Carefully Constructed Gardens Replace Cement, Prevent Flooding and Attract Pollinators
A movement is afoot to beautify cities and populated areas by removing substantial amounts of excess cement and planting greenery that is aimed at reducing stormwater runoff and flooding.
Water-absorbing “rain gardens” are popping up around the world and are part of the “Soak Up the Rain” effort by the US Environmental Protection Agency.
Individuals can make rain gardens on their own properties while communities can create this green infrastructure in their cities and public areas.
Historical Fights Against Flooding
In the Middle Ages, the Dutch dealt with the encroaching waters of the North Sea by building dikes, dams, and a canal system to stop, then harness the sea.
Since then, countries across the world have been using innovative methods to combat the catastrophic risk of flooding. In Jakarta, Indonesia, for instance, the government plans to build a 20-mile artificial island in Jakarta Bay—in the shape of its national emblem, the eagle-like Garuda bird—to protect its capital city from storm surges.
Meanwhile, in the borough of Enfield, London, 80 hectares (about 197 acres) of empty land has been transformed into a natural defense system against flooding in nearby towns. New woodlands have been planted that contain 50 ponds to absorb rainwater.
Excess stormwater and flooding can be a common problem in modern population centers. For instance, in the United States’ Great Plains states, its great swaths of prairie grasses and woodlands would have once absorbed the rush of water from heavy rainstorms. But today, when the clouds open up over a typical Midwestern city, the gushing water has less soft soil to slow down its flow, and it instead races over roofs, parking lots, sidewalks, and roads.
Runoff is then funneled towards storm drainage systems into rivers, lakes, and streams, even though this can increase the risk of flooding.
The Natural Solution
Rain gardens are a beautiful solution to mitigate impervious, man-made, urban landscapes.
Rain gardens return the land to something approximating its naturally porous state; it can again capture and filter stormwater before it runs off into storm drains, thus reducing the risk of flooding. Moreover, not only are rain gardens beautiful to look at but maintaining them is good for the gardener’s health.
Rain gardens are not just a garden. They are designed to collect water in shallow hollows in the yard that have been filled with appropriate vegetation.
This practice, also known as bioretention, is designed to mimic the mechanisms of natural systems that reduce water volume and pollution removal. The rain water is encouraged to slowly seep into the ground. By reducing the velocity of the flow, this process reduces the potential for erosion as well as cutting the amount of pollutants pouring from a yard into a storm drain and waterways.
Another benefit of rain gardens is their help in refilling groundwater in aquifers; they capture runoff in the shallow hollows of up to 2 feet deep to avoid soil compaction and then let it soak deeply into the ground.
Furthermore, their design helps them act as a pollution and sediment filter by catching almost the first inch of runoff, which contains the highest concentration of pollutants. Thus, rain gardens transform stormwater from a destructive carrier of pollution into a source of sustenance for plant and wildlife habitats—the plants thrive on nitrogen and phosphorus that is picked up by their roots.
Thus, rain gardens transform stormwater from a destructive carrier of pollution into a source of sustenance for plant and wildlife habitats—the plants thrive on nitrogen and phosphorus that is picked up by their roots.
Although conventional gardens on one’s property are a valuable asset, they are not a rain garden unless stormwater runoff is directed into the garden.
Rain gardens can be difficult to maintain because it is necessary to have a proper grasp of all planted species throughout all seasons to ensure none are accidentally weeded.
There are also additional upfront costs, such as the size of a rain garden being 5 to 10% of area where stormwater comes from. Rain gardens also incur additional costs if drainage is needed instead of soil as the filtration medium.
Six Components of a Rain Garden
A rain garden typically has six basic components (see image above)—growing medium, vegetation, rock trench, perforated drain, above-ground storage zone, and overflow—according to Kerr Wood Leidal, a Canadian engineering consulting firm.
The growing medium supports plant growth and holds water. Vegetation promotes the regeneration of the infiltration surface and supports evaporation and transpiration. A rock trench holds water and releases it after a rainstorm, while a perforated drain protects plant roots from flooding and maintains adequate oxygen in the space. The storage zone above ground holds rainwater after a heavy downpour until the growing medium is able to accept the water. Finally, the overflow protects any nearby buildings when heavy rainfall or freezing of the ground overwhelms the rain garden by safely steering the water to a nearby location.
Growing One’s Own Rain Garden
If the deep-rooted plants in the rain garden are native to the region, they will not need special attention once they are established. Rain garden plants may be trees, shrubs, and perennials depending on their tolerance to wet or dry soils. Plants may include hornbeam, birch, red and black chokeberry, and big bluestem, depending on the growing zone or region. Of course, non-native plants can be used, provided they are also pest-free and not invasive.
The best soil type is sandy soil that drains well, but rain gardens can even be built within gardens with less permeable soils, such as clay, as long as they can absorb the stormwater runoff from the house or garage.
The best soil type is sandy soil that drains well, but rain gardens can even be built within gardens with less permeable soils, such as clay, as long as they can absorb the stormwater runoff from the house or garage.
Potential rain gardeners can test the infiltration abilities of their soil by digging a hole 8 inches wide by 8 inches deep and filling it with water. If the water level recedes at 1 inch per hour, then the area is perfect for a rain garden without any extra soil preparation.
The size of the garden will be determined by the amount of storm runoff that needs to be absorbed and the permeability of the soil, with a sandy soil rain garden needing less space than a clay soil garden.
During a rainstorm, watch the flow of the water to find the best place for the garden, bearing in mind it should be at least 10 feet from building foundations and 25 feet away from septic system drain fields. Call the local services provider beforehand to avoid digging into buried cables and pipes.
Although a rain garden might look unkempt compared to an immaculate lawn, they do need a degree of maintenance, including regular weeding. A newly planted rain garden should have a mulch, such as wood chips or compost between the plants, to help prevent weeds and erosion, and reduce watering needs. The mulch needs to replenished as necessary and spread by hand to avoid damaging the plants.
The Front Yard Initiative
The City of New Orleans is working to cope with runoff. After a heavy rainfall, the water in this growing metropolis on the Mississippi River delta has few places to go, due to the extent of development.
To resolve its runoff problem, New Orleans is helping residents get rid of excess garden paving and encouraging the creation of rain gardens.
The city has “had a problematic, unhealthy relationship with water,” according to Dana Eness, executive director of Urban Conservancy (UC), a nonprofit organization that fosters environmental and economic resilience in a warm weather climate.
Eness said that after the devastation of Hurricane Katrina in 2005, the city started a conversation with water experts from the Netherlands, a country with water management expertise developed after the devastating North Sea Flood of 1953.
The Dutch experts encouraged the city to look for natural solutions to support their infrastructure problem. “They told us what they have learned, which is you can’t engineer your way out of this situation. You have to look at biodiversity, you have to take your lead from Mother Nature by identifying a nature-based solution,” she said.
As the city looked for ways to support the green infrastructure, UC started hearing complaints from residents about too much paving in neighbors’ yards, causing water to flood their properties, and so the Front Yard Initiative (FYI) was born.
The program encourages people to remove excessive paving and replace it with their own rain gardens. FYI pays residents $2.50 per square foot of paving they lift from their front yard in an initiative designed to let water seep into the ground and reduce the risk the city faces from further flooding.
FYI pays residents $2.50 per square foot of paving they lift from their front yard in an initiative designed to let water seep into the ground and reduce the risk the city faces from further flooding.
In the last 10 years, UC has provided financial and technical assistance to over 150 homes and lifted more than 93,000 square feet of paving from front yards. This allows, at a conservative estimate, 125,000 gallons of water to be diverted from the city’s pumping system and streets and instead absorbed into the ground after a heavy rainfall. This adds up to 4 million gallons annually.
Eness explained: “For the individual on their lot and their neighbors, it can be a major quality of life changer. They can now use their backyard that might have held water for three days and can walk from their car to their house without getting their ankles wet due to rainstorm water.
“That is a dramatic and immediate improvement,” she said.
Eness added that rain gardens also expand the biodiversity of the area: When heat-reflecting pavement is replaced by lush greenery that cools the environment, it attracts butterflies and bees. The runoff in neighborhoods is improved by even one rain garden in the area; this encourages others to build their own bioretention spaces, creating a virtuous circle.
While the level of water absorption in rain gardens is small compared to a citywide scale—the city’s pumping system has to deal with 450 million gallons in the first hour of a rainstorm—rain gardens keep significant amounts of water out of the system at an important moment.
“What this is doing in a volume scale is dwarfed but in that first critical hour, every drop counts,” Eness explained.
*Gordon Cairns is a freelance journalist and teacher of English and Forest Schools based in Scotland.