Porosity, Permeability, Perviousness and Infiltration

Although literal definitions of all these terms are different, the Stormwater industry treats porous paving, permeable paving, and pervious paving as interchangeable. The terms themselves have different meanings. Below are the the definitions of these words and how they relate to our products.

PorousPavingrecharges
Porous-permeable-pervious paving allows rainwater to pass through the cross section, and back to the ground water supply. This mimics the natural water cycle reducing stormwater runoff volume and rate.

Permeability

The rate at which a fluid flows through a porous substance under given conditions.

Porosity (void space)

The portion of a volume of material that is not solid.

Infiltration

Movement of a fluid into the surface of a porous substance.

Effective Imperviousness Information

We have information regarding our Grasspave2 and Gravelpave2 porous pavers for Effective Percentage Imperviousness, Effective Imperviousness and the Rational Method, and Effective Imperviousness and TR-55, download our pdf here.

 

The Permeability (infiltration) of Grasspave2

  • grasspave2_102 Sand permeability = 8.27 inches/hour
  • Grass in Sand root zone= 9 to 25 inches per hour (various USGA mixes)
  • Base course poor draining = 2.63 inches/hour* (sandstone with 10% fines)
  • Base course common = 7.37 inches /hour* (limestone with 3% fines)
  • Base course mixed = 38.55 inches /hour***** (66% GP and 33% GW)
  • Subsoils need to infiltrate at least 0.5 in/hr to be considered permeable** and recommended soils would be loam, sandy loam, or loamy sand.

Our system would deliver 2.63 to 38.55 inches of water per hour to the subsoils.

 

The Permeability (infiltration) of Gravelpave2

  • gravelpave2_102 Open graded aggregate, 1/4” = 2500 inches/hour***
  • 0.1” to .2” inside open-celled grids = 40+ inches per hour****
  • Base course poor draining = 2.63 inches/hour* (sandstone with 10% fines)
  • Base course common = 7.37 inches /hour* (limestone with 3% fines)
  • Base course mixed = 38.55 inches /hour***** (66% GP and 33% GW)
  • Subsoils need to infiltrate at least 0.5 in/hr to be considered permeable** and recommended soils would be loam, sandy loam, or loamy sand.

Our system would deliver 2.63 to 38.55 inches of water per hour to the subsoils

The Porosity (void space) and Water Storage of Grasspave2

  • 1grasspave2_1023 inch cross-section
  • One inch Grasspave2 with Sand = 20% void
  • 12 inches base course = 20% void (16%-and-up depending on composition)
  • 13 inches x approx. 20% void space =

2.6 cubic inches of Water Storage

The Porosity (void space) and Water Storage of Gravelpave2

  • gravelpave2_10213 inch cross-section
  • One inch of Gravelpave2 with Open Graded Aggregate at 3/16” – 3/8” = 35%
  • 12 inches base course = 20% void (16-35% depending on composition)
  • (One inch x 35%) + (12 inches at 20%) =

2.75 cubic inches of Water Storage

Download a pdf of this information

Infiltration and Permeability are used interchangeably in reference materials. For runoff coefficient, click here (pdf).

GW = Well graded, clean gravels, gravel/sand mixtures
GP = Poorly graded, clean gravels, gravel/sand mixtures
All rates are approximate and actual installed rates will vary depending on local materials and other conditions.

If existing site soils infiltration rates are below .5 in/hr (silt loam, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, or clay), additional drainage is recommended below Grasspave2 and Gravelpave2.

*Permeability of Pavement Base Course, SAM I. THORNTON & CHIN LEONG TOH, Civil Engineering Department, University of Arkansas, May 1995
**Guidelines set by the EPA
***AASHTO, 1993, p I-19, extracted from page 144, Porous Pavement, Bruce Ferguson, Taylor and Francis, 2005.
**** Pratt et al 1995 extracted from page 144, Porous Pavement, Bruce Ferguson, Taylor and Francis, 2005.
***** Data from “Civil Engineering Design Manual”, 1995