HydroCAD Infiltration Cell Modeling

[craleigh]

I have a storm water scenario I'm having trouble with. I have a wet pond, nwl with an 8" outlet at elevation 100. At 102, the pond overtops (weir flow) into an infiltration cell whose bottom is 101. So at elevation 102.1, my wet pond is 2.1' above the outlet and there is 1.1' of water in the infiltration basin. I have 27" horizontal orifice overflow at elevation 103. Let's just say my wet pond surface areas are as follows: 100=10000 sf, 101=11000 sf, 102=12000 sf, 103=20000 sf (wet+ area above infiltration), 104=22000 sf (wet+area above infiltration), 105=24000 sf (wet+area above infiltration). The infiltration cell areas are 101=5000 sf, 102=6000 sf.

How do I model this in HydroCAD?

Here's what I've tried: I used 2 ponds (1 wet, 1 infiltration). My wet pond had a primary orifice (8") at 100, a secondary outfall (weir) at 102, and another primary outfall (27" horiz. orifice) at 103. My infiltration pond only had exfiltration as an outlet because what doesn't exfiltrate will keep on filling up with the wet pond. However, when my infiltration pond filled up, I couldn't model it to go "back" into the wet pond as it was a circular reference.

So then I tried modeling it as one big pond. 100=10000 sf, 101=11000sf+5000sf, 102=18000sf, 103=20000sf, 104=22000sf, 105=24000sf. I had my 8" outlet at 100, my 27" outlet at 103, then exfiltration between elevations 100 and 102 and I used a discharge multiplier of 0.3333 (since the infilt pond was about 1/3 of the size of the whole pond system.) This at least 'worked' in the model but it didn't give me very high infiltration volumes so I question if that was the correct way to do it.

Does anyone know the 'right' way for a scenario like this? I know HydroCAD has provisions for embedded storage but I'm not sure if that is what this would be.

 

[craleigh]

Now using Dyn Stor Ind and based on some other warnings I've gotten, I changed the dt to 0.01, finer routing to 3, end time of 40 hours. Here are the warnings I still get:

(87) Warning: Pond 2P Oscillations may require Finer Routing or smaller dt
(80) Warning: Pond 3P Exceeded Pond 2P by 1.49' @ 21.43 hrs (0.00 cfs 2.299 af)

 

[psmart]

A number of conditions can trigger oscillations. I really need to see the project file to comment any further. You can post a link here or send it to HydroCAD support.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

Also see

http://www.hydrocad.net/oscillations.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

Dealing with oscillations is one of the hardest things about stormwater modeling, in any software package. There are many knobs you can twiddle to try and get rid of them, but be careful not to twiddle them irresponsibly. Peter's the expert obviously. One thing you may try, if twiddling the knobs doesn't get rid of them, is inserting small amounts of dummy storage at other nodes in your network, such as inlets or catch basins that have some but not a lot of storage. That will tend to dampen the oscillations some, as well as round out the inflow hyetograph some, which can help.

I remember one XPSWMM model on which I just couldn't solve the oscillation problem, and after about four hours of struggling with it, I discovered my design actually created a condition where the water sloshed back and forth. So in the end, I was modeling it properly, but the design created a sloshing condition.



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

http://www.campbellcivil.com

 

[Twinkie]

Can you post a link to your file?

I'd like to take a crack at this if you don't mind, and I think it would help us all get a better visualization.

I'm just curious, why is your infiltration basin downstream of the wet pond? Why not use the infiltration basin upstream, take care of the lower level storms, and then the wet pond handles the larger storm events.

 

[psmart]

FYI, this question was resolved directly with HydroCAD support. The setup is a bit unusual in that the infiltation area is basically to the side of the wet pond, and only receives flow at a higher WSE, and then reverses and drains back into the wet pond as the outlet devices are able to pass the flow.

The final solution was to model the infiltration volume as part of the wet pond's storage. Using a separate storage definition for the infiltration area, you can deselect the option to "allow exfiltration" in the wet pond volume definition.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[craleigh]

Twinkie I don't really have a spot to upload this file to the web so I'm sorry about that. But to address your question as to why my infiltration is downstream of the wetpond, it is so that we can settle out a bunch of the TSS prior to infiltrating. If I were to have my infiltration before the wet pond, I'd be infiltrating 'dirtier' water, my infiltration basin would clog up much quicker, and the solids that stay in the bottom of the infiltration basin would re-suspend every larger rain event.

As psmart says, we did resolve this. It's a strange design as the infiltration basin doesn't receive flow very often but a coworker had asked me if it could be modeled so I was just trying to figure out a way.

 

[Twinkie]

I always do some form of pre-treatment prior to an infiltration BMP (snout, forbay, etc.)

I was just curious as to the thought process. Glad you got this resolved.