multiple flows to one outfall

[PMAB32]

Hello. I am looking at multiple ponds, some of which are interconnected but then at the end of the line I have like 3 or 4 outflows from those ponds going to one outfall so when I'm looking at how much water is being introduced to that single outfall how do I combine those flows? I can't simply add them all together right? but they are not connected, it's like 3 pipes discharging at the same point in a river and I want to compare pre- to post-development and I have the flows out of each of those outlet pipes but how do I combine them to say "this much is entering the river at this point" ?? Thanks in advance for your help.

 

[psmart]

It depends on the software you're using...

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Twinkie]

What program are you using? Most programs will allow to input some form of a junction or combination node or link.

 

[gbam]

To be conservative you can add your peak outflows from sll of the contributing basins. If the downstream hydraulics work you know that anything less (typically) will also work.

The scenario sounds like a typical hydrologic pond routing exercise; unless, the downstream conditions can reverse flow. Then you will need the software like ICPR or the like to route the hydrographs through the basins.

 

[PMAB32]

Peter,

It was hydrocad but I guess I was asking a more general hydrology question - it's not proper methodology to just add them all up to say that's the flow that's coming into the outfall, is it? WE're not talking about volume, I could see adding up volumes but adding up flows oversimplifies things.

 

[cvg]

depending on the outflow hydrographs, you likely will add all the peaks together. or add the ones that coincide. or if you want to be more accurate, add the outflow hydrographs together

 

[psmart]

You can combine the flows by routing them to a single reach, pond, or link. No description is needed - just set the name to "Combined Flow" or whatever you wish. This will add the individual flows at each point in time. The volumes will add exactly. (Conservation of mass) The peaks will add directly only if they occur at the same time. Otherwise the combined peaks will be somewhat less, depending on the relative timing and breadth of the peaks.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

Never add peak flows, especially in a pre/post analysis. You need to add the hydrographs themselves, because the hydrographs peak at different times. In HydroCAD, if you don't care about variable tailwater in the river, you should route all of your ponds / etc to a single link, and that link will add the hydrographs and show you the total hydrograph leaving your site.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[PMAB32]

That's the info I was looking for! Thanks so much!

One more Question to you guys on this subject- what would you do without a computer program - like by solving by hand - how would you figure out cumulative peak flow for a post-development condition when the goal is to compare pre-development to post-development? My coworker suggested breaking up the pre-development in the pieces that you broke up the post-development and doing a new pre-development peak flow for each of those areas to compare to the separate post-development peak flows. Do you agree with that?

 

[Twinkie]

Do they all drain to the same point of interest? Really tough to say without seeing your topo/drainage maps. But if your pre/post all drain to the same channel, inlet, whatver...I probably would not break down the pre...you are only breaking up the post due to the multiple SWM features, correct? But overall, the end "link" is your POI, and total overall draiange area size should be very close, if not equal in size.

As for doing this stuff by hand...I did a small watershed once or twice in school, I couldn't imagine doing what I do now by hand.

 

[peter246]

if you did the TR-20 tabular method, you could manually summ the hydrographs.

if you did the TR-55 graphical method, it only gives you the peak so you could only add the them together, which yields a conservative combined peak flow.

 

[beej67]

Hydrographs are graphs of flow rate vs time. A composite hydrograph is just the ordinates of two or more graphs summed together at equivalent times.

So say your hydrographs are in one minute increments. In order to find the total flow 93 minutes in to your storm, you find the flows of each of those hydrographs 93 minutes in, and sum them, and that's the total flow 93 minutes in to your storm. You do that again for each time step in your storm. Obviously, this is a task for spreadsheets, if you're doing it by hand.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com