Modeling Off-Site Areas
Modeling Off-Site Areas
(OP)
Can someone explain an acceptable way to model off-site drainage areas without modeling the entire upstream network?
I have the total offsite area and an estimated rational flow. I'm using StormCAD for Microstation V8i if that helps at all.
If I assume a full capture inlet or inject the flow it is ignoring the fact that some flow is bypassed to the outfall and never entered the pipe network.
If I assume the flow makes it to the first on-site inlet, most of the flow would be bypassed.
Is there a rule of thumb regarding how much I can assume is captured upstream and how much bypasses to the outfall?
Thanks!
I have the total offsite area and an estimated rational flow. I'm using StormCAD for Microstation V8i if that helps at all.
If I assume a full capture inlet or inject the flow it is ignoring the fact that some flow is bypassed to the outfall and never entered the pipe network.
If I assume the flow makes it to the first on-site inlet, most of the flow would be bypassed.
Is there a rule of thumb regarding how much I can assume is captured upstream and how much bypasses to the outfall?
Thanks!





RE: Modeling Off-Site Areas
Your captured flow into the system should take into account bypass. If the majority of the R/O reaching your first inlet will bypass it you need to account for that and design conveyances accordingly for both the captured flow and bypass flow. Its been a while since I have used StormCad; but I do not remember being able to code a rating curve into the program. You may want to search the help section to see if you can enter a rating table. I recommend creating a rating curve at your inlet to determine the captured vsw bypass flows. If it is somewhat linear you may be able to adjust your drainage area to reflect the actual captured flow.
RE: Modeling Off-Site Areas
This is for a roadway drainage system. The offsite areas are mostly residential with the two larger areas producing about 30 cfs. I don't have survey info to model the entire system. I estimated the drainage areas from a USGS Quad Map.
RE: Modeling Off-Site Areas
RE: Modeling Off-Site Areas
If you're catching water from an upstream pipe system, and you're worried about not knowing the characteristics of the pipe system, there's a way to do that. In StormCAD, there's a way to enter an additional "upstream CA" and an "upstream Tc" if I recall, a few tabs over from the inlet watershed tab. I think it was called "additional flow." ..? StormCAD is going to use modified rational method and recalc the entire basin at each inlet anyway, taking the longest Tc and applying the I from the IDF curve at each inlet instead of summing the flows, so just get a good estimate of C, A, and Tc, to input into that tab, and you should be fine.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com