Evaluation of Permeable Pavement at the Kortright Centre

This multi-year project, which concluded in 2015, evaluates the performance of different permeable pavements installed in the Kortright visitor’s centre parking lot at the Living City Campus in Vaughan, Ontario. The project was undertaken by researchers at the University of Guelph in collaboration with the Sustainable Technologies Evaluation Program (STEP), a multi-agency program led by the Toronto and Region Conservation Authority (TRCA).

With temperatures rising and the frequency of extreme precipitation events expected to increase under climate projections, there is a need to develop and implement stormwater management best practices using new paving technologies to mitigate these impacts and adapt our built environment to a changing climate. Various stormwater best management practices have been devised, including permeable pavements.

Despite much research on permeable pavements, there is a lack of long-term performance evaluation using climate and geologic data from Ontario. This project was conducted over two phases, with phase 1 evaluating the hydrologic, water quality, and functional performance of different concrete permeable pavements. Phase 2 extended the monitoring and evaluation with a focus on assessing the long-term performance of the pavements and documenting the direction and magnitude of changes in performance over time.

Results of this project indicate that permeable pavements are an effective practice for restoring natural infiltration functions and present a valuable stormwater management tool for reducing future impacts of climate change. Over the monitoring period, the permeable pavements reduced runoff volumes by 45%, and effluents were generally much cleaner than runoff from the asphalt pavement.

Understanding and Assessing Impacts

Traditional materials used to pave our roads and parking lots absorb more solar radiation than vegetated surfaces resulting in enhanced warming in the built environment. Materials such as asphalt also increase the imperviousness of land surfaces, resulting in increased volumes and rates of stormwater runoff, increased accumulation and distribution of warm polluted water and increased erosion. Climate change is likely to amplify these impacts as future climate projections suggest increases in temperature, precipitation and the frequency of heatwaves and extreme precipitation events.

Various stormwater best management practices have been devised to mitigate these impacts utilizing one or a combination of different treatment processes, such as sedimentation, filtration, infiltration, and bio-degradation. Permeable pavements are unique in that they replace existing hard surfaces. Therefore, they do not require additional space that is either unavailable (e.g. in older developments) or can be alternatively used for greenspace or buildings.

The application of permeable pavement in stormwater management is an example of low impact development (LID), which is meant to mimic natural hydrological processes by increasing infiltration, groundwater recharge and lower surface runoff volumes and flow rates that cause damage to both the natural and built environment.

The main environmental benefits of permeable pavements are:

  • Reduction of runoff, which reduces flood risk, stream erosion and damage to downstream infrastructure;
  • Removal of contaminants from infiltrated stormwater; and
  • Reduction in heat flux from the pavement surface to the atmosphere, which helps to mitigate against the urban heat island effect under a warming climate.

Identifying Actions

To help better understand the performance of permeable pavements, the TRCA constructed a permeable pavement research facility on the visitor’s center parking lot at the Kortright Centre for Conservation in Vaughan, Ontario. A variety of different types of permeable pavements are available.

These are typically categorized into three main types:

  • Modular Interlocking Concrete Block Pavement consists of impervious concrete blocks that allow water to infiltrate into a reservoir through inter-block or intra-block voids. These voids may be filled with gravel or soil and grass. Gravel is the most common filler as it is less susceptible to clogging.
  • Porous Asphalt or Porous Concrete consists of standard asphalt or concrete mixes from which the finer aggregates have been removed. Removal of these fine materials results in the pavement with a matrix of pores that allows water to permeate through the surface.
  • Plastic Grid Systems consist of plastic interlocking units with no impervious surface area. Grid spaces may be planted with grass or left unplanted and filled with gravel. The grids provide structural stability and prevent settling while allowing a large amount of void space for stormwater infiltration.

Implementation

The research facility consists of four 230–233 m2 pavement cells. Two cells are constructed with permeable interlocking concrete pavers, one cell is made with porous concrete, and one is built with traditional asphalt.

The open-graded aggregate base for the permeable pavements provides storage roughly equivalent to a 100 mm rain event. Each permeable pavement cell is drained by a perforated pipe placed 500 mm below the surface at the interface between the open-graded aggregate subbase layer and the native soil. The asphalt cell is surface drained via a catchbasin in the center of the plot. Infiltrated water collected from the three cells and runoff collected in the catchbasin is directed separately in sealed pipes to a downstream monitoring vault where automated samplers, flow meters and temperature sensors are housed.

The initial and ongoing long-term monitoring program includes coordinated measurements of precipitation, surface and subsurface infiltration, flow rates, and water quality. Precipitation is measured with a tipping bucket rain gauge located approximately 200 m south of the facility. A second precipitation gauge located 500 m north of the facility served as a backup.

Water temperatures were recorded to assess the effectiveness of permeable pavements in mitigating the thermal impacts of runoff into stream ecosystems. The quality of water discharged from the underdrains of the three permeable pavements and the asphalt surface were monitored during rain and snow events throughout the year. Water quality samples were prepared and delivered to the Ontario Ministry of the Environment (OMOE) Laboratory in Etobicoke for analysis following OMOE lab preparation and submission protocols.

Outcomes and Monitoring Progress

Climate data was collected during the project included rainfall and air temperatures. This data was combined with stormwater data (e.g. water temperature, water quality, runoff volume and rates) to evaluate the stormwater management performance of the permeable pavement.

Results of this project indicate that permeable parking lot pavements are an effective practice for maintaining or restoring natural infiltration functions while reducing warming and contamination of built and natural systems, even in areas with low permeability soils. Performance data was compared against Intensity-Duration-Frequency (IDF) curves for the region. The evaluation found that the permeable pavement installation could accommodate rainfall from at least the 100-year storm without overtopping but was not designed to match pre-development peak flows for flood flows.

Key findings of the extended monitoring program include the following:

  • Thermal loads: Results of this study showed that permeable pavement generated considerably lower thermal loads (45% reduction) to receiving waters than the asphalt pavement during hot summer days, primarily due to lower outflow volumes.
  • Runoff volume reduction: The pavements were found to reduce runoff volumes consistently over the course of the study, despite the presence of fine-grained native soils.
  • Surface infiltration: The rate of infiltration through the surface of the three permeable pavements was initially very high but declined rapidly over the first two years as sediment accumulated in surface voids of the pavements.
  • Surface Water Quality: The permeable pavement effluents had lower concentrations of most pollutants relative to asphalt runoff.
  • Surface movement: Results showed that permeable pavement surfaces have been relatively stable over time with no obvious signs of heaving or slumping.

Next Steps

This monitoring and evaluation project provided several key recommendations:

  • Monitoring should be continued to further characterize performance trends and provide one of the only long-term datasets for permeable pavements available in North America.
  • Improving methods for winter snow and ice maintenance of permeable pavements. The first phase of this study demonstrated that current maintenance practices are only partly effective in restoring infiltration properties and may not work at all on pervious concrete.
  • Future work needs to be done to quantify road salt application rates as research suggests less salt may be required on permeable pavements.
  • Continuous monitoring and fate tracking of road salts and other contaminants need to be carried out to better understand the impacts on natural environments and water quality.

The results of this project can be used to refine policies and guidelines on the appropriate use and design of permeable pavements for stormwater management.

Resources

Link to Full Case Study

Additional Resources: