Sub catchment Question

[Jmeng1026]

Attached is a sketch of a subdivision plan where I am trying to determine the size of a detention basin needed. Should I model the entire area as one sub catchment or should I have three sub catchments (one for the roadway, on for lots 1 and 2 and a third sub catchment for lots 3, 4 and 5?

 

[psmart]

Normally the lot boundaries have no hydrologic significance. It depends more on the distribution of ground covers and how the water is conveyed to the pond. You need to step back and look at the general criteria for subcatchment delineation, flow paths, Tc, etc. I would start by identifying separate tributary areas and define each as a subcatchment. Have a look at the topo rather than the lot boundaries.

Also note there are several techniques available for handling intermixed CN values. For details see

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

I'm sure the seasoned engineers on this site will have further suggestions. What you're asking is a general H&H question and not something that is specific to HydroCAD.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Jmeng1026]

I have attached a different plan showing how I divided up the areas and my time of concentration information on it. Am I modeling this correctly? My basin contours are not finished so disregard those.

 

[psmart]

If you want to model the sewer network then you would typically define a separate subcatchment for each inlet.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Jmeng1026]

I entered all the areas and ran through the time of concentration.

My time of concentration started with sheet flow then went to shallow concentrated flow then went to channel flow in the ditch then went through pipe flow under the road and then back to channel flow through the ditch. Is that how I should do it?

 

[Jmeng1026]

Psmart,
Can I email you my hydrocad file? That way you could see how I actually entered everything to see if I'm doing it right.

 

[psmart]

The Tc calculation path should generally end at the next reach or pond in the HydroCAD diagram. This means that the Tc will end at the point where the runoff enters the ditch or sewer. (Assuming that the ditch or pipe is being modeled as a separate node.) For details please see

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

You can post your file here or send to support@hydrocad.net


Peter Smart
HydroCAD Software

http://www.hydrocad.net