June/July 2008

Stormwater Management Technical Article

What I learned in Paver School

The role of permeable paver systems as a stormwater management technique

Several million square feet of permeable pavers with the open-graded aggregate system have been installed in the Chicago area over the past 20 years. I visited several sites in various stages of completion with the paver instructor who installs pavers in the Midwest.  One site was a private road under construction in a large-lot subdivision.  The road, if built using traditional methods, would have been the majority of impervious area and the greatest source of

Photo: Model of aggregate layers for stability
and detention capacity

runoff.  Since the road was constructed with permeable pavers over the aggregate layers, a detention pond and conveyance infrastructure that is typically required to handle road runoff was not needed.

A college in the Chicago area installed permeable pavers on an 8% slope and combined the paver installation with curb cuts directing stormwater into sumped landscape areas. Block pavers can be plowed and are less prone to black ice and other surface freeze/thaw problems.  In addition to the void space of the open-graded aggregate system, there is more air moving through the system to keep the surface free of ice and snow when compared to impervious surfaces such as asphalt and concrete. 

Photo: Pavers with the open-graded aggregate system in
Illinois and swales for added detention capacity (Michell DeLaria)

Cost Benefit

The cost benefit analysis is variable.  For example, at an installation in Florida, permeable paver systems broke even after 22 years when comparing the materials, construction and maintenance to concrete and asphalt surfaces.  On another site in the Chicago area, after 50 years an asphalt surface would have cost 10 times as much as pavers to maintain.  Additionally, comparing costs of materials and installation is not a complete and perhaps not an appropriate evaluation.   In the Denver area for example, a concrete parking lot would cost approximately 50% more than asphalt and a permeable paver system with the full open-graded aggregate system, would cost two to three times as much as asphalt.  Based upon initial investment, asphalt or concrete appear to be more cost effective than pavers.  However, the cost of asphalt or concrete does not include the costs of inefficient use of land and associated cost if a detention structure is required.  Also not included are costs associated with managing offsite impacts that are generated such as:  excess stormwater runoff rate and volume, pollutants washing off of impervious area into receiving waters, and future waterway stabilization needs.  Pavers may have a larger initial investment, but the cost of detention is included and offsite impacts are reduced.  

While permeable paver, porous concrete and porous asphalt systems provide infiltration and can be designed for detention capacity, there is a difference that may be important to some property owners.  Both systems have similar recommended annual maintenance needs such as removing surface grit by using a sweeper or vacuum truck.  However, it is possible that the subsurface may accumulate enough solid material resulting in decreased infiltration and detention capacity that cannot be regenerated by a power vacuum.  At this point, porous asphalt or concrete needs to be remove and discarded.  However, paver systems are completely modular. The system can be deconstructed, the aggregate layers cleaned and all the products reinstalled with minimal waste.  The reduced waste in a paver system may be preferable.

Additional examples of low impact development site planning strategies and construction projects can be found on the Meza Construction website at www.mezaconstruction.com

Summary

While many LEED elements tend to focus on the actual building structure, at Meza Construction we extend the innovation to the entire site with high performance infrastructure.

Photo: Pavers can be plowed. Courtesy Chuck Taylor.

From a stormwater management perspective, either  porous asphalt, porous concrete or permeable pavers (all with the open-graded aggregate system) are techniques that can restore permeability and infiltration as well as provide large storm detention volume. Parking lots, alleyways, driveways, fire lanes, and parking lanes on streets are common examples of impervious flatscape areas that can instead be porous or permeable to reduce runoff.  Communities can retrofit highly urban areas to help retain the economic benefits of developed land while reducing offsite impacts.

We know how much money communities collect in stormwater fees to fund drainageway stabilization projects and water quality programs.  We know that total maximum daily loads (TMDLs) for temperature or E coli are going to be expensive and cumbersome to correct

Photo: Standing water and freeze/thaw cycles create
an auto and pedestrican hazard and shorten the life
of the paving material. Michelle DeLaria

with our existing drainage-based land development design and infrastructure. We even knowof communities that are adopting low impact development standards and retrofitting their urban environments to protect receiving waterways.

Perhaps instead of discussing the expenses and maintenance of runoff reduction and infiltration best management practices, we should examine the collective costs and ramifications of not using these techniques.

Photo: Pavers are largely free from ice and snow
accumulation. Courtesy Interlocking Concrete
Pavemenet Institute

About the author: Michelle DeLaria is a project manager for Meza Construction Company in Denver, Colorado, specializing in Low Impact Development (LID). Prior to joining Meza Construction, Michelle developed and managed Jefferson County's Stormwater Quality program for five years and has an additional 10 years of experience in water quality and environmental compliance. Michelle holds a Master's degree in Environmental Science and Engineering from Colorado School of Mines and is a Registered Environmental Health Specialist. Michelle is also the stormwater quality committee chair person for Colorado Association of Stormwater and Floodplain Managers.  To learn more about Low Impact Development, you can contact Michelle at 303.778.8381 or via email at MDeLaria@mezaconstruction.com

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CBG /June.July 2008