Storm Water Detention vs Retention

Just a quick note about storm water detention or as we are calling it these days, “storm water control facility”.  People often confuse the meaning of storm water “detention” and “retention”.  They think a retention pond it is a storm water detention area that is always wet or looks like a pond.

True storm water retention is rarely used in Northern Illinois.  A storm water retention facility captures storm water runoff and holds all or part of it until it infiltrates or evaporates or is used for irrigation.  Storm water retention is a common practice in arid regions and locations with surface soils that allow water to enter quickly.  These soils have a “high infiltration capacity”.   Sand is an example of a high infiltration capacity soil.

Much of Northern Illinois has soils with high clay content near the surface.  Clayey soils have a low infiltration capacity and may not be well suited for “retention” facilities.  Other reasons much of Northern Illinois is not easily suited for storm water retention include a high water table during wet seasons and frozen ground during winter months.  Storm water retention does have several potential benefits for Northern Illinois.  These include recharging our drinking water aquifers, reducing the discharge of pollutants to our streams and reducing flood flows.  These benefits may lead to future state and local storm water retention regulations.

So what do we call a storm water detention facility that always has water in it?  It is often referred to as a “wet bottom storm water detention facility”.   The three common types of storm water detention/control facilities in Northern Illinois are:

(1) Dry bottom (grass, concrete, and landscaping)
(2) Wet bottom (open water pond with dumped rock, grass, or landscaped banks)
(3) Wetland bottom (flat bottom holding a few inches of water with wetland vegetation)

Many storm water control facilities are a combination of all three types.


About Mike Gingerich

Engineer Writer Lecturer
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