By Claude Goguen, P.E., LEED AP
When we construct roads and buildings, it disrupts a sensitive ecosystem. Not only are we displacing fauna and flora, we are disrupting the hydrologic cycle. Stormwater surface runoff increases with every added roof and concrete or asphalt surface. Less stormwater is absorbed into the soil and more is conveyed to collection systems.
When sewers were first built, they could handle the amount of stormwater runoff. However, as development expands, systems are now pushed beyond their capacity. One of the main consequences of this is combined sewer overflows that convey both wastewater and stormwater. Under normal conditions, the combined system transports the water it collects to a sewage treatment plant before discharging it to a water body. During heavy rains or snow melt, the volume of stormwater and wastewater can sometimes exceed the system’s capacity. When this occurs, untreated stormwater and wastewater discharges directly into nearby streams, rivers and other water bodies. It’s hard to imagine raw sewage flowing directly into our rivers and lakes, but according to the United States Environmental Protection Agency, this causes major pollution concerns for nearly 800 cities.
As a result, cities are mandated to address these CSOs as quickly as possible. There are many strategies to address CSO management that may involve precast concrete structures. One strategy that is gaining popularity in urban areas is to employ green infrastructure to mimic nature by storing and soaking up stormwater runoff near the source. Precast concrete is well-suited for this green infrastructure.
Hopping on the green infrastructure train
The U.S. EPA defines green stormwater infrastructure as “a cost-effective, resilient approach to managing wet weather impacts that provides many community benefits. While single-purpose gray stormwater infrastructure – conventional piped drainage and water treatment systems – is designed to move urban stormwater away from the built environment, green infrastructure reduces and treats stormwater at its source while delivering environmental, social and economic benefits.”
This green infrastructure trend hasn’t simply caught on. It’s growing by leaps and bounds and presents a potential product line for precast concrete producers. Commonly used terms for green stormwater infrastructure systems include rain gardens, and bioretention or bioinfiltration cells. These are shallow, vegetated basins that collect and absorb runoff from rooftops, sidewalks and streets. Planter boxes or filtration planters can often be used to function as rain gardens. There are also bioswales, which are vegetated or mulched channels that provide treatment and retention as they move stormwater from one place to another. Vegetated swales slow, infiltrate and filter stormwater flows. As linear features, they are well-suited for placement along streets and parking lots.
How soil absorbs the water will dictate the planter’s size and type. Some are bottomless, allowing water to filter through. Others have a bottom that treats and retains runoff water before discharging it into the sewer system.
According to the City of Portland, Ore., special attention should be paid to the structural waterproofing if the planter is constructed adjacent to building structures. Infiltration planter areas should be clearly marked before site work begins to avoid soil compaction and sedimentation to preserve infiltration capacity during construction. No vehicular or foot traffic, except that specifically used to construct the facility, should be allowed within 10 feet of infiltration planter areas.
Green Stormwater Infrastructure Success Stories
The U.S. Department of Transportation awarded Ranson, W.Va., federal funding as part of its “Green Corridor Revitalization” project to improve a 1.5-mile stretch of Fairfax Boulevard, a two-lane road providing residential access and parking. In addition to adding trees, sidewalks, a center landscaped median and new street furniture, the project scope implemented cost-effective and sustainable stormwater management techniques. This was an important feature since the community is located in the Chesapeake Bay watershed.
The stormwater management initiative included installing bottomless precast concrete filtration planters along the western edge of the southbound lane along Fairfax Boulevard. Each planter, manufactured by Midwest Block & Brick in Bridgeton, Mo., has a geo-membrane liner to prevent concentrated infiltration points at the installation site. Perforated underdrains and an overflow inlet also help to accommodate large stormwater events.
The precast concrete planters consist of a segmental planter wall and curbing system. The modular system allows the filtration planters to expand and contract in size and shape based on block-by-block conditions and stormwater capture needs.
The precast concrete system can be built in a day versus a week with cast-in-place concrete. In addition, the modular system can be removed and reused. It can also be reconfigured to accommodate changes to size and shape – based on localized needs – and repaired if accidental damage occurs.
Levi’s Stadium stormwater solution
The bioretention project at Levi’s Stadium in Santa Clara, Calif., home to the San Francisco 49ers, provides an excellent example of a simple runoff mitigation system that uses the power of nature to achieve its purpose. Ghilotti Construction and second-phase contractor Turner-Devcon built a bioretention project to handle runoff for more than five acres of concrete.
A large construction project such as a major league sports stadium can include significant expanses of runoff-generating hardscape. A solution to drain the area and treat the runoff was the project’s goal. To meet the needs, designers selected a low-impact development solution that was also aesthetically pleasing. The modular bioretention system collects runoff and treats it using natural biofiltration, and includes attractive landscaping elements and customizability.
Precast concrete is the ideal solution
Landscape architects, urban planners and designers are devising innovative ways to manage stormwater runoff while adding beautiful elements to the area. With ease of installation, durability and versatility, precast concrete is the ideal material for these green infrastructure elements.
Claude Goguen, P.E., LEED AP, is NPCA’s director of sustainability and technical education.