If you need to assign separate Tc values to the pervious and impervious areas, they can be modeled as separate subcatchments.  But in many cases the sub-areas and flow paths are intertiwined, so a single subcat (and Tc) must be used for the total area.  That's the intended use of the option you described.  For details see www.hydrocad.net/impervious.htm

Peter Smart
HydroCAD Software
www.hydrocad.net

Could you give an example of when this would be desirable?

If you are modeling a subcatchment with both pervious and impervious area, why would you need to separately calculate the runoff generated by each?  I'm also having trouble believing that this is even a hydrologically-sound principle.    How can the software accurately calculate the runoff generated from each without separate times of concentration?  If you have to create separate subcatchments anyway, why include that option?

I checked the link you posted before I started this thread, and it does not address the issue I am referring to.  Maybe I'm not being clear enough?

This option is used primarily in conjunction with the Santa Barbara Urban Hydrograph (SBUH) runoff procedure, although it is sometimes used with the SCS/NRCS Unit Hydrograph procedure as well.

It is intended to address the fact that the runoff volume generated by a composite CN value is not the same as the sum of the runoff from the separate CN values.  This is because of the non-linear relationship of the SCS/NRCS runoff equation.  (Which is also used with the SBUH procedure procedures.)

For example, the runoff volume from 1 acre of CN=98 added to the runoff from 1 acre of CN=58 is NOT the same as the runoff from 2 acres of CN=78 (the composite CN).

By calculating the runoff volume separately for the impervious area, it ensures that it will produce its full runoff potential, and not be understated because of the composite CN process.

Remember, this is just an option designed to meet specific circumstances, and is used almost exlcusively with the SBUH procedure.  As the design engineer, you need to determine if it is appropriate for your specific situation.

Peter Smart
HydroCAD Software
www.hydrocad.net

I should have added:  This option is needed primarily when the pervious and impervious areas are interspersed along the same flow path, such that it is not possible to calculate a separate Tc for each sub-area.  If it is possible to calculate a separate Tc, then by all means model them as separate subcatchments with separate Tc and CN values.

Peter Smart
HydroCAD Software
www.hydrocad.net

Years ago when I was working in Washington State I was using Intellasolv's Hydrograph program which I liked a lot. But several of the local jurisdictions checked our submitted calcs using an old DOS program called HYD which calculated previous and impervious runoff separately as you've described above. Hydrograph used composite CN's and therefor I couldn't get the same results and was forced to use that tedious and unfriendly HYD program just so I could get the same results. Since I started using Hydrocad I haven't run across anybody doing it that way since.

Gary Van Dyke,
Or. P.E.

Where I've noticed this can be an issue is when the impervious is NOT spread throughout the drainage catchment.  For instance, I was reviewing a set of plans once for a condominium development.  The pre-development was all woods, good condition.  In one of the post-development subcatchments all of the newly developed area was at the top of the drainage area, about 15% developed (most impervious) with the remaining 85% still woods below it.  

A composite CN resulted in peak flows that were somewhat less than pre-developed (the total area above this evaluation point was less than in the predeveloped state as the development roads and drainage system cut off the top third of the original area and diverted it towards other evaluation points (which had detention).

However, if just that 15% developed area were calculated draining into a reach through the woods and terminating at the evaluation point, the predevelopment peaks would be exceeded.  This was not an unlikely scenario based on the proposed finish grading of the site.  In this instance it was a pretty easy fix to adjust some of the grading and break it up so there was more sheet flow towards different sections of the woods.  But there was definite potential for the post-developed grading to divert water to a single point going into the woods that over time would create its own flow path through the woods to the eval. point.

It certainly opened my eyes a bit towards that type of scenario in my own design work and I try to be a little more sensitive to possibly breaking up my designs into more, smaller subcatchments where such circumstances could have negative impact on an abutter.