channels, and pumping stations to name a few. Why the change?

A: All these features help capture, convey (move), and clean water that moves over the surface of the earth: what we call “surface runoff.” Surface runoff occurs whenever the rate of precipitation exceeds the ground’s ability to soak it up.

As civilizations continued to create buildings and roads, and as towns grew into cities, more of the earth’s surface was covered with impervious materials. This created a need for systems to manage the runoff.

Consider a highly urbanized area that might have more than 95% of the area covered by buildings or pavement. These surfaces essentially absorb nothing. All the rainfall becomes surface runoff. The amounts can be significant. A standard city block (1/8 mile by 1/8 mile, about 10 acres in size) creates nearly 300,000 gallons of surface runoff from a one-inch rainfall if none of it enters into the soil. Even residential areas can have more half the surface covered with prohibitive surfaces. Now imagine a major storm that drops five inches of rain or more. Storm water management is important!

Increasing the size of streetside storm drains helps water flow during intense events in Manhattan, KS. Credit: Gary Pierzynski, Kansas State University

History is a good teacher. Under-designed storm water systems contribute to urban flooding during heavy precipitation events. The design has to handle the total amount of normal runoff as well as the peak amount. Five inches of rain falling over 24 hours creates a much different storm water need compared to five inches of rain falling in two hours!

Concrete stone lined drainage way in Manhattan, KS. Credit: Gary Pierzynski, KSU

Newer developments are correcting previous mistakes. This includes concrete-lined channels to prevent erosion at high flows and pumping stations to keep water moving if the natural landscape does not provide gravity flow.

There is also an increased focus on cleaning the runoff. Everything you see on streets and sidewalks–salt, petroleum products, pesticides, plant nutrients, litter, and other contaminants–moves with the runoff. Often it ends up in local lakes and streams as pollution. Rain gardens and retention basins allow water to flow more slowly and naturally through soil. This facilitates the removal of many of these undesirables. (See previous blog posts: Soil: Largest Reactor on the Planet? and How do Rain Gardens Help with Storm Water?)

Finally, city planners know to look to the future. One prediction with global climate change is less frequent but more intense storm events. This suggests a need to enhance our storm water management capabilities, before we need it!

Answered by Gary M Pierzynski, The Ohio State University

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