Modeling a Vegetated Underdrained Soil Filter 1

[Chaskell27]

I have come across some results recently that seem a bit odd when modeling an underdrained soil filter in HydroCAD 10. I have always modeled my primary outlet as exfiltration only above the invert elevation with a velocity input depending on the soil media. My secondary outlet is the grate on top of my emergency overflow structure which also collects the underdrain before discharging offsite. I can see two problems with my latest models:

1) My outflow volume does not match my inflow volume (roughly 50% of the inflow) even though I have the exfiltration specified as a primary outlet. Theoretically, shouldn't the two volumes match assuming no loss to the surrounding soils?

2) My Outflow rate exceeds my Inflow rate. In one case I am showing an inflow of 4.05 cfs and an outflow of 6.55 cfs. What am I missing?

Any help would be appreciated.
Chip

 

[TerryScan]

Without seeing your actual data, it could be hard to say. That being said:

1. the most common reason for an outflow volume to be half of the inflow would be that the modeling time has not extended until flow is complete.

2. the most common reason I have found for outflow greater than inflow is that modeled water surface elevations have exceeded those I have defined. This can create a erroneously high head condition which produces an erroneously high outflow.

 

[psmart]

This looks like the same question sent to HydroCAD support today.

You'll get an answer either way, but if you use both channels we have to spend extra time responding to both inquiries.

Please post only in one place.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Chaskell27]

My apologies, I was not aware that this forum was monitored by HydroCAD support as well. I appreciate the timely responses though and will check into my model when I get a chance.
Thanks,

 

[TerryScan]

The great thing about posting here is that everyone gets the benefit of seeing solutions to what may be common problems.

Also, if one searches this forum, one may find the solution and save Peter support time, allowing him to keep improving this great product!

 

[beej67]

Echo Terry's comments above.

First thing I'd do, is add some dummy storage above the crown of all your detention ponds, to make sure you're not pegging them out, and see what the head in those ponds actually comes to. If that fixes the inflow/outflow problem, then your ponds are too small.



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

http://www.campbellcivil.com

 

[psmart]

Have you reviewed all the warning messages?

I assume you're getting a Qout>Qin warning - click the message for complete details and solutions.

If you're getting an "overfilled storage" message also see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net