Calculating percentage impervious values

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