Soil Permeability

The porous pavement, rain garden, gravel grass, and bioswale are infiltration-based stormwater treatment tools. This means that they collect runoff, hold it, and allow it to slowly infiltrate into the ground. Many people believe that this is not possible because of the abundance of clay soils in the greater Midwest. A clay soil does not, however,  automatically equate imperviousness. We are blessed with a great variety of different clay soils, some of which infiltrate water better than others.

In the attempt to quantify the runoff reduction benefits, I had to determine the infiltration capacity of my clay soils. I set out to investigate infiltration capacity in the rain garden, porous pavement, gravel grass, and bioswale. I removed the topsoil down to the depth of the clay subgrade, which was about 8” to 24” deep and easy to distinguish from the rich dark topsoil due to its light beige color.

I conducted infiltration tests by soaking the subgrade for 24 hours and then testing how quickly the water level dropped down when I filled it up full with water. You can browse IMAGES of the testing.

The results were very encouraging. For an animated representation of the results click on TEST RESULTS link. For text based results refer to the bullet points below:

  • Gravel grass infiltration capacity: 0.37 inches per hour
  • Porous pavement infiltration capacity: 1.00 to 2.25 inches per hour
  • Small front rain garden infiltration capacity: 1.10 inches per hour
  • Large rain garden infiltration capacity: 2.25 inches per hour
  • Bioswale infiltration capacity: 2.25 inches per hour


What do these numbers mean?

Take for example the large rain garden with an infiltration capacity of 2.25 inches per hour. It could rain at a rate of 2.25 inches per hour into the rain garden (this would be a pretty significant storm*) and cause no puddling! The clay soil would be able to soak all the water into the ground.

What happens if it rains harder than 2.25 inches per hour, or if runoff from the roof starts to drain into the rain garden? At that point it is likely that the amount of water reaching the rain garden exceeds the infiltration capacity. The excess water is now stored as surface water in the rain garden depression – the rain garden slowly starts to fill up.

Once the intensity of the rain recedes, surface water levels in the rain garden will begin to drop. At this point the infiltration capacity of 2.25 inches per hour starts to exceed the amount of water reaching the rain garden. Having said all this, I had no standing water in my rain gardens, even with all the storms and rainy periods we’ve had - until September 13, 2008. That Saturday we received a total of 6.63 inches of precipitation. This storm event filled the rain gardens about 1/4 to 1/3 (see images). The standing water completely drained into the soil within 2 hours after the heavy rain changed to lighter rain.

To better demonstrate the infitration capacity of the rain garden, I flooded it with water that I pumped out of the cistern during a very wet spring week end. You can see the process and results under RAIN GARDEN FLOODING. Because of the infiltration capacity of many clay soils, and the surface water storage volume in the rain garden, significant amounts of precipitation can be infiltrated and put back into the natural water cycle.

*This type of rain event, at 2.25 inches in one hour, is estimated to reoccur only every 10 years in the Chicago area. (Bulletin 71, MCC Research Report 92-03)

Why does the infiltration capacity vary so much?

You may have noticed that the infiltration capacity results varied among the test locations, with the lowest capacity of 0.37 inches per hour recorded under the gravel grass followed by 1.00 inches per hour under the small front rain garden. All other test locations showed a fairly consistent infiltration capacity of 2.25 inches per hour.

There is a good reason for the lower infiltration capacity under the gravel grass and small rain garden – and an important lesson to learn:

The gravel grass area was used as access for construction traffic during the last remodeling of the house, and may have been for other remodeling projects prior to that. The construction traffic compacted and damaged the soil down into the subgrade to the extent that the infiltration rate was reduced to a mere 0.37 inches per hour.

While digging the hole for the infiltration test in the front rain garden, we came across some construction debris. This suggested to us that construction work on the house (probably the addition and enclosure of the front porch) resulted in contamination, compaction, and damage to the soils, reducing the infiltration capacity on 1.00 inch per hour.

These observations show that it is imperative to protect soils (particularly clay and silty soils) from heavy equipment, compaction, and damage during construction operations. If not, then a previously highly permeable clay or silt soil may be rendered almost impervious.


Soil Permeability - What do these numbers mean? - Why does the infiltration capacity vary so much?
Soil images – Infiltration test imagesAnimated infiltration test resultsRain garden flooding

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