Spring 2006

Reasearch shows that rain gardens can be a highly effective way to funnel rainwater into the ground
UW Water Resources Research

Flower Power
Rain Gardens Found to Be Highly Effective

By John Karl

If only all woes of urban development could be addressed so simply and beautifully.

The buildings, roads, and parking lots covering the ground in urban areas prevent rainwater from soaking in, and that can be hard on a community’s water supply and water quality. The impervious surfaces prevent rainwater from refilling the underground storage areas, or aquifers, from which households, businesses, and industries pump millions of gallons of water every day. So water tables fall. And, as rainwater flows off rooftops and concrete, it picks up contaminants and heads directly into sewers—and from there into rivers and lakes—bypassing the natural filtering that happens when water slowly seeps though the tiny pores of underground rock, gravel, and sand.

New research, however, shows that rain gardens—essentially depressions in the ground filled with water-loving plants—can be a highly effective way to funnel rainwater into the ground. And the most effective rain gardens are quite modest in size, making it feasible to incorporate them into new developments.

The findings come from Ken Potter, a UW–Madison professor of civil and environmental engineering. With support from the UW Water Resources Institute, Potter worked with former graduate student Alejandro Dussaillant and others to develop a theoretical model to estimate how the size, depth, and soil characteristics (especially “hydrological conductivity”) of rain gardens affect the amount of water they channel into the ground. They then built an experimental, real-world rain garden to test the model’s predictions.

The actual rain garden supported the model’s predictions, yielding two happy conclusions. Across most of Wisconsin, a rain garden on developed property can funnel two to three times more water into the ground than comparable undeveloped property. And the optimal area for a rain garden is only 10 to 20 percent as large as the impervious surface area it drains. (That’s because larger gardens lose more water to evaporation.)

Potter’s “enhanced” rain gardens feature three layers. The top layer is simply the depression itself, where rain water collects when it runs off rooftops and other impervious surfaces. This is the “ponding zone.” Below that is the root zone, where plant roots help the infiltration process by maintaining channels that water can flow through. The bottom layer is a storage zone. It’s filled with sand or crushed rock, which supports the layers above and, when it rains hard, it holds large amounts of water until it can seep into the surrounding soil.

If local topography permits, an underdrain pipe at the top of the storage zone leads to a ground-level opening, allowing water to drain out of the ponding zone when the storage zone fills up. This prevents water from remaining in the ponding zone longer than the plants can tolerate.

The Wisconsin Department of Natural Resources allows developers and landscape architects to use Potter’s model software, called RECARGA, to demonstrate compliance with new regulations requiring large-scale commercial and residential developments to meet targets for infiltrating runoff. To help developers and others use the software, Potter and his former students Linda Severson and Dustin Atchison have published a manual, called Design Guidelines for Stormwater Bioretention Facilities. Both the manual and the software may be ordered or downloaded free of charge from the ASC Publications Store (aqua.wisc.edu/publications).

More information on rain gardens is available from the Wisconsin Department of Natural Resources at www.dnr.state.wi.us/org/water/wm/nps/rg.

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