Filtration basin modeling 2

[hydroponder]

I am trying to model a filtration basin where I have 24" of planting medium (50% sand, 25% topsoil, 25% compost) and a 4" draintile at the bottom. I am wondering if anybody has an estimate on the void ratio of the aforementioned planting medium? How does 30% sound?

Also, has anybody had any problems with routing exfiltration to another pond, i.e. through the 4" draintile. Sometimes my volume out is greater than my volume in...

 

[hydroponder]

Another question to add: Assume that I am modeling a filtration basin with bottom of ground elevation at 950. I have two storages set up with about 2 feet of storage above ground (952-950) and 2 feet of storage below ground (950-948 w/ 30% voids). The outlet consists of the follow; a behive catch basin, feeding into a 12 inch culvert, and a 4 inch draintile feeding the same 12 inch culvert. I am trying to model the filtration through the 24" planting medium to the 4 inch draintile and then to the the 12 inch culvert. Should I have "allow exfiltration" checked on the below ground storage? I am not really sure how to make this work. I am thinking that the filtration is the controling factor, not the 4 inch orifice, but I could be wrong. I read the "modeling raingardens" on the HydroCAD site but still have these questions. Thanks to anyone with some ideas.

 

[TerryScan]

In the past I have used a void ratio of 40% for bioretention media. (one can verify this from numerous sources using a google search for "bioretention media void ratio".

Read thread:

http://www.eng-tips.com/viewthread.cfm?qid=209756

We had a detailed discussion on the routing issues you mention.

 

[psmart]

If you are getting Qout>Qin there is a problem, which will be flagged by a warning message. For usable results, you must resolve all warning messages - or at least understand them and make your own determination that the results are still valid.

Please click the warning message for details and recommendations. Until you fix the warning, do not proceed or draw any conclusions from the model.

BTW, 40% voids is generally the best you can get from clean stone, so I suspect the value is much too high for bioretention media.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[kmatte]

Generally I would not model the filter media as part of my storage. In hydrocad, it uses this storage as instantaneous storage, which is not very accurate, as the flow would come from the top down. I would model this by only using the bouce storage. One outlet is exfiltration, but do not discard it, instead route it to a link and use a delay time. The delay would be based on the depth of media and your flow rate. For a cleam manufactured media, I use 1.8 inches per hour. So you would set a delay time of 13 hours in the link.

Modeling beehive orifice using a horizontal oriface using the square foot opening of the casting to determine the opening diameter.

Model the 12-inch pipe as a reach. route the oriface and the delay link to the the reach. Hope this helps.

P>S. As mentioned by psmart, try to fix the in/out error. You could try to decrease the dt or increase the size of the live storage.

 

[psmart]

I agree with kmatte about the complexity of modeling the filter media. Since it doesn't saturate immediately with water, it's not really part of the "pond" storage, nor is it an instantaneous outlet device. So you'll have to make some choices about how to setup your model. Try to keep it as simple as possible. And make sure you identify the exact objectives of your HydroCAD model.

BTW, the article mentioned in the OP is at

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net