Kortright Centre Study
The Toronto and Region Conservation Authority (TRCA) released a two-year study on the performance of a permeable pavement parking lot built at their Kortright Centre in Vaughn, Ontario, in metropolitan Toronto. The pavements consisted of two types of permeable interlocking concrete pavement (PICP), pervious concrete and an impervious (conventional) asphalt pavement as a control surface. The contiguous parking areas each are about 3,400 sf (320 m2).
The permeable pavements did not produce surface runoff throughout the 22-month monitoring period of this study. The permeable systems reduced the outflow volume via underdrains by 43 percent, and completely captured rainfall events up to 1/4 in. (7 mm) deep. Not surprisingly, the permeable pavements delayed and reduced peak flows existing underdrains throughout all seasons by an average of 91 percent, compared to surface runoff from the conventional asphalt pavement. The slower, controlled outflow closely mimics nature, and reduces the flooding risks and downstream erosion in receiving waters.
Cold, hard facts
During freezing temperatures, the permeable pavements functioned well and did not exhibit significant surface heaving or settlement. A substantial spring thaw occurred in March 2011 and the permeable pavements delayed the outflow of snowmelt by three days, greatly reducing peak flows. Increases in outflow volume happened occasionally during the winter and spring due to delayed release of stormwater stored within the aggregate reservoir.
Monitored median and mean concentrations of several pollutants in the permeable pavement outflows were significantly lower than those from the asphalt, including suspended solids, extractable solvents (oil and grease), ammonia-ammonium nitrogen (NH3, NH4+), nitrite, total Kjeldahl nitrogen (TKN), total phosphorus, copper, iron, manganese and zinc. The permeable pavements also generated a net reduction in total pollutant mass for all of these constituents, in addition to dissolved solids, chloride, sodium, phosphate, and nitrates.
In the winter, deicing salt-related pollutants were considerably higher from the asphalt runoff than in the permeable pavement outflows. The reduction in concentration is from detention and diluting winter stormwater. Water quality data collected below native soils indicated that sodium and chloride migrated into them. Further investigation is needed to determine how the presence of these constituents may affect the mobility of other stormwater contaminants, such as metals, as well as impacts on groundwater.
Surface infiltration measurements indicated substantial reductions in permeability over the monitoring period. However, reduced surface permeability still provided sufficient infiltration rates to rapidly infiltrate all rainfall from the storms. Between June 2010 and May 2012, permeability reductions of a narrow-jointed PICP, a wide-jointed PICP, and the pervious concrete were 87, 70 and 43 percent, respectively. The pervious concrete was installed about 6 months after the PICP systems, so the comparison in reductions is a bit uneven.
The pervious concrete maintained high surface infiltration capacity even after two years, with a median rate of 422 in./hr (1,072 cm/hr) at the end of the study in 2012. The infiltration rate of the narrow jointed PICP was 8 in./hr (20 cm/hr) after two years. Vacuum sweeping provided partial restoration of surface permeability for the PICP surfaces. Interestingly, no benefit from increased infiltration rates was observed from vacuuming the pervious concrete. The researchers found that vacuuming all of the permeable pavements produced highly variable surface infiltration results, and did not provide consistent removal of embedded fines within them.
As part of the study’s recommendations, the researchers suggested that further tests using different techniques for loosening or dislodging compacted material should be conducted in permeable pavements prior to cleaning. Further experimentation could likely improve the effectiveness of regenerative air and vacuum sweeping trucks. Based on maintenance practices evaluated in this study, annual vacuum cleaning of PICP is recommended to increase their operational life.
The two PICP cross sections consisted of 3 1/8 in. (80 mm thick) concrete pavers, 2 in. (50 mm) bedding (similar to ASTM No. 8 stone), 4 to 5 in. (100 to 125 mm) of material similar to No. 57 stone, and about 8 in. (200 mm) of material similar to No. 3 subbase stone. The pervious concrete was 6 in. (150 mm) thick, placed over 2 in. (50 mm) of No. 57 stone and about 14 in. (350 mm) of No. 3 subbase stone. The asphalt pavement was 3 in. (75 mm) thick over almost 15 in. (370 mm) of dense-graded aggregate base. The silt to silty-clay soil subgrade was covered with geotextile prior to placing subbase aggregates. The soil’s clay content ranged from 7 percent to 30 percent.
The study is among several over the past three decades that underscore the ability of permeable pavements to reduce runoff and stormwater pollutants. In addition, the maintenance study confirmed the ability of highly clogged PICP surfaces to be rehabilitated by using vacuum equipment, whereas highly clogged pervious concrete presents additional challenges regarding restoration of surface infiltration.