Calculating percentage impervious values

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the percentage impervious value defines the proportion of

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the catchment that has been developed as roads,

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roofs

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and other hard standing areas.

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This is crucial because it is assumed that only rainfall landing on these

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areas will run off over land and end up in a manhole.

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In a typical housing or urban development,

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roads make up about 30% of a catchment area,

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roofs make up another 30% and the remaining 40% is permeable land,

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such as grassy areas, gardens,

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grass verges

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and open undeveloped land from which there is

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assumed to be no contribution to the runoff.

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If you were designing for structures such as an airport,

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a large parking lot or a large warehouse facility,

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the roads and roofs values could be as high as 100%.

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However, for the example, you see here,

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these average urban development numbers will suffice

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the roads and roofs values together make up

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the percentage impervious value or the pimp value.

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This could be an assumed value or you could

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use digitized data for a more accurate analysis.

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There are G. I. S. Systems that can help you to calculate more accurate pimp values.