Heavy summer rainfall is common in the hot and humid subtropical climate of the Southeast. What happens to all that water? Until communities flood or their water sources become contaminated, most of us don’t think about it. In fact, excess stormwater runoff can damage land and buildings, parks, playgrounds and more. As stormwater flows over land, it picks up contaminants including debris, oils, grease, metals and excess nutrients from fertilizers. If not mitigated, it can contaminate habitats and waterways.
Stormwater runoff refers to the stormwater that does not soak into the ground after a rainstorm, but rather runs into a stormwater drain, ditch or other catchment, picking up contaminants along the way. In an urban environment, this is exasperated by the much higher amounts of impervious areas such as rooftops, roads and parking lots in which stormwater is unable to penetrate into the ground, collecting silt, grime and waste on its way. By virtue of its concentrated flow and the materials it contains, runoff erodes river banks and other natural barriers that protect water quality in rivers, wetlands and estuaries; the physical and chemical components of runoff degrade and contaminate groundwater, streams and rivers.
Effective stormwater management is a dynamic and ongoing municipal responsibility that is regulated by the federal and state agencies. Stormwater management mitigates the damaging effects of runoff in the following ways:
- Reduces/prevents the physical, biologic and economic impact of flooding.
- Filters pollutants from runoff before they reach the water supply.
- Reduces environmental damage from flooding by reducing erosion and decreasing silt accumulation.
- Captures rainfall that can be used for drought mitigation, landscape and sports field irrigation, cooling towers, toilet flushing and other purposes, thus reducing pressure on existing freshwater supplies.
Sustainable Approaches to Stormwater Management
Greater awareness of human impacts on the environment as well as population density and climate change have pushed sustainable stormwater management to the forefront of urban planning and design.
In the Atlanta region, water issues are managed by the North Georgia Water Management District and the design of stormwater best management practices, or BMPs, are outlined within the Georgia Stormwater Management Manual, better known as the “Blue Book.” One example of sustainable stormwater management is the creation of green infrastructure (GI) which is a Low-Impact Development (LID) BMP that uses the approach of mitigating stormwater at the point source by creating stormwater treatment areas such as rain gardens and bioswales that emulate the natural water cycle, slowing down runoff and allowing the stormwater to infiltrate or soak into the land, thus not burdening the stormwater pipe system. In addition to controlling stormwater, GI filters out pollutants before they reach groundwater sources and rivers. Many green infrastructure practices are designed to filter the first inch of rainwater, which carries the highest concentration of pollutants.
The Southface Institute’s Green Infrastructure and Resilience Institute (GIRI) and Atlanta CREW offers home builders, contractors, landscapers and others training in the implementation of GI practices that address stormwater challenges in the low-lying areas of Georgia cities by teaching the techniques of how to install and maintain GI. GIRI also works with government, the private sector and community organizations, like the West Atlanta Watershed Alliance, to better integrate green infrastructure into public and private development plans.
Here are five examples of green infrastructure that sustainably capturing or treat stormwater in an urban environment:
1. Rainwater harvesting
One of the simplest water-harvesting solutions, capturing rainwater from rooftops, reduces the quantity of stormwater collecting on the ground or gushing down stormwater drains. It can be implemented on any scale, from the smallest dwelling or outbuilding to large factories and shopping malls. All that is required is a channeling mechanism (like a gutter or a downspout) and a storage option (like a barrel, cistern or tank). Furthermore, it can be scaled according to roof surface area, average rainfall and/or your budget. Captured rainwater can be used to water lawns, gardens and houseplants; to flush toilets; to wash cars; to add to compost; and to store as fire protection in fire-prone areas. Harvested rainwater does need to be filtered and the storage container emptied occasionally to prevent “fouling.” Treated rainwater has even more uses, including toilet flushing.
2. Green roofs
Vegetated green roofs allow rainwater to infiltrate into a soil media, where it is absorbed by plants and slowly released back into the atmosphere by evapotranspiration. Vegetated green roofs also create additional habitats for wildlife and mitigate the urban heat island effect, reducing the overall air temperature of cities. There are many varieties of green roofs, including active and passive, depending on the intended use of the roof. Innovations over the past few years have allowed green roofs to be designed and installed on existing buildings in addition to new builds.
3. Rain gardens
Rain gardens are shallow planted depressions that collect and hold stormwater from impervious surfaces such as driveways, pathways, patios and rooftops, allowing it to slowly infiltrate the ground. The soil and plants act as a natural filtration system that improves the quality of water entering local freshwater systems; at the same time, filtered runoff replenishes local groundwater sources rather than flowing into stormwater drains.
Bioswales, like rain gardens, retain water in soil and serve as a filtration system as the water seeps back into the ground. Additionally, they convey rainwater to a particular place, like a cistern or bioretention area, that stores larger volumes of water and allows it to slowly filter back into the ground. Bioswales are typically designed with long, narrow, slanted slopes that guide rainwater from a large area, like a parking lot or roadway. Because they must manage a higher volume of water, bioswales are deeper than raingardens and may use soils engineered to be more absorbent.
5. Permeable surfaces
Permeable hardscapes, such as permeable pavers and pervious concrete, are alternatives to typical streets, parking areas, driveways and walkways. Unlike impervious surfaces, permeable hardscapes allow water to pass through cracks in the surface material and into an area of large gravel aggregate. From this storage area, the water ultimately seeps into the ground to replenish groundwater supplies.
An underutilized renewable resource, stormwater can play a key role in improving the water sustainability of many water-stressed urban areas. Harvesting this valuable resource not only mitigates drought, water scarcity and flooding but also improves water quality while limiting habitat degradation.
To see how these BMPs can be integrated into a site development, you can visit our campus in midtown Atlanta for a free tour, and to learn more about GI training and workforce development opportunities, see our green infrastructure operations and maintenance trainings at Southface Institute.