How to Model a Filtration Bioretention System

[MrNYPE]

I'm trying to figure out how to model this type of practice so I can understand how the Stage Voids.



I have the pond setup as follows, so that the lower four feet is the 40% porosity soil and the upper 1.5 feet is open water. In the outlets I have an orifice/grate at 523.5 and the culvert at the bottom.



When I do this, I get an error stating "First Voids Must Be Zero". I searched this forum and google and haven't been able to figure out what that means.

Would appreciate suggestions on how to model this/fix this error.

 

[TerryScan]

Line #1 should have voids =0
Line #2 should have voids=40 (voids from previous line to this elevation)

 

[MrNYPE]

Thanks! Was speaking with a colleague and he suggested the same, which at least fixes the error code.

However, it still doesn't model correctly as a single node filter practice in Hydrocad. It looks like hydrocad fills a basin from the bottom up, regardless of the elevation of the inlet pipe and the pond layers. So this doesn't account for the travel time through the media.

Turns out I have to use two nodes- first is for the pool at the top, which exfiltrates into the second node which is another pond w/ 40% porosity and it's own exfiltration rate through the media into the underdrain/culvert.

 

[psmart]

It is assumed that all the pond storage is interconnected so that it will fill from the bottom-up, always maintaining a level pool. The outlet devices, including exfiltration, regulate the flow of water out of the defined storage.

The problem with your setup is that you are attempting to include the storage volume in the media, which is not part of the free-flowing pond storage. The media voids are downstream of the exfiltration control point, so they are not part of the same level-pool, and should not be included in the storage definition.

For details see www.hydrocad.net/raingarden.htm