To the untrained eye, it looks like a peaceful creek. But the water flowing through the UW Arboretum's West Wingra Marsh is a deep gully cut by unmanaged storm water.
Urban areas struggle to control storm water because many surfaces, such as pavement and rooftops, are impervious to rain water. When it cannot soak into the ground, storm water becomes runoff, causing erosion, sedimentation, excess algae growth and the spread of invasive species.
Since the Arboretum and Lake Wingra sit at a low point, they receive runoff-an average of 470 million gallons a year-from the surrounding areas of Madison and Fitchburg.
David Liebl, a waste-reduction management specialist at UW-Extension, said the gully through the West Wingra Marsh is caused by a storm drain at Manitou Way, which forms the western border of the Arboretum. The gully carries 40 percent of runoff entering Lake Wingra each year. Such a high volume of flowing water creates multiple problems.
\The water cutting the channels is not only degrading the landscape, but it's tearing up all the sediment, and it ends up in the lake. It's filling Lake Wingra,"" said Kenneth Potter, UW-Madison professor of environmental and civil engineering.
""We're getting dry land forming in the lake. There's actually an island with plants growing on it,"" Liebl said.
Civil and environmental engineers have devised solutions that make urban surfaces more permeable, preventing runoff and the associated damage.
One effective storm water management device is called a green roof, which is a rooftop covered with soil and plants.
""When rain hits a green roof, it stays there and is slowed down,"" said Patrick Eagan, a UW-Madison associate professor of engineering and member of the Arboretum's storm water committee.
If gardening on the roof does not seem appealing, homeowners and businesses can create rain gardens instead.
""A rain garden is a depression where you plant water-tolerant species, hopefully with deep roots, at a place where water infiltration rate is reasonably high,"" Potter said. ""If the plants are about 20 percent of the size of the roof, you can successfully infiltrate the water coming off that roof.""
Plants can prevent rooftop runoff, but they cannot compensate for large areas of impermeable pavement.
Water can infiltrate asphalt that is constructed with small porous areas and an absorbent layer underneath. For example, a layer of sand beneath the surface can take in rainwater and then slowly release it into the ground.
Pervious pavement costs more than regular pavement, so few examples exist. However, Eagan said the expense, as well as the expense of other storm water prevention strategies, is a good investment because it prevents the costs of future storm water-related cleanup.
""We're going to have to invest in these kinds of practices and find even better practices than we have now,"" Eagan said.
