Q: I have heard on the radio that some of the recent river flooding in the Midwest could have been prevented if people had used rain barrels and rain gardens in their yards. I don't have either of those products but was wondering if they would help.

A: While I think these are wonderful tools for a gardener to use, I believe a little math will show that they are less useful in the face of large rainfalls.

Rain barrels typically hold between 30 and 50 gallons. They rapidly fill up with even a small amount of rainfall. When they are full, they will start to overflow.

Let's run through a quick example. A 20-foot by 50-foot roof area is 1,000 square feet in size, meaning one-quarter of an inch of rain from the roof will produce 155 gallons of water. If we had five of the 30-gallon barrels, we would still not catch all the rain falling from the roof. Most people only have one rain barrel; therefore, we would have only saved 30 gallons.

The same one-quarter inch rainfall on an acre of land gives us 6,777 gallons of water. A 30-gallon rain barrel holds less than 0.5 percent of that total. Unfortunately, the rains in the Midwest were not just one-quarter inch rainfalls; some places received more than 10 inches of rain in a few days. That is 40 times the example listed here - an acre received 271,080 gallons of water, while the full rain barrel still only held 30 gallons from the first rain. It would take thousands of rain barrels to make a difference on each acre of land.

In areas with limited yearly rainfall, a cistern that holds thousands of gallons may be a wise choice for collecting rainfall. In areas that typically receive a plentiful amount of rainfall, collecting this free water can still be useful. Urban gardeners can give the water to planters who may not have access to an available hose. House plants such as African violets and orchids will do better with water that doesn't have chlorine and fluoride in it.

Rain gardens are created in landscapes where water is retained long enough to keep dry land plants surviving. In other words, instead of letting the water run off the lawn, you can create a low spot for the water from the roof gutter to accumulate for a while, letting it soak into the ground. Instead of having water quickly leave a landscape and flow into storm sewers and then into rivers, it can slowly sink into the soil where it replenishes groundwater aquifers and wells.

The depression that is designed into a rain garden can only hold a limited amount of water. It is a known rule that you can only put 2 gallons in a 2-gallon bucket; rain gardens will overflow when it rains more than they are designed to hold. Rain gardens work best when there are small amounts of rain that fall from roofs into these shallow depressions - large rains will just overflow the rain garden.

Wetland and shoreline plants grow well in rain gardens, and give gardeners a chance to grow a different kind of flower bed.

Many communities have required that new subdivisions create retention areas to act as community rain gardens. Even though many are not planted with wetland plants, they can slow the water flow and allow some to sink into aquifers.

The recent rains affected areas that were supposed to be flooded. Imagine a 5-mile wide river valley with a river channel in the middle of the valley that is only a half mile wide. The low and normal flows of water stay within the river channel, but the high water levels are going to spread across the whole valley. When the river is kept from flowing 1 foot deep over a 5-mile wide area, it will become deep inside the much narrower river channel that is bordered with levees.

Instead of trying to block the natural flow, it seems wiser to use the flood plain of the river valley for a flood relieve zone. Every so often down the river, there should be an area that is designed to accept the excess flow from the occasional large amounts of rain. These flooded wetland areas would act as the rain gardens of the whole river system.

Government agencies have purchased many locations throughout the country from the landowners to allow an area that floods often to become a natural wetland again. This makes more sense than trying to get more people to buy rain barrels.

You may still want to buy a rain barrel and plant a rain garden in your landscape. Although they have a small impact on flooding from abundant rains, they will still be beneficial to you. If more people had a rain garden, the important result may be the increase in aquifer levels.

E-mail questions to Jeff Rugg, Kendall County unit educator, University of Illinois Extension at jrugg@uiuc.edu.
© Copley News Service