Modeling Off-Site Areas

[NJHighway]

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!

 

[gbam]

If I understand your scenario correctly, I would say it depends on the facilities located within the watershed. For instance if there is large detention facilities it may warrant modeling them to reduce the peak flow to your specific point of concentration. If not, the assumption that all R/O will make your should be okay.

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.

 

[NJHighway]

Thanks gbam. You can create an inflow-capture curve in StormCad. I'm not sure what assumptions I would make to develop one. Would it be easier to assume a percent capture? Would 80% capture be reasonable?

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.

 

[gbam]

We should never assume, you need to calculate the capture rate. If your inlet is in a sump use weir/orifice flow to compute a stage vs discharge for the inlet and then compute the bypass flow based on the stage developed for the inlet. Now the two discharges added together is your inflow discharge and you have Qtotal(Qbp & Qcap) vs stage, some interpolation may be required. Badabing, there is your curve; you have Qin vs Qcap vs Qbypass all based on stage. If the inlet is on grade, StormCad can compute that directly. This type of analysis in known as diversion or split flow.

 

[beej67]

Okay, first off, if you're catching 30 cfs worth of surface runoff from offsite areas in your system, you need to change your design to keep that runoff from getting into your road. It's going to blow your gutter spread calcs all to hell and create a problem in your system. Catch it in a head wall or a pedestal top inlet before it jumps the curb into your road. Once you do that, just treat the headwall or drop inlet, whatever you use, as a node and input the watershed as you ordinarily would. USGS for watershed boundary and Google Earth for land cover should be good enough to characterize the basin.

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.


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