if i want to model my drainage network using rational, and use the design event tailwater elevations from detention ponds, that are developed using SCS, is there any way in SSA to tie them together, as in one same model? i looked up some sample models that install with the program, and it seems like for the ones with inlets+pipes and detention facilities (the pre/post with detention sample model for example), it is modeled using SCS. is that something everyone's been doing these days? or should i keep the rational drainage and SCS storm separate? please advise.
They way I was taught, you never mix methods. Each of the methods has it's strengths and weaknesses, and they are really not all that compatible.
Rational method is best used for pipe sizing, since it was designed primarily to determine peak Qs. But, it will begin to grossly overestimate peak Q in large basins. Our guidelines limits Rational to 25 acres, but we rarely use it for more than 10. We are not to use it for pond routing, period.
SCS is better for hydrographic modelling and pond routing. It can determine peak Qs better in large basins over 10 acres.
Pick a method and stick with it. Otherwise, you will get unreliable results.
I would stick with SCS, add your pond, and run the gambit of storms. If you need to run rational, save the model, change the hydrology enginge and proceed.
thanks for the replies guys. yeah, that is how i have learned it as well. but then fairly recently i started using SSA, and saw some of the sample models with both detention and drainage components in it, running SCS. made me wonder if SSA has the capability to do both at the same time, rational for the pipes and SCS for the detention in the same model, and use SCS detention water surface elevation as tailwater for rational! again, thanks for clarifying, and bringing me back to reality
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