Modeling underground storage as infiltration

[Anonymous]

This is my first experience trying to model an underground infiltration/storage vault. This seems like a pretty easy model but the data I'm getting back doesn't make sense. I have attached the file if anyone wants to open it. The issue I'm having is I'm getting a Qpeak rate for a vault that is underground. The vault elevation does not fill completely, and there is no outlet for the stormwater to discharge to other than infiltration. At some point I will need to add an overflow orifice that will throttle flow, however I'm not at that point yet. Unless I'm looking at this wrong, why wouldn't the Qpeak be 0 if the elevation doesn't fill up? unless it's assuming that there are outlets or discharge points from this underground vault. Any thoughts on this? 

[Anonymous]

I know this was a few months ago but hopefully I can still help:

• The infiltration rate is 3.75 in/hr, which is pretty good. Here, we generally use 0.5 in/hr as a minimum and don't often see above 2 in/hour. No need to change this, but it will decrease your peak flows if...
• You have "extract from hydrograph" checked- this will take whatever is infiltrating into the ground and remove it from the pond discharge, thus lowering to 0 cfs in this case. Try unchecking this in preliminary design to be conservative. You can always check it again later you need to show more peak flow reduction 
• There are a lot of barrels for such a small site. I usually start sizing my systems by checking the 100 year storms and the difference in volumes. For your site, this is around 4,100 CF. Try lowering this system to 21-25 barrels, which is more in the 4,000-5,000 CF range.

Even with these changes, the model is going to show a low cfs because the only thing discharging is the gravel bed. Once you input a discharge culvert, the peak flow will increase. 

[lynn_zhang]

Hi @Anonymous Welcome to Autodesk Community!

It's never too late to share your knowledge 🙂 Thanks for sharing your solution! 

@Anonymous Have you tried the suggestion yet? Let us know if it helps to solve your problem.

[fcernst]

It sounds like you're wondering about the Stage-Discharge curve... That shows what peak flow you can expect at that pond stage per your outlet configuration.

If you turn off the extract hydrograph setting, you'll see it peaks at 0.5 cfs with a elevation of 1203.06... which makes sense when you look at your stage-discharge curve.

[jwagoner7V9QG]

@fcernst  I have a similar situation I would love to get your input on, I keep seeing your helpful feedback on this forum. The condition in question is the underground sandfilter/detention module notes as SCM1A. The runoff is routed through a splitter box that contains a WQV overflow weir. When the sand filter stages up the WQV it backflows into the splitter and overflows the WQV weir into detention. Both boxes then equalize to detain the 10-yr storm using outflow/overflow weirs at the same elevation and work together to safely pass the 100-yr storm. The reviewer is concerned that the diversion methodology is not producing accurate results. Thoughts on how this is modeled? The design is not intended to be in series, but is in sequence or in tandem. 

[fcernst]

I agree with the Reviewer..there's no dynamic equilibrium capability between the sand filter and the detention pond in this model setup.

I would have you model this in a dynamic model such as SWMM or SSA.