University of Minnesota-Duluth Rain Garden
One of the goals of UMD's storm water pollution prevention program is to look for ways to improve the quality of storm water leaving the campus. Installing a rain garden was our first choice for improving storm water from UMD's largest parking lot.
We chose a rain garden for this location because:
- the existing lot shed most of its water to a single point on the surface,
- a garden would enhance appearance of this site
- The existing trees could be preserved and incorporated into the garden.
- And the location provided a good opportunity to educate the public on the benefits of rain gardens.
The UMD Rain Garden covers approximately 1/3 of an acre and can hold over 60,000 gallons of water. Storm water from 2.5 acres of the parking lot is directed to the settling basin where sand and debris sink to the bottom. As the settling basin fills, water then overflows into the rain garden. Some of the water is used by the plants, some will evaporate, some will soak into the ground, and some will be released into the storm sewer.
The depressed area of the garden can hold about a foot of water before it will begin to flow out through the overflow pipe. Because the native soil is mostly clay, we installed drain tile in the bottom of our garden. Water not immediately absorbed by the soil filters down through the soil and into the drain tile. Since we want to hold as much water in the soil as possible, we installed a dam which allows us to control how much water leaves through the drain tile.
The planting soil in the rain garden is made up of 40% peat and 60% topsoil. The peat helps absorb more water than normal topsoil would. The soil treats the water in two ways; bacteria in the soil breaks down oils and greases, while the soil particle themselves bind to phosphorus and heavy metals removing them from the water.
There are more than 75 different species and 3,800 plants in the UMD Rain Garden. The plants you see were chosen for more than just their color and texture. The majority are native plants which can survive the differences in water availability. The deep roots break up the sub soil to promote infiltration and take up water into the plant. The stems and leaves put water back into the atmosphere through evaporation and help hold the mulch in place as the garden floods.
By directing storm water through the UMD Rain Garden we are decreasing the quantity of water that will reach Oregon Creek, and the water that does arrive, will arrive slower and be cooler and cleaner than when the water was flowing directly off the parking lot surface into the storm sewer.