How does TR-20 add hdrographs?

[rrowe1]

I have 3 tributary areas delineated for use in TR-20. A flows into B. B flows into C. Ponding occurs in C because of a restrictive culvert. Is it accurate enough to ADDHYD A and B, then ADDHYD A+B and C, then route the final ADDHYD through a RESVOR created by the culvert? Or should I route the runoff from A through a REACH in B, then route the runoff from A through a REACH in C and route the runoff from B through a REACH in C to the culvert then route the 3 seperate hydrographs through a RESVOR created by the culvert? Thank you for your help.

 

[psmart]

You can use either approach. The results may be similar - or not. The only way to be sure is to compare the two options.

If you use the intermediate reach routing, then the Tc for each subcat should extend up to, but not include the reach itself.

If you eliminate the reach routing, then the Tc for each subcatchment should include the entire flow path up to the pond.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[rrowe1]

Thank you Peter. To make sure I understand: if I just ADDHYD the sub-areas together, I am not introducing the lag in peaks that is actaully realized in the watershed, right? But, if I increase the Tc's to reflect the travel times through the sub-areas to the outlet, I am introducing the lag time that is missing when I simply ADDHYD everything together. I think this is the route I will take since I don't have much information on the geometry of the reaches. Thanks again.

 

[psmart]

Yes. The result isn't exactly the same a full reach routing, but it's a reasonable approximate, especially if the reach travel time is fairly short in relation to the Tc.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[rrowe1]

My travel time will be about half my Tc. Think that's short enough? No matter how much reading/research I do, it never seems to be as effective as conversing with someone with experience! Thanks again.

 

[psmart]

Try it both ways and see how they compare. That's the only way to know for sure.


Peter Smart
HydroCAD Software

http://www.hydrocad.net