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# Curve Number for Residnetial districts (TR-55)

## Curve Number for Residnetial districts (TR-55)

(OP)
When using HydroCad or other TR-55-based model, the Curve Number for residential areas can be calculated using Table 2-2a in the TR-55 documentation.  The "average impervious area" for the various residential densities are given, and can be used to calculate a weighted CN for the residential area.

Question: Does the "average impervious area" include the adjacent residential street or is it just the residential lot area?

### RE: Curve Number for Residnetial districts (TR-55)

It could be used either way.  As long as your impervious fraction matches the stated value, you're OK.

If you impervious fraction is different, enter the separate CN values and let the program calculate the composite CN.

Peter Smart

### RE: Curve Number for Residnetial districts (TR-55)

(OP)
Thanks but..

What if you are modeling a future scenario and all you have is zoning information?  You know the lots will be 1/4 acre.  Table 2-2a of TR-55 assumes a 38% average percent impervious.  Should you assume that 38% includes the adjacent road?  or add in the assumed road area with a CN of 98?

### RE: Curve Number for Residnetial districts (TR-55)

If in doubt, I would avoid using the pre-averaged CN values.  In addition to the road issue, the average CN values include assumptions about building footprint, garage size, roadway width, sidewalks, and other factors that are subject to significant regional and local variation.  Houses have also gotten a lot bigger since these tables were produced.

If you really want to use a pre-averaged CN value, I would verify the impervious value against what you expect to be built on a typical lot.

Peter Smart

### RE: Curve Number for Residnetial districts (TR-55)

Zuver,

The Residential Districts by average lot size does include streets etc.  Of course there are variables that can make "your" district vary from the numbers provided in TR55, which is why it is handy that they give the %impervious so one can check your model if desired.

When making assumptions on future development, one can never truly predict the configuration someone may come up with.  One approach might be to look at an aerial photo of an area already developed in your area with the same zoning and check the impervious cover percentage of what one would a typical portion of this example.

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