By Claude Goguen, P.E., LEED AP
It seems like we see more news every year about floods ravaging areas throughout the country. It would be logical to assume that if we are getting more floods, we are getting more rain. However, it’s not the overall rainfall amount, but rather the increase in heavy downpours that is partially to blame. In the United States, the amount of rain falling in very heavy precipitation events has been rising significantly. This increase has been greatest in the Northeast, Midwest and upper Great Plains, which are seeing increases of 55%, 42% and 29%, respectively, in heavy rain events since 1958.1 In terms of flooding events causing more than $1 billion in losses, there have been 29 since 1980.2
The other contributing factor to increased flooding is the ever-expanding use of impermeable surfaces in the form of roofs, roadways and parking lots. When you mix higher rainfall with larger amounts of surface runoff, you get more water moving quickly to streams and rivers.
According to the U.S. Environmental Protection Agency, when rain falls on an undisturbed natural area, 50% is infiltrated into the soil, 40% is absorbed by plants and trees and 10% remains as surface runoff. When rain falls in a developed urban area, there is 15% soil infiltration, 30% absorbed by plants and trees and 55% runoff.3 This means 45% more water runs over the surface to the closest sewer or body of water. To make matters worse, the runoff captures sediment, pesticides, nutrients, bacteria, viruses, road salts and other pollutants, and takes them to rivers, ponds and lakes. This constitutes or creates a negative impact on the community in terms of environment, social and economic pillars.
NOW FOR THE GOOD NEWS
In response to the increase in surface water pollution and flooding due to stormwater runoff in urban areas, many cities throughout the U.S. are realizing the old ways of managing stormwater may not always work. They are looking at approaches to land development or redevelopment that work in tandem with nature to manage stormwater as close to its source as possible. In other words, communities are using systems that mimic the natural environment and treat stormwater as a resource rather than a waste product. This has launched a green infrastructure initiative known as low impact development.
Recently, National Precast Concrete Association professional staff attended an American Society of Civil Engineers conference devoted to LID education. Hundreds of stormwater management professionals from cities large and small attended the event to discuss LID strategies. It quickly became apparent these professionals were serious about the practice and were hungry for new technologies and ideas. This could represent an opportunity for precast concrete manufacturers to supply products that enhance LID systems’ functionality and durability. These LID strategies include simple measures such as disconnecting rooftop downspouts to more complex measures such as land-use optimization. Here are a few LID initiatives that could benefit from the use of precast concrete.
LID MITIGATION STRATEGIES
Bioretention systems, also known as filtration planters or bioretention cells, are shallow, vegetated basins that collect and absorb stormwater runoff. Smaller versions of these systems are called rain gardens. These systems are highly suitable for new construction or can be retrofitted in commercial and residential sites.
These systems mimic natural hydrology by infiltration and evapotranspiration of runoff water. Physical, biological and chemical processes help remove harmful pollutants from stormwater before discharging it to underground aquifers or surface waters. The planters also act as temporary storage for runoff water, helping minimize discharge rates. A percentage of water captured can also provide irrigation for vegetation in the planters, further contributing to the overall reduction in the volume of stormwater runoff. Typical components found in bioretention systems include grass buffer strips, sand beds, an organic layer, planting soil and vegetation.
The ability of the soil to absorb the water will dictate the size and type of planters. 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. Precast concrete can enhance these systems by encapsulating the basin and adding to its efficiency and durability while reducing maintenance demands. Some producers already offer all-in-one-type systems that can be used to hold trees, filters and additional treatment components.
Precast concrete biofiltration systems can be also be manufactured modularly. Read the Precast Solutions Winter 2015 article, “Where Does the Rain Go?” about a filtration planter installed in Ranson, W.Va.
Another LID strategy is to collect rainwater runoff from a structure or other impervious surface and store it for later use. These uses could include water for laundry, flushing toilets and urinals, irrigation, outdoor ponds and water features, and even for potable water when treated and tested.
Traditionally, this involves harvesting the rain from a roof. The rain will collect in gutters that channel the water into a storage vessel. Precast concrete can serve as a durable and strong rainwater harvesting tank for residential and commercial sites.
Permeable pavement has become a popular LID strategy. It allows water to infiltrate into underlying soils, therefore reducing storm runoff and promoting pollutant treatment and groundwater recharge. There are several types of permeable pavement systems that offer different infiltration capacities and structural strengths. Permeable pavement materials can be used for parking lots, driveways, sidewalks, utility access and residential roads.
There are two main types of permeable pavements. One is solid pavers, referred to as segmented permeable or interlocking pavements, separated by small joints or gaps to let water through. These systems allow rainwater to pass through the paver joints and seep into the ground naturally, acting as a stormwater retention and treatment system. A bed of open-graded stone is often used under the pavement to collect and store water before discharging. The nature of the underlying soil will dictate whether the water can be allowed to seep through or be collected by a network of drain pipes and routed to another system.
Another type of permeable pavement is to manufacture pavers of permeable concrete. This type of concrete is made to incorporate a system of highly permeable interconnected voids. It uses a mixture of cement, water and chemical admixtures to form a paste that forms a thick coating around coarse aggregate particles. Little-to-no sand is used. The concrete sits on a bed of stone to allow for retention and discharge into the underlying soil or a pipe system.
This technology has been dominated by the ready-mix industry; however, more precast manufacturers have been entering this market.
Green roofs are vegetated roof systems that help reduce the number of impervious surfaces in an urban landscape, and can partially or fully replace rooftop gutters and drains that feed into sewers. The green roof’s soil helps to retain rainwater and filter out contaminants in the rainwater while the vegetation removes water from the roof structure through evapotranspiration. Precast concrete roof systems are ideal for these applications as they are strong, durable and versatile, and enable designers to incorporate a variety of green roof ideas. StructureCast, an NPCA member in Bakersfield, Calif., contributed structures to a parking garage at Pomona State College that features a soccer field on the roof. This essentially met two objectives of LID: a parking garage helps reduce the impermeable area compared to a single-level parking lot and the green roof helps reduce stormwater runoff.4
Although some LID strategies may involve replacement of traditional precast stormwater structures such as pipe and manholes, there are many other opportunities for precast in this burgeoning movement. The first step is to educate yourself on LID and research what is being done in your market area to assess if this may be a viable new product line to explore. For more information about LID systems, visit U.S. EPA’s website5 or contact NPCA at (800) 366-7731.
Claude Goguen, P.E., LEEP AP, NPCA’s director of outreach and technical education.
1 ClimateCentral.org : More Downpours: Increase in Heaviest Precipitation Events, Feb 2018
2 NOAA (National Oceanic and Atmospheric Administration) – National Centers for Environmental Information – Billion-Dollar Weather and Climate Disasters: Summary Stats: As of October 9, 2018.
3 Environmental Protection Agency (EPA) Document 841-F-03-003 – Protecting Water Quality from Urban Runoff. February 2003.