Bio-retention systems

Who hasn’t observed a flotsam of debris in a rainbow-streaked pool of water swirling around a storm sewer drain during a heavy rain?

In addition to the aesthetic insult, that scene signals a danger to the quality of water in the lakes and streams to which it will eventual flow. During periods of heavy rain, overloaded storm water systems often dump directly into waterways rather than directing the runoff through areas where it can be filtered and treated. If the trash, pesticides, fertilizers, oil, grease and bacteria that are collected by the rain as it drains from rooftops and scours across parking lots enter a waterway without treatment, the results are predictable – degraded water quality. 

A second negative impact created by the quick runoff is loss to the local aquifer. Rain that rushes from parking lots, down a storm sewer and into a stream, doesn’t have the opportunity to filter into and replenish the local water table.

According to David Riddell, biologist for The Davey Tree Expert Co.’s natural resources consulting division, property owners are taking a new look at bio-retention installations in light of concerns about water quality, storm water management issues, governmental regulations and a heightened interest in sustainable landscapes.

Although the available bio-retention design techniques can be used on either commercial or private property, Riddell makes the distinction between private and public installations based on how they are most commonly employed. He divides bio-retention systems into those that are mandated by governmental regulations, such as bio-retention basins and bioswales on commercial properties, and a rain garden that a private land owner might install.

Obviously, collecting water that has run off from a vast commercial parking lot has a larger payoff than from the roof of a private home, but the concept is the same.
“Bio-retention areas collect and/or hold runoff to reduce the pressure on watershed and local ecosystems,” Riddell says. “They work because they release water into the storm water system at a slower rate to avoid overwhelming capacity.”

Another approach is one that takes advantage of the filtering capability of soil and plants and bypasses the storm water system. Instead, water is collected in a rain garden – a constructed area that contains plants selected for their ability to tolerate  a wide range of moisture conditions. The rain garden is located so that it naturally collects runoff and holds it until it can percolate into the ground. The water that reaches the aquifer is purified by the plants and soil that it has trickled through. Not only does this method reduce the load on treatment facilities, but it also returns the water to the ground near where it fell, renewing the local water table rather than rushing it away.

Rain gardens, bio-retention swales and basins all require thoughtful construction and regular maintenance to work effectively, but a combination of concern for a sustainable environment and a push from governmental agencies means that the beneficial results outweigh the costs.
 

Bio-retention options.
The options for property owners interested in installing a retention system depend on the configuration of the other features on the property and local regulations.  A bio-retention system is by no means a maintenance-free project, but it is one that has benefits that extend beyond the property on which it is located.

A property owner may opt for a rain garden incorporated into the existing landscape, or a bio-retention basin or bioswale in the area that water naturally collects on a developed commercial property. A public park or similar facility may develop a planting mix in a flood plan to interrupt soil erosion and silt runoff to the waterway.

Green roofs also are considered bio-retentive, but because of the engineering questions associated with the load-bearing capability of an existing structure, the installation is more complicated and much more expensive than ground level installations. Green roofs require a careful mix of impervious base material, soils, plants and irrigation systems. An incorrectly installed or poorly maintained green roof could mean water damage in the building or structural failure.

Interest in bio-retention is relatively new, but the practice is common enough that a growing number of contractors and specialty nurseries are emerging to support property owners who want to install a bio-retention cell. Each type of installation has specific requirements for successful construction.

The existing drainage patterns, soil type, structure configuration and ordinances all have impact on the size and type of bio-retention system that will be the best choice. The most common bio-retention systems are rain garden, swales and basins. Here is an overview of each:
 

Rain garden.
“A correctly constructed rain garden can sequester approximately 0.62 gallons of water per square foot,” Riddell says. And he added that private homeowners can realize tax rebates in some jurisdictions. A typical design requires a flat-bottomed area filled with engineered soil and planted with deep-rooted native species that can survive periodic submersion.

“The design, plant selection and site location are all important,” he says. “And once installed, regular maintenance is essential for the area to operate properly.”

Plant selection is critical to success as well. One business that has developed plant collections for rain gardens is Ohio Prairie Nursery in Hiram, Ohio. Its owner, Bob Kehres, says, “We’ve helped establish rain gardens in several Ohio locations, and we advise on maintenance practices. We offer selections of native plants that can handle the swing in moisture levels, which is essential for successful operation of the system,” he says.

The most salient benefit is that water collected in a rain garden may never enter the storm water system, but instead be held until it can slowly percolate into the ground refreshing the local aquifer. The water that reaches the ground water will have been cleansed by the filtering action of the root systems of the plants in the garden, mimicking a process that occurs naturally. A properly-sited rain garden also limits local flooding that can be a problem for homeowners.
 

Retention basin.
A retention basin is constructed by taking advantage of a natural depression and shaping it as a holding pond or creating a depression in an area where water flows.

“To work properly, a basin should be constructed with drainage, and a mix of engineered soil, aggregate materials and vegetation,” Riddell says. “A basin can do its job well only if the drainage outlet is maintained at the proper level and the soils and other materials are kept clog free.” He adds that often basins in commercial developments are initially constructed correctly, but over time they have lost their effectiveness by poor or absent maintenance.

Another basin configuration is one used in parking lots or areas where drainage is surrounded by impervious surfaces. Curbs or barriers are positioned into a pool-like structure to collect and hold water before slowly letting it empty into a storm drain.

“Government regulations requiring protection of water quality have been in place for years and developers have complied, but often with limited knowledge of how to install a facility. What many people don’t recognize is that these utilities require expertise to install and maintenance to function as they were intended,” he says.
 

Bioswale.
A bioswale works on the same principle as a retention basin and is constructed with some of the same techniques, although a swale takes advantage of a natural slope and is less a holding installation than one that reduces runoff speed. The  vegetation in a swale reduces the “gullywasher” effect by absorbing some of the water as it moves downward. As in all of the retention installations, the vegetation serves to slow runoff speed, filter impurities and trap some moisture that is released later through evaporation or transpiration. 
 

What’s ahead. 
If the benefits of storm water management are carrots, governmental regulations are the stick. National regulations under the clean water acts of 1972 and 1999 require outreach to the public, control of runoff to public sewer system and from construction sites, and good management practices. 

Governments are increasingly taking enforcement of management practices seriously as they struggle to meet the conditions of the acts. Additional regulations, such as those that require separation of storm water and sanitary sewer lines and  more rigorous standards for water quality, are making runoff management a high priority for both commercial developers and private property owners.

The bottom line on retention systems is that when they are properly installed and maintained, they alleviate stress on the water management systems and move toward a more sustainable landscape.

Pat Sarikelle is a master gardener. She spent 16 years with The Davey Tree Expert Co., most recently as a project manager in corporate communications. She retired in 2010.