ATHENS, Ga. – When it’s dry, the rock-filled swale in front of the University of Georgia’s physical plant grounds department office looks like it might be leftovers from a construction project. But when rain sends water from nearby roofs and a parking lot flowing into the depression, it becomes a showpiece for a popular concept in stormwater management known as a raingarden.
Part reservoir, part filtration system, raingardens are a smaller version of what are also known as detention ponds. Like ponds, raingardens catch rain runoff from impervious surfaces such as asphalt and concrete, helping keep the water out of conventional drainage systems such as sewers.
But raingardens – technically known as “bioretention cells” – can do more. They help cleanse water before it goes into the ground or into a drainage system. They also beautify landscapes and can improve safety and health by reducing flooding and standing water.
Expect to see several of them along Lumpkin Street as part of the extensive stormwater and drainage improvement work Athens-Clarke County is carrying out on that busy thoroughfare.
UGA will maintain the Lumpkin Street raingardens, and more will be built around campus, says Dexter Adams, head of the grounds department. The garden at the grounds department office was installed as a demonstration to help UGA workers become familiar with them, says Adams, who is a big raingarden fan.
Basically, a raingarden is a large hole in the ground, several feet deep. Its size is based on the amount of water it’s expected to handle; its shape is aligned with the contours of the surrounding landscape.
The bottom has a “cell” composed of uniform-sized gravel and surrounded with a special porous fabric that allows water through but filters out sediment. On top of the fabric is a layer of organic material such as sandy loam. Above that, on the ground surface, are gravel and rocks of varying sizes that trap water in pockets and release it slowly. An overflow pipe buried in the hole rises about a foot above the rock surface. The garden is surrounded by grass, shrubs, trees and other amenities.
When rain falls, runoff from parking lots, sidewalks, streets, roofs and other impervious surfaces is channeled into the raingarden instead of going down the sewer drain or pooling along the curb. As the water seeps down through the rocks, organic material and porous fabric, dirt, oil and other impurities are leached out and the clean water is absorbed into the ground.
Adams says a raingarden can treat about 70 percent of the annual rainfall amount in Athens (the amount that occurs in approximately one inch or less of rain). For bigger downpours, the excess water goes into the overflow pipe, which may open into a storm sewer or nearby stream.
“Rains of an inch or less carry the most concentrated runoff of pollutants, trash and sediments,” says Adams. “For these normal rainfalls, a raingarden will really drink up the water quickly so there’s rarely any standing water.”
The demonstration raingarden at the grounds department, roughly rectangular in shape, is more than 12 feet long and several feet wide. Rocks on the surface vary from golf ball-size stones to heavy mini-boulders. It’s surrounded by native plants including broomsedge, iris and serviceberry trees.
If a deluge sends more runoff into the garden than it can handle, the excess water goes in nearby Trail Creek and “will actually tend to be cleaner due to the earlier work of the garden,” says Adams.
The university has a couple of other devices that just hold water, including a berm-like area at the ecology building, and a retention pond at the intramural fields. “But this is our first proper raingarden that brings together all the elements and amenities,” Adams says. “It’s not just a tank.”
Raingardens are an integral part of the stormwater management work underway on Lumpkin Street. Adams says gardens about the same size as the demonstration garden will be installed at several locations between Baxter and Bloomfield streets, including near the auxiliary services building and near the Chi Omega sorority house.
The forested area between Cedar Street and Clark Howell Hall will be a “stormwater treatment museum” with several kinds of treatments, Adams says. They include bioretention areas (raingardens), filter strips (basically a grassy swale), an enhanced swale (an area with plants and rocky dams to reduce water flow), and an infiltration trench (an underground pipe with holes that allows water to percolate into the subsoil).
Gardens will also be included when the work moves north on Baxter toward Broad Street next year.
Raingardens are more expensive to install than conventional sewer drains. But Adams says they pay off in reduced water pollution, stormwater control, improved aesthetics and enhanced safety.
“As an educational institution, we should set an example by using best management practices,” he says. “Physical plant is excited to be working with Athens-Clarke County to take advantage of these new approaches to stormwater management.”
