Hydraulic Design Factors
MEASURING SITE INFILTRATION
On-site infiltration testing should be done whenever possible to determine site values. Oaks recommends following the protocols laid out in Appendix C of the TRCA/CVC Low Impact Development Stormwater Management Planning and Design Guide. Double-ring infiltrometre or Guelph Permeametre testing should be used as the results are more accurate (they estimate the vertical movement of water only). The test should be done at the bottom elevation of the proposed subbase, which is where sub-grade infiltration will take place in the finished pavement. It should also be verified that the depth to the seasonably high water table (SHWT) is not within 1 metre (3 feet) of the subbase. ASSESSING INFILTRATION RATES OF SOILS As part of its Waste Water Flow Management Plan, the City of Toronto summarized the distribution of rainfall events for 16 rainfall stations across the city. The study concluded that 54% of daily storms in Toronto produce less than 5mm of precipitation, and 98% less than 35mm. This table shows sample infiltration rates for different soils. Precipitation of 1.5mm/hour for silty clay may not seem enough to work with a PICP system. But, comparing the daily total (36mm of water infiltration) to the results of the previous study, and it becomes apparent that this infiltration rate exceeds even the 98th percentile of storm events in Toronto. In other words, even over silty clay storm water will infiltrate into the sub-grade within the same day as the storm event in all but the most severe storms.
Double Ring Infiltrometre
Proposed Subgrade
3 ft.
SHWT
INFILTRATION RATE (mm/hour)
SOIL TYPE
SAND
210mm (8.27")
SANDY SILT
26mm (1")
SILT
7mm (0.27")
SILTY CLAY
1.5mm (0.06")
CLAY
0.5mm (0.02")
Source: Porous Pavements
CONTRIBUTING WATER TO PICP Many agency regulations allow PICP systems to receive run-on from roofs (see adjacent photo), adjacent impervious pavements, and/or stabilized pervious areas (such as lawns). Although these regulations typically specify a maximum run-on ratio (compared to PICP surface area), Oaks recommends that you perform a water balance analysis to determine if the system can accommodate the additional storm water. Adjust the design details as required. To quantify the run-on, define the total area of each run-on source and estimate the contributing runoff from each source using the adjusted design storm(s) based on standard run-off practices. Be sure to also consider the potential for increased sediment and contaminant loads associated with the additional run-on. A sediment control chamber may be needed. (See Page 16)
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