Pre vs Post Subcatchment development with partially developed and undeveloped upstream

[Andrew Toole]

I have a question about "best practice" for modelling an area in HydroCAD.
My situation is that I have a drainage area which runs from the top of a hill, down to and existing developed area. My development will develop only a small portion of the lower end of this subcathment.
In the past I have tried several approaches to modelling this, and am unsure which is most "correct".
I have attached a screenshot of a subcatchment. The yellow area is being developed, the orange area will remain treed.
My question is the best way to model this. I have tried making this all a single subcatchment which results in a single Tc and average land use (which is not totally true), the other option is to break it into two subcatchemnts, but where does the upper one get directed?
My real issue is that in the pre development, the Tc of the larger area gets say 10 minutes. My two smaller ones, or one combined and averaged one gets a Tc of about 5 minutes which seems overly quick? This also results in an apparent significant difference between pre and post.
Any suggestions would be greatly appreciated.
Thanks,
Andrew

 

[psmart]

This really depends on how the upper flow is being conveyed through the lower area. If you break it into two subcatchments, you can use a reach to route the flow from the upper area through the lower area. For details see

http://www.hydrocad.net/overland.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net