Storm and Sanitary Analysis - Modeling Outlet Structures

[Anonymous]

I'm still relatively new to using Storm and Sanitary Analysis and one thing I've been struggling to understand is how to properly model an outlet structure that has multiple components housed in the structure. 

Now, I know in HydroCAD, you can tell the program that you have a culvert outfall from a pond but then you can also communicate to the program that this culvert is connected to a box which has an orifice opening into the box that controls the flow during smaller storms and then a grate at the top of the box which of course allows a larger flows to enter the box but regardless of whether the flow enters the box through the orifice at the bottom of the box or through the grate at the top of the box, it ultimately has to pass through the culvert before it outfalls.

I could just be completely missing something but it seems to me that there is not a good way to model the above situation using Storm and Sanitary Analysis. I don't see a way to house all compents as part of one outlet and if I create multiple outlet components, how does the program understand that these are connected in series and not each having their own separate flows from the storage node (and therefore adding them together rather than determining the limiting component depending the storm/scenario)?

Thanks in advance for your help, looking forward to your input!

[wfberry]

JB:

Not very good at this myself, but one of our Forum members, Brian Hailey, has a nice YouTube that may help you.

https://www.youtube.com/watch?v=xzgONT9ekpE

Bill

[Hidden_Brain · Accepted solution]

What if you do this:
1. use a storage node for pond - P1
2. use a junction (or another storage node) for outfall structure - OS1
3. connect P1 to OS1 with 1 side orifice (primary - possibly circular), another side orifice (secondary - possibly rectangular), and one bottom orifice (riser grate). If you have multiple orifices at an elevation (or different sizes), add more as necessary.
4. then have a pipe connecting OS1 to an outfall (OUT1), this will be the pond barrel.

With this arrangement, when the pond starts to fill, water is going to leak through the primary side orifice first, until water reaches the secondary orifice, and finally through the riser orifice. All leaked water will pass through OS1 and ultimately through the barrel.

Add a weir that will directly connect from P1 to OUT1 (bypassing OS1) that will act as your emergency spillway.

[Anonymous]

As far as I can tell, this is working! Thank you!

[Anonymous]

I did this and got an error, which thankfully the SSA User's Guide had a solution for:

"ERROR 141: Outfall nnn has more than 1 inlet link or an outlet link.
An outfall node (model terminal node) is only allowed to have one link
attached to it, and there cannot be a downstream link attached to it. Convert
the outfall node to a junction and place a “dummy” outfall node downstream
of it, and connect it with a link that will not cause backwater effects."

To address the last sentence, I made sure to name the link and dummy outfall node as "Dummy", and to assign them rounded whole number properties, i.e. diamter of 30", slope of 10%, IE=390, etc. This made sure that no backwater effects occured in the upstream system. 

Not the best way to model a common hydraulic configuration, but it's a good workaround.

[Hidden_Brain]

good catch, the outfall cannot have two connecting links, barrel and emergency spillway. another way to get around this would be to have two outfall nodes with identical tailwater condition, each connecting to the barrel and the spillway (provided both outfall into the same waterway).

[Matt.Anderson]

Outlets in the SWMM world is where water leaves a system.   Hence the limiter in SSA, InfoSWMM, PCSWMM, and EPASWMM.

Pond Outlets should not be connected directly to outlets - as Pond Outlet performance in many cases are impacted by tailwater conditions of the downstream geometry that OUTLETS structures cannot provide.