Modeling of stormwater exfiltration system performance for a residential development in Richmond Hill, Ontario, Canada

An innovative stormwater exfiltration system was developed in Toronto, Canada.  It consisted of two 200 mm perforated pipes with end capped below a storm sewer system. Storm runoff from catch basins would enter these two perforated storage pipes, fill it up, exfiltrate to the sewer trench and eventually exfiltrate to the surrounding soil. Larger rainfalls can overflow to top storm sewer. Performance monitoring indicates up to 28 mm of rainfall with a duration of 22.5 hours was almost completely captured without overflowing to the upper storm sewer.  In order to determine the long-term performance of this innovative system, a modelling study was conducted for an existing residential site.  The residential site has a drainage area of 10.4 ha, impervious percent ranging from 10 to 88%, sandy-loam soil, slope about 2%, a minor storm sewer system and a major quantity/quality pond for stormwater management. Design of the stormwater exfiltration system follows the planning and design manual prepared by the author (www.civil.ryerson.ca/seminar).  The closest long-term weather station has 54 years of hourly rainfall and temperature record. Continuous hydrologic modelling of the runoff quantity and quality characteristics at the residential site with and without the exfiltration system was conducted using the PC-SWMM model and the “orifice-storage-pump” method in SWMM. With the exfiltration system, continuous modelling results indicate: (1) the average annual runoff volume from the site was reduced by 77%; (2) runoff flow rates at various durations were reduced almost half; (3) the exfiltration system provided an extra 22% infiltration; and (6) the annual TSS loading could be reduced by 77.2% with the exfiltration system only.  It can be concluded that the exfiltration system can achieve the same performance of stormwater quantity and quality pond.