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 surface drain.
• A garden would enhance the appearance of this site.
• The existing trees could be preserved and incorporated into the garden.
• The location provided a good opportunity to educate the general public on the benefits of rain gardens.
The Lot B 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 (perforated piping) in the bottom of our garden. Water not immediately absorbed by the soil seeps down through the soil and into the drain tile which then directs this relatively “clean” water into the City of Duluth’s storm water system that discharges into Oregon Creek. Since we want to hold as much water in the soil as possible, to allow for infiltration and plant uptake, we installed an outlet structure which allows us to control how much water is held back.
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 three major ways:
1. It acts as filter with fine pore spaces that retain mineral particles and organic debris.
2. Naturally occurring microorganisms in the soil break down organic matter like oils and greases, bits of dead animal and plant detritus, disease causing organisms from animal and human wastes, and some anthropogenic contaminants such as pesticides and petroleum derived compounds (“biodegradation”).
3. By the capacity of soil particles to chemically adsorb (i.e. bind to) most heavy metals, and the nutrient phosphorus (in its phosphate form) which in excess can stimulate too much algae growth in downstream areas and in the near-shore waters of Lake Superior.
There are more than 75 different species and 3,800 plants in the Lot B Rain Garden. The plants were chosen for more than just their color and texture. The majority are native plants which can survive the typical range of water availability and temperature. The plants are important because the roots break up the subsoil and promote infiltration. The roots also help hold the mulch in place if the garden floods. During the day the plant’s growth acts to remove water via evapo-transpiration.
By directing storm water through the Lot B Rain Garden we are decreasing the quantity of storm water that will drain into Oregon Creek. The water that does arrive will arrive slower and be cooler and cleaner than if it flowed directly off the parking lot surface into the storm sewer.
Design by Barr Engineering
Constructed Summer 2005 by LKO Contracting / UMD Grounds
RG Construction Pictures