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Unconnected impervious

Unconnected impervious

Unconnected impervious

Version 9, which I recently upgraded to, includes several CN values for impervious and unconnected impervious.  A house out in the woods of a 2 or 5 acre (or however large)parcel that is sloped towards the woods seems a pretty straightforward candidate for using "roofs - unconnected impervious", and I can reasonably see the same for certain driveways.  But how about a portion of a road where it's runoff is collected in open ditches, routed through a treatment device (say a water quality swale), and ultimately outlet via a level spreading device into those same woods?

Is it reasonable to consider that impervious also as unconnected?  I'm not talking about a level spreading device that is situated just 20 or 30 feet or so above a stream channel, intermittent or perennial, but where it's 100 or several hundred feet from a stream or property line.

RE: Unconnected impervious

Using the CN adjustment for unconnected impervious areas would seem appropriate only if the level spreader is able to spread the flow over the entire pervious area.  Othewise the effective pervious area will be somewhat less than is being allowed for in the CN adjustment.

But in the end, this is a matter of what your reviewing agency will accept.

For details see www.hydrocad.net/pervious.htm  

Peter Smart
HydroCAD Software

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