Calculating percentage impervious values

Any referenced datasets can be downloaded from "Module downloads" in the module overview.

Video quiz

When the Percentage Impervious value is set to 100, which of the following is likely the purpose of the design?

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Step-by-step:

The Percentage Impervious value defines the proportion of the catchment area that has been developed as roads, roofs, and other hard-standing areas. It is assumed that only rainfall landing on these areas will run off overland and end up in a manhole.

Typical housing or urban development catchment areas consist of:

  • roads ≈ 30%
  • roofs ≈ 30%
  • permeable land ≈ 40% (from which there is assumed to be no contribution to the runoff)

Roads + Roofs = Percentage Impervious (PIMP):

  • may be an assumed value
  • may use digitized data for a more accurate analysis
  • may employ GIS systems

The definition of percentage impervious and a simple graphic showing a typical housing or urban development catchment area.

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