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RiverSides Toronto Homeownders` Guide to Rainfall
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Calculate Your Infiltration Rate

Once you have selected the preferred location for your landscape solution (see Choose a Location), you should determine how fast water will infiltrate the soil at that location.  Generally, water should not be standing or pooling on your property for longer than 48 hours. If you want to install a stormwater management technique such as a soakaway pit, trench or swale, you’ll need to have a high enough infiltration rate to manage your stormwater capture target within this time frame.

Some soil types are better than others for stormwater management.  A quick guide to the infiltration of different soil types is given in the table below.  You can also measure the infiltration rate of your soil directly.  Toronto’s soils tend to have high levels of clay, but differ according to where you live.  Soil is greatly affected by slope, vegetation, and the history of modifications (e.g. construction on your property may cause significant soil compaction, or you may have already altered the soil structure for a garden).  If the rate of infiltration of your soil is too slow it is possible to modify it. For more information on soil types and composition, including the “Feel Test” for soil, See Ideal Soil Types.

Soil Infiltration Rates

Soil type

Min absorption rate in mm/hr

Sandy soils
210 mm/hour
Sandy Loam
25 mm/hour
Loam
15 mm/hour
Clay soils
1 mm/hour

Test your soil

One way to discover your infiltration rate is to measure it with this simple test.

  1. Dig a hole in the ground by hand (see Call Before You Dig) that is at least 30 cm (or 1’) deep. Be careful not to disturb the soil horizons or to compact the sides while digging, as this will affect the infiltration rate. Note: this can be the same pit dug for the Water Table test for choosing the best location if you did not reach the water table.
  2. Fill the hole with water to moisten the soil and allow it to drain completely.
  3. Fill the hole with water a second time, and place a ruler or stick in the hole. Note the water level and time, and after 15 minutes check the water level again*. Multiply this by 4 to get the number of centimeters or millimeters of infiltration in an hour.

*If you have a very high clay content or heavily compacted soil, you may need to wait even longer as infiltration rates in clay soils like Toronto can be 5mm/hour.

Modify Your Soil 

If your infiltration rate is too slow, mix sand, fine gravel or organic matter (wood chips, compost and soil) into the ground. Be careful not to compact the soil significantly while digging or installing your system. Ensure that soil is permeable to a depth of 0.5 - 1 m below the stormwater system. If the infiltration rate of your soil is too fast, mix clay and soil into the ground in the desired location. Consider testing the soil again after your modifications and before installing the stormwater management system to ensure that the infiltration rate is appropriate.

Refer to "Get to Know Your Soil" (PDF) in the Canadian Mortgage and Housing Corporation’s 'About Your House' series for tips on understanding and modifying your soil.

 

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Did You Know...

71% of the total flow of the Don River is from stormwater.