Paving the Way for Long Term Stormwater Mgmt

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PICPs offer cost-effective, sustainable solutions for all climates

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Development brings changes to the hydrologic cycle. With development comes paved surfaces, which may well be the most ubiquitous structures ever built. In the United States alone, paved surfaces cover more than 43,000 square miles—an area nearly the size of Ohio—according to research published in the June 2004 issue of Eos, the newsletter of the American Geophysical Union. But not all pavement is created equal. Impervious pavements can be concrete or asphalt, roofs or parking lots, but they all have at least one thing in common—water runs off of them, not through them. They collect and accumulate pollutants which run directly into water bodies. Because impervious surfaces promote less infiltration, peak flows of stormwater runoff are larger and arrive earlier, increasing the magnitude of urban floods. By contrast, pervious pavement is designed to allow percolation or infiltration of stormwater through the surface into the soil below where the water is naturally filtered and pollutants are removed. Segmental pavements have been used since the Romans built the Appian Way over 2,000 years ago. In recent years, flexible segmental paver systems, notably permeable interlocking concrete pavements (PICPs), have provided environmentally sound engineered solutions for municipal, commercial, and industrial applications. This article will explore the impacts of development on the hydrologic cycle, explaining low-impact development as a way to mitigate negative effects, and assessing the importance of permeable pavement systems in achieving sustainable stormwater control. Also discussed will be the LEED categories to which permeable pavement contributes, and examples of how PICP systems function in real-world applications.

The Hydrologic Cycle Explained

To understand the contribution of permeable segmental pavements to sustainable sites, it is important to understand the dynamics of the hydrologic cycle.

The natural hydrologic cycle involves recycling of water, the continuous circulation of water between the oceans, atmosphere, and land. Precipitation events produce stormwater, which can take a number of paths. It can become part of the groundwater, which feeds streams, wetlands, and underground aquifers and supplies much of our drinking water. It can form lakes or enter topsoil and evaporate, or it can be absorbed by plants and eventually evaporate from plant tissues. This cycle is contained to a degree within a watershed, which is naturally bounded by hills or ridges.

Managing stormwater runoff is critical and it constitutes a major part of site design. Nature has always provided a solution to manage stormwater runoff, naturally. Man, on the other hand, does not always get it right. As it was put in Understanding Stormwater Management: An Introduction to Stormwater Management Planning and Design, Ontario Ministry of the Environment, ©Queen’s Printer for Ontario, 2003, “Humans interact with the hydrologic cycle by extracting water for agricultural, domestic, and industrial uses, and returning it as wastewater which may degrade water quality. Urban development also interferes with the natural transfers of water between storage compartments of the hydrologic cycle. There is decreased infiltration (seepage into the soil) of precipitation and snowmelt which leads to increased stormwater runoff.”

When land is developed and covered with impervious surfaces like roads, buildings, and parking lots, the rain no longer infiltrates, or nourishes plant growth. In a post-development scenario, infiltration and evapotranspiration, both drop dramatically and runoff increases by 45 percent. Additionally, the runoff is often picking up pollutants which might include motor oils, gasoline, fertilizers, and pesticides. This type of pollution is called non-point pollution, and according to the U.S. Environmental Protection Agency (EPA), it is the leading remaining cause of water quality problems.

The initial flow of stormwater that runs off a surface or catchment area typically contains a much higher pollutant load than the stormwater that follows. This first flow is called the “first flush.” The ability to catch and treat this first flush before releasing it into a stormwater system controls most of the pollution. Cities are particularly concerned with the quantity of urban stormwater discharge and its impact on streams and rivers. Not only does this large volume of water erode stream banks and streambeds, change the shape and dimension of river channels, and alter aquatic habitat and channel stability, in extreme cases it leads to floods and will affect the built environment through washed out roads and bridges.

Stormwater runoff also carries dust, dirt, and debris that can also degrade aquatic habitats. In some climates, collecting stormwater via impervious surfaces will raise the temperature of the water, which when released to streams and rivers, can harm aquatic species.

The Total Suspended Solids (TSS) measurement calculates the amount of dirt and some pollutants in runoff for a value used by regulating bodies to ensure the cleanliness of stormwater discharge. This is expressed as a percentage of removal where test results of surface removal of TSS were impacted by various sizes of chips in the voids, per a Florida Gulf Coast University study. According to the EPA, a Total Maximum Daily Load, or TMDL, is “a calculation of the maximum amount of a pollutant that a water body can receive and still safely meet water quality standards. The most common pollutants coming from stormwater sources include sediment, pathogens, nutrients, and metals.” TMDL levels are set by regulating bodies.

Stormwater Management

Managing stormwater supports sustainability goals, and municipalities and developers have created strategies to prevent stormwater pollution and designed systems that treat stormwater and route it safely back into the natural environment. Traditional stormwater management techniques have taken a collect, convey, and centralize approach that views water as a waste product. The methodology focuses on collecting stormwater in pipes and transporting it off site as quickly as possible, either directly to a stream or river, to a large stormwater management facility, or combined sewer system linked to a wastewater treatment plant.

Common components of this method are impervious surfaces and retention ponds, both of which have their drawbacks. Traditional retention ponds use valuable surface area and have proven to be expensive to maintain, usually requiring draining and dredging. Impervious pavements have a high cost of maintenance, they transfer pollutants and sediment to sewers and waterways, and increase winter maintenance expenses, providing a poor life-cycle cost.

Ecological Strategies

A more modern and sustainable view of stormwater management maintains the natural hydrologic cycle, prevents increased risk of flooding and stream erosion, protects water quality and the health of water bodies, and provides human uses of water. Low-impact development (LID) promotes these methods. Working to minimize impacts of development on the hydrological cycle of a watershed, LID addresses stormwater concerns through a variety of techniques, including strategic site design, measures to control the sources of runoff, thoughtful landscape planning, preserving and recreating natural landscape, and minimizing imperviousness to create practical, appealing site drainage systems that treat stormwater as a resource rather than a waste product. Examples of these techniques include bioretention facilities, rain gardens, vegetated rooftops, rain barrels, and permeable pavements. By implementing LID principles and practices, stormwater can be managed to reduce the impact of built areas and promote the natural movement of water within an ecosystem or watershed. Applied on a broad scale, LID can maintain or restore a watershed’s hydrologic and ecological functions. (For more information on LID see: www.lowimpactdevelopment.org/ index.html.) 

Best Management Practices

Best Management Practices (BMP) have been developed in order to help municipalities and other entities effectively control stormwater runoff. Comprising stormwater management and conservation practices proven to effectively control the movement of pollutants and prevent degradation of soil and water resources, BMPs can be divided into two categories: structural and nonstructural. Performance-based BMP tools allow for monitoring and adjustment to achieve wastewater volume and quality goals that will:

• Work within the landscape
• Focus on prevention
• Micromanage stormwater
• Keep it simple
• Allow for multifunctional landscape designs
• Capture and treat stormwater runoff at its source
• Maintain and sustain LID tools such as permeable pavement systems

 In short, LID involves the three D’s of stormwater control: disconnect, distribute, decentralize, that is, disconnect storm sewers, distribute the stormwater on-site, and decentralize this runoff by promoting groundwater recharge where possible. Central to this system is permeable interlocking concrete pavement (PICP), which is able to capture rainwater for full exfiltration into the soils below or for capture and reuse locally.

PICPs—An Ecological Approach

Defined as a system of concrete pavers with joints that allow for infiltration of water through the pavement, PICPs are an engineered ecological system that captures, treats, and stores stormwater. The pavement uses an opengraded base and sub-base for water infiltration and/or storage. These systems can be designed for full exfiltration of the captured water or complete storage. They can stand alone or be used in conjunction with swales, ponds, or storage tanks.

PICP systems can be designed for 100-year hydrological events, with treatment zones that can also be included to encourage the use of naturally occurring enzymes to establish a bacteria colony that will break down the first flush pollutants within the system. In contrast to impervious systems, PICPs have demonstrated ability to provide competitive capitalization costs, reduce winter maintenance costs, and improve long-term savings by providing a 50-year pavement design. Used in conjunction with bioremediation, PICP is a BMP treatment train used effectively in the U.S. for the past 10 years and in Europe for the past 20 years to mitigate peak flows and improve water quality. A method of removing pollutants using microorganisms in soil, bioremediation can occur naturally or it can be spurred on via biostimulation, which is the addition of fertilizers to increase the bioavailability within the medium. Recently, the addition of microbe strains matched to the medium have successfully enhanced the resident microbe population’s ability to break down contaminants. Bio-swales have proven effective in this regard as well.

Designing a PICP System

PICP systems are very site-specific, engineered systems, as are all site solutions involving water, soils, and loading conditions. However, generalizations can be made for purposes of discussion. It is important to note that every PICP system requires a set of site parameters and must be designed and reviewed by a design professional—civil engineer, geotechnical engineer, or other qualified land planner/ designer. PICP systems are most effective if treated as a first design strategy by engineers or other qualified professionals. 

Soil Considerations

The first step in designing a PICP system is a hydrologic analysis of in-situ soils or the sub-grade, with an additional structural analysis required for vehicular applications. Sub-grades should be at 92 percent density. For full exfiltration, the sub-grade should not be compacted during installation. If soils do not demonstrate a percolation rate greater than ½ inch per hour, a partial or no exfiltration strategy should be considered. This would require compaction of sub-grade to 95 percent as well as use of an underdrain engineered to clear water from the loaded areas of the subgrade within 24 (36) to 36 (72) hours. Clay soils lose strength when saturated; soil strengths are measured at a 96-hour saturated condition and will not perform as designed if saturated for a longer period.

Also, municipal peak discharge rates are set at 24 hours, and mitigating peak flows will be met by a permeable pavement system without compromising soil strength within this timeframe. If a longer time of concentration is required, the water should be moved to a non-vehicular loaded area or a structural tank, or a deeper aggregate cross-section should be considered to reduce excessive loading on the soils.

Aggregate System

PICP systems have three layers of aggregates, increasingly larger stones for the subbase, base, and bedding layers. The aggregates play an important part in filtering pollutants.

Most pollutants should be filtered out of the stormwater before it is discharged into the ground. In areas of higher pollutant levels or with high water tables, capture and treatment of the water are necessary. Depending on the pollutant level, a liner and biofiltration system may be used for a no-exfiltration system. A minimum of 12 inches of soil under the subbase is necessary and will encourage removal of chlorides and some heavy metals with a full ex-filtration system.

Pollutants can be removed by the setting bed and void fill materials, or via the filtration action of the water passing by the aggregates laden with enzymes that will create a bacteria colony.

Void areas of the base and sub-base aggregate material play a key role in the system’s reservoir capacity. Stormwater runoff is calculated for a site prior to development and expressed in a volume, e.g. cu-ft or ac-ft. When the site is developed, another hydrological analysis determines the increase in volume of runoff, the difference being the volume required for a detention or retention pond based on local regulations for a 2-year or 25-year or 100-year storm. The base and sub-base depth is designed to hold this increase in volume of water within the voids of the aggregates.

The void area or porosity of the sub-base aggregate will vary based on quarries and material runs and should be known prior to design. Typically, ASTM #2 develops voids at 41 to 45 percent, with a conservative value of 40 percent recommended to determine the appropriate detention/retention volume. The base material, #57, usually is credited at 32 to 35 percent void value. These values are also used to determine the depth of the aggregate sub-base based on area covered.

PICP System Design

ASTM #2 aggregate is used to build the subbase. This will bridge clay soils without the use of a geotextile and provide a detention area of 40 percent suitable for detention/retention requirements. The base layer will act as a choking layer using ASTM #57 washed fractured stones that do not exceed a 4-inch depth. The setting bed layer will be washed and screeded to a level condition to receive the permeable pavers. Concrete permeable pavers that have been designed with ¼-inch minimum joints (and may or may not have non-structural openings larger than the joints called voids) when filled with ASTM #8, or #89 or #9 washed aggregates, will provide load transfer and infiltration and act as a filter. A concrete curb or edge restraint is required, and a bio-swale may be used as well to better enhance water quality. This system will provide a 20-year hydrological design value of 10 to 15 inches per hour. Specifications and other guidelines may be reviewed at www.icpi.org; a design manual for PICP by ICPI is available as well as other design software for permeable pavement design.

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