Infiltration: Sizing and modeling

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The depth of an infiltration facility is dependent on the native soil infiltration rate, porosity (void space ratio) of the gravel storage layer and the targeted time period to achieve complete drainage between storm events. The depth of the reservoir for designs without an underdrain can be calculated using the following equation:

d = (i × ts)/ Vr

Where:
d = Stone reservoir depth (mm)
q = Infiltration coefficient for native soils (mm/hr) (See Infiltration for details on measurement and safety factors)
Vr = Void space ratio for aggregate(typically 0.4 for 50 mm clear stone)
ts = Time to drain (48 hour recommended)

Notes:

  • The infiltration coefficient (q) used above should incorporate a safety correction factor
  • On highly permeable soils (e.g., infiltration rate of ≥ 45 mm/hr), a maximum depth of 2 m is recommended to prevent soil compaction and loss of permeability from the mass of overlying stone and stored water.
  • For designs that include an underdrain, the above equation can be used to determine the maximum depth of the stone reservoir below the invert of the underdrain pipe.


Once the depth of the stone reservoir is determined the water quality volume (WQV), can be used to determine the area needed using the following equation:

Af = WQV /(d × Vr)

Where:
Af = Area of facility (m2)
WQV = Water quality volume (m3)
d = Stone reservoir depth (m)
Vr = Void space ratio for aggregate (typically 0.4 for 50 mm clear stone)

The ratio of impervious drainage area to infiltration area should be between 5:1 and 20:1 to limit the rate of accumulation of fine sediments and clogging.