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# Modeling Disconnected Impervious

## Modeling Disconnected Impervious

(OP)
TR-55 states that the composite CN value for disconnected impervious greater than 30% should be computed from Figure 2-3.

In the attached screen capture there is a 20ac site with 10ac unconnected pavement and 10ac pervious. HydroCAD is taking the weighted average to compute a composite CN of 69 instead of using Figure 2-3.

Is there a step I am missing?

### RE: Modeling Disconnected Impervious

Per TR-55 and NEH, this adjustment only occurs when the total impervious portion of the subcatchment is less than 30%. Your example is 50% impervious, so there is no adjustment.

Try 4 acres unconnected with 10 acres of grass. Please note that the data entry screen will continue to show the standard average value, but the subcat summary and all the reports will show the adjusted value.

Peter Smart

### RE: Modeling Disconnected Impervious

If you look carefully at figure 2-3 it's gust giving you a straight algebraic weighting without any special adjustment for UI. That's exactly what HydroCAD is doing.

Peter Smart

### RE: Modeling Disconnected Impervious

(OP)
Ok, I see it now. Thanks

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